2.5L TBI Jeep YJ Wrangler Running Rich and Won't Idle

[JP88YJ]

Hi,

Just joined but have been using these forums for a while for information. 

SO I have an 88 YJ with the TBI. My issue all started when I went to get it smogged last year. Short story it failed 3 times for high NOX @ 35mph. By the third failure I had replaced every sensor I could replace, including the EGR. The cataliytic converter only has about 1000 miles. I do have the snap-on diagnostic tester, just looking for some direction to narrow my troubleshooting. Currently the jeep wont idle on its own and I can smell that it is running very rich and it seems to run a bit rough.

Thanks!

[Moses Ludel]

JP88YJ...The rich and rough idle are a clue.  NOx is handled by the EGR under normal conditions.   Sounds like your EGR valve vacuum is not applying correctly and/or you have an EVAP canister issue.  This could be a vacuum hose routing problem, a defective/missing EGR Solenoid or no current to the EGR Solenoid at an idle. 

Assuming that the pressure regulator is holding TBI pressure normally at the TBI unit and the fuel return from the TBI is not restricted, check for a vacuum routing issue or defective EVAP canister.  (See our many forum discussions about the regulator pressure, injector leaking, fuel supply and fuel return troubleshooting.) 

Your EGR is controlled by the EGR Solenoid.  It should not get direct vacuum from the intake manifold, rather the manifold vacuum routes through the EGR Solenoid.  If the EGR valve is open at an idle, you should be able to disconnect the vacuum supply hose at the EGR and see a difference in the idle.  If the idle stabilizes and what seems like a rich condition disappears, the EGR valve is open when it should be closed. 

I have attached two PDF vacuum hose schematics for your YJ Model 81 2.5L TBI engine.  Note the hose routing.  The diagrams are similar, the upper PDF also shows a 4.0L for comparison.  Note the 2.5L details, including the EVAP canister Tee and other vacuum features:

Jeep YJ Wrangler 2.5L TBI Engine Vacuum Circuit.pdf 

YJ Four-Cylinder TBI Vacuum Schematic20200406_12500982.pdf

Confirm vacuum routing through the EGR Solenoid.  With the engine idling, check the vacuum at the supply side (from manifold) of the solenoid.  This should be 15 in.hg. or higher.  Now check the vacuum at the EGR side of the solenoid.  If the engine is idling, that reading should be zero. 

Disconnect the EGR Solenoid wire connector:  The vacuum should read the same on both sides of the solenoid.  The EGR valve solenoid is closed by electrical current. Disconnecting the electrical supply will allow the solenoid to open.  If the EGR Solenoid is open at an engine idle, either the solenoid is defective or there is no current to the solenoid.  Check for supply current at the EGR Solenoid wire connector with the engine idling.

 If the solenoid vacuum has been bypassed (vacuum going from the manifold directly to the EGR), your EGR is open at an idle and low speeds.  That will cause a rough idle.

Also check vacuum routing to the EVAP canister.  Check the EVAP function.  Check the fuel filler EVAP system and the gas cap seal.  Also, your YJ 2.5L TBI engine does use a PCV valve as part of the crankcase ventilation system.  Make sure the PCV Valve is in good condition with vacuum applying correctly.  The valve closes at an idle and opens as engine speed increases.  A defective PCV valve will cause a rough idle.  The PCV valve is not expensive to replace periodically.

Moses

[JP88YJ]

Just wanted to give an update on my issue. Found a couple issues that I was able to resolve. The EGR solenoid was not functioning found the ground was bad. fixed that and now it works. I also discovered, after checking my fuel pressure, that I didnt adjust it after I put a new regulator in and it was too high.  I still have the issue with too high of NOX during the 25mph test, Not sure where to go now. Could it be excess carbon buildup?

[Moses Ludel]

JP88YJ...NOx is usually a high upper cylinder temperature issue.  The purpose of the EGR system is to reduce combustion temps below 2500 degrees F, which reduces NOx.  Make sure the EGR is functioning properly and actually recirculating exhaust at the engine speed you describe.  The EGR valve could be sticking closed or clogged.

Another item would be spark timing.  Make sure the base timing is correct, even though timing is actually a function of the ECU.  Use a timing light to verify timing at an idle and also at the 25 mph engine speed.  Make sure the spark timing is not too far advanced.  Avoid the fan when increasing engine speed and checking the timing.

I would also check for an exhaust restriction (muffler, pipes, cat, etc.) or a clogged catalytic converter.  Make sure the oxygen sensor is working properly as well.  If NOx is the only measurement out of specification, my primary focus would be a non-operative EGR or exhaust restriction.

If you do suspect excess carbon buildup, do a compression check.  Higher than normal compression readings would indicate high carbon buildup.  In any case, Sea Foam would do wonders here.  I do a Sea Foam injector cleaning at routine intervals.  Here's my MPI/EFI approach.  You can do a similar approach with TBI.  Sea Foam also makes a nozzle spray cleaner that you can simply spray down the TBI unit's throat.  Follow directions for carbon cleaning:

https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

I do a walk-through of Sea Foam products in this video, beginning at 6:47-minutes.  You'll see the spray cleaner:

https://www.4wdmechanix.com/curing-an-engine-knock-with-surr-tools-and-sea-foam/

Let us know what you discover...

Moses 

[JP88YJ]

Hi

been a while since I last posted. Tried the sea foam, replaced some hoses.

decided to pull the head. Pic attached there was a bit of carbon build up on the pistons and on the head by the valves. Also not sure why the one valve on the rear most cylinder is white. Even inside the head on the valve is the same. Any advise on what to do since I have the head off? Also added a couple more pic taken from the troubleshooting tool. Before I pulled the head.

[JP88YJ]

Ment to send this one also 

[Moses Ludel]

JP88YJ...Did you run a compression test or, better yet, a leakdown test before pulling the cylinder head?  What do the cylinder walls look like?  Is there noticeable taper?  A cylinder ridge above the top ring(s)?  The fourth cylinder looks like low compression, which could be from valve leakage and/or worn piston rings.

The wear I describe is part of diagnosis.  Disassembly of the valves will reveal any valve seat, valve stem or valve face wear.  However, before removing any valves or loosening springs, run a quick check of the valve sealing.  With the intake/exhaust ports facing upward, carefully fill the ports with a solvent.  Watch the valve seat-to-face areas for liquid leaking between the valve faces and seats.

Some photos of the cylinder walls with the pistons lowered in each cylinder would be helpful.  We can assess the condition of the block, which in turn indicates piston ring wear.  (Without a leakdown test, this is the remaining means for assessing ring wear.)  Also check the piston to cylinder wall clearances with the pistons lowered about 3/4-inch below the block deck. 

Moses

[JP88YJ]

I will perform the leak test. The cylinder walls show some wear, can still see the cross-hatching. I beleive there is a ridge at the top of the cylinders but will check again. Will take some more pics also.

[Moses Ludel]

JP88YJ...Curious if the valves are sealing.  Was the engine using oil? 

Good sign about the cross-hatch.  "Wayne" and I had a constructive, sensible approach as he rebuilt his 2.5L, a later MPI type though much like what you would be doing.  Since this appears the direction you've taken, I recommend going through our exchange on Wayne's engine rebuild, which turned out very well without an out-of-chassis boring.  Wayne did meticulous micrometer reads and held honing to precisely the amount needed and nothing more...You may be able to do this if wear is similar to his 2.5L, your measurements will tell:

The cylinder head work will be a sublet.  Depending upon head decking, check the pushrod lengths with a CompCams gauge after the cylinder head work and installation—before selecting new pushrods.  A new camshaft, lifters and timing set is always part of the build.  You may need more machining and parts, depending upon the wear you find.  That will determine whether the engine stays in the chassis or not.

Moses 

[JP88YJ]

So I finally got the Jeep back together, new head, new cam, new lifters, new push rods. Now it won’t start when turning the engine over I get a pop out of the TBI at regular intervals. Thinking it’s a timing issue. The jeep shop manual is a little confusing on installing the distributor. I double and triple checked the timing marks on the timing chain. 
 

 

[Moses Ludel]

JP88YJ...This does sound like ignition timing...At least for now, let's assume that you got the valve timing (chain and sprockets) aligned properly.  With the valve cover and distributor cap removed, key OFF and coil-to-cap lead removed, rotate the crankshaft slowly with a ratchet and socket in the crankshaft's normal direction of rotation.  Watch the #1 intake and exhaust valves closely.  When the intake valve begins to open, keep rotating the crankshaft slowly and watch for the timing marks to come into alignment.  Do not pass the TDC/0-degree mark.  Stop at the TDC or 0-degree advance mark.

Note precisely where the rotor points.  It should be pointed directly toward the #1 spark plug wire in the distributor cap.  If not, either the distributor housing and/or the distributor shaft and rotor are misaligned.  Follow the shop manual distributor installation and rotor alignment instructions with the crankshaft in this position.  You may need to rotate and align the oil pump tang with a large screwdriver to allow the rotor and distributor shaft to drop into the correct position.

The firing order is 1-3-4-2.  Note that the rotor moves clockwise.  Make sure the cap wires attach to the spark plugs in a clockwise rotation, the leads running to the 1-3-4-2 spark plugs.

Let us know if this solves your backfire problem...

Moses

[JP88YJ]

Following the shop manual the rotor is at 6 o’clock when the timing mark is at TDC. 
the two marks on the cam gear and crank are aligned. 
Intake valve, cylinder 1 starts opening once I pass TDC.

is that right?
something seems off to or maybe I’m overthinking it?
Could the alignment pin be in the wrong hole on the cam causing it tobe180 off?

hope I’m wrong

[Moses Ludel]

JP88YJ...Just past crankshaft TDC should be either the power/expansion stroke or the beginning of the intake stroke.

The TDC mark on the damper is top dead center for #1 (cylinder) piston.  As the piston moves down from here, it will be either on the intake stroke or the power stroke.  (The camshaft rotates at half the speed or degrees of the crankshaft.)

For ignition timing, you want to ignite the fuel mix at the top of the compression stroke.  This is the piston rising up to TDC before the power stroke.

Sounds like you have the spark timing off 180 distributor degrees.  The seated distributor should have the rotor pointing toward 6 o'clock with the piston at TDC on the compression stroke.  You now have the piston at TDC on the exhaust stroke, ready to fire.  This is the wrong rotation of the crankshaft.  The ignition fires near TDC of the compression stroke, which requires rotating the crankshaft another 360 degrees.

Your cam/valve timing may be smack on.  Your ignition firing is off.  Rotate the crankshaft until you are confident that the #1 piston is rising on its compression stroke (not the exhaust stroke).  The compression stroke follows the intake stroke in the normal rotation of the crankshaft.  This will be 360 crankshaft degrees from where you are now.  Park the crankshaft at TDC on the compression stroke.

If I'm correct, the distributor rotor will be pointing toward the #4 plug wire.  Loose the distributor and rotate the shaft and rotor approximately 180-degrees or until the rotor points toward #1 spark plug wire at 6 o'clock.  #1 spark wire should be at the 6 o'clock position. 

Make sure your spark plug wires are 1-3-4-2 in clockwise rotation at the distributor cap.  The engine's ignition will now be in time with the valve opening events and not 180 distributor degrees apart.

With the 2.5L TBI engine, the distributor housing locates in one position (pinned).  You do not move the distributor housing to change base spark timing.  All spark timing is controlled by the ECU.  The only concern you have is positioning the distributor housing and rotor correctly.  The rotor points to #1 spark plug terminal at 6 o'clock when the #1  piston is at TDC of the compression stroke.

Let us know how this works...

Moses 

[JP88YJ]

I think I’ve got it now. Thanks for the help

 

[Moses Ludel]

I think you'll be driving the Jeep shortly...

[JP88YJ]

Good new Jeep fired up on first try! Seems to be running much smoother. Got to button up some things but I think im in the home stretch to passing the smog test.

thanksfor all the help

[Moses Ludel]

Good news, indeed...You did have the valve timing set correctly.  Spark timing is now on the mark, and you will pass smog!

[JP88YJ]

Quick update, I’m getting ready to take the Jeep in for smog. Doing some checks and noticed something. The egr inst functioning. Checked egr solenoid has power . No vacuum on output. Disconnect power have vacuum and EGr moves when throttle let off. Apply power to solenoid no egr function when letting off throttle. Is the ECU controlling power to solenoid   . What if I bypass solenoid will that be a smog issue?

 

[Moses Ludel]

JP88YJ...Yes, the solenoid function is controlled by the ECU.  "When energized by the ECU the solenoid closes and prevents vacuum from reaching the EGR valve.  When not energized, the solenoid is open and vacuum is applied to the EGR valve."  On circuits and devices controlled by the ECU, think of the ECU as the ground source.  You have a hot source to the solenoid that's active as you discovered.  The ECU provides the ground signal.  The #5 pin on the 35-way ECM/ECU connector is a blue wire (BL) and 14 gauge.  This is the ECU signal for the EGR solenoid.  Your vehicle grounds are important, the reason everyone discusses the body-to-engine and frame grounds, including the classic dipstick ground or body strap ground(s).

An emissions check, if "visual", will test the function of the EGR solenoid, i.e. whether the EGR valve opens and closes at the right time.  I would want the system to function normally, otherwise the tailpipe readings will be wrong at the emission test's rpm thresholds.

On your YJ Wrangler, the evaporative canister is also on the vacuum circuit with the EGR.  Below is the vacuum diagram and FSM troubleshooting steps.  Do you have an FSM for your YJ?  A CD reprint of the 1987-90 FSM is practical.  I found this at eBay with a quick search.  The source is reputable, and the CD enables faster navigation plus printing of individual pages to take into your garage.  A CD avoids the paper deterioration and getting smudge prints on an older (used) book.  I use Mopar OEM/FSMs, not aftermarket manuals like Haynes or Chilton:

https://www.ebay.com/itm/152798258131

Moses

[JP88YJ]

So finally had time to smog the Jeep and it passed. One thing though, in order to get it to pass the guy , when doing the 35 mph synonym test he had to vary the rpm within the test limits. This kept the NoX low. If he used constant rpm it would have failed . This the same failure since the beginning of this saga.

Lin happy he got it to pass, but still want to fix this.

[Moses Ludel]

Glad it passed, JP88YJ...If you need to pursue this further, try these:

1) Check the exhaust pressure/back pressure.  Make sure the cat and muffler are unrestricted and that the cat is working normally.  An infrared, non-contact temp gauge or FLIR test is very helpful with the cat.  

2) Run a vacuum leak test ("smoke test" if possible).  Check vacuum pull at the EGR.

3) If you haven't done so, clean and check the EGR valve for carbon and a sticky plunger.  Make sure the diaphragm is not overstretched.

Moses

[Julio]

I have a very similar issue with my 2.5 TBI YJ. Mine idles at 1000 fixed rpm. Smooth. Changed every possible sensor including a NOS tach and a new CAT. I'm unable to pass the emission test. Altough the tach is showing a steady 1000 rpm, the SNAPON scanner shows 730 rpm. 

 

 

[Moses Ludel]

Julio, what does the smog shop say is the cause for the failed test?

Moses

[Julio]

14 hours ago, Moses Ludel said:

Julio, what does the smog shop say is the cause for the failed test?

Moses

Thanks a lot for your message.

I dont remember the numbers exactly. Mixture was too rich and the jeep was spitting gas and black particles from the escape. This was May 2021. Eversince I have changed everithing that I have read and watched on forums and videos. (TPS, IAC, MAP Sensor, 02 Sensor, TBI gaskets and Coolant T Sensor, distributor cap, sparks and wires, air and fuel filter, even a new TACH. The las thing I ve replaced was the injector.) Nothing happens, iddles evenly at 1.000. Like it was programed to do that. Besides that the jeep runs great. 

I tried what I read in one of your posts about disconecting the egr solenoid and watch for a reaction. It makes a difference.

I have looked around the vacum lines not thoroughly. But havent fount any parent leak using wd40 and cigarete smoke. There is one line missing on the font axle. The one that goes into the dashboard line. Also, the in the same 4wd harness there is a blue cable (I believe is vent) plugged with an iron ball. 

 

 

IMG_9048.HEIC IMG_9047.HEIC IMG_9045.HEIC

[Moses Ludel]

Julio...I took your images into my photo edit software then rotated and sharpened them for viewing.  They are very helpful, and below each picture, I have make observations about obvious issues:

 

RPM normal; Fuel Trim short term looks okay; LT (long term) fuel trim very high:  total for fuel trim high.

Nothing of obvious concern here...

Exhaust rich...

For a quick observation, the rich exhaust and high LT fuel trim hint that either the fuel flow is enriched or the exhaust is restricted.  The catalytic converter could be clogged or not functioning properly—or both.  The idea is to find the issue and not replace parts until you know they are defective.

First, I would check the fuel pressure at the TBI fuel test port with a fuel pressure gauge.  Run the test at idle and 2,500 rpm.  (Stay away from the engine driven fan, do not lean over it when revving the engine.)  Make sure the pressure is normal or 14-15 psi.  If pressure is high, that could be the cause for running rich.  If pressure is high, the issue could be 1) the TBI pressure regulator, 2) leaking regulator gaskets or 3) a restricted return line from the TBI unit back to the fuel tank.

The pressure regulator can be adjusted if slightly out of range.  If defective, the regulator needs to be replaced.  You want the pressure to be a constant 14-15 psi at the TBI test port.  Another cause of high fuel pressure can be a fuel pump that has been replaced with a 1991-up MPI fuel pump instead of the lower pressure output 1987-90 style TBI type.

The concern is pinpointing the cause of high fuel pressure.  If pressure is normal at the test port from an idle to 2,500 rpm, I would consider an exhaust restriction.  Flow out the tailpipe can be measured.  A simple test is to feel the actual pressure at the end of the tailpipe.  Have someone accelerate and decelerate the engine during the test.  Use a glove to protect your hand.

You did not share the engine's cranking compression.  I would run a compression gauge test for a quick overview.  Disconnect the coil high tension wire to prevent an engine start.  Hold the throttle open...If there is more than 10% difference between the highest and lowest cylinder psi, the engine balance is off.  If one or more cylinders have low compression, run a cylinder leak down test on the low cylinder(s) to see whether the valves, rings or a blown head gasket are at fault. 

Vacuum circuits do need to be correct.  At these forums, I have posted the vacuum circuit diagram(s) for the 2.5L TBI engine bay if you need to check the hose routing.  Search the forum with keywords "vacuum", "2.5L" or "2.5L TBI" to find one of the exchanges that includes YJ 2.5L vacuum diagrams.  Usually these are PDF attachments that you can open.

As for tachometer readings, I would trust the Snap-On scanner.  Be aware that the tachometers were different for 2.5L fours and six-cylinder engines.  Did you install a 2.5L (four-cylinder) tachometer?  If your diagnostic scanner or another tool has an induction pickup that clamps over the #1 spark plug lead and provides a tachometer signal, you can get the most accurate rpm reading that way. 

The reading you get on the Snap-On scan tool is simply information/data from an ECU/ECM/PCM.  An rpm signal is usually data  from the crankshaft position sensor.  The only way the information could be inaccurate would be a problem with the CPS (sensor) or the ECU.  Does it sound like the engine is idling at the speed displayed on the scan tool?

Let us know the TBI test port fuel pressure reading...

Moses

[Julio]

On 11/25/2022 at 1:49 PM, Moses Ludel said:

Julio...I took your images into my photo edit software then rotated and sharpened them for viewing.  They are very helpful, and below each picture, I have make observations about obvious issues:

 

RPM normal; Fuel Trim short term looks okay; LT (long term) fuel trim very high:  total for fuel trim high.

Nothing of obvious concern here...

Exhaust rich...

For a quick observation, the rich exhaust and high LT fuel trim hint that either the fuel flow is enriched or the exhaust is restricted.  The catalytic converter could be clogged or not functioning properly—or both.  The idea is to find the issue and not replace parts until you know they are defective.

First, I would check the fuel pressure at the TBI fuel test port with a fuel pressure gauge.  Run the test at idle and 2,500 rpm.  (Stay away from the engine driven fan, do not lean over it when revving the engine.)  Make sure the pressure is normal or 14-15 psi.  If pressure is high, that could be the cause for running rich.  If pressure is high, the issue could be 1) the TBI pressure regulator, 2) leaking regulator gaskets or 3) a restricted return line from the TBI unit back to the fuel tank.

The pressure regulator can be adjusted if slightly out of range.  If defective, the regulator needs to be replaced.  You want the pressure to be a constant 14-15 psi at the TBI test port.  Another cause of high fuel pressure can be a fuel pump that has been replaced with a 1991-up MPI fuel pump instead of the lower pressure output 1987-90 style TBI type.

The concern is pinpointing the cause of high fuel pressure.  If pressure is normal at the test port from an idle to 2,500 rpm, I would consider an exhaust restriction.  Flow out the tailpipe can be measured.  A simple test is to feel the actual pressure at the end of the tailpipe.  Have someone accelerate and decelerate the engine during the test.  Use a glove to protect your hand.

You did not share the engine's cranking compression.  I would run a compression gauge test for a quick overview.  Disconnect the coil high tension wire to prevent an engine start.  Hold the throttle open...If there is more than 10% difference between the highest and lowest cylinder psi, the engine balance is off.  If one or more cylinders have low compression, run a cylinder leak down test on the low cylinder(s) to see whether the valves, rings or a blown head gasket are at fault. 

Vacuum circuits do need to be correct.  At these forums, I have posted the vacuum circuit diagram(s) for the 2.5L TBI engine bay if you need to check the hose routing.  Search the forum with keywords "vacuum", "2.5L" or "2.5L TBI" to find one of the exchanges that includes YJ 2.5L vacuum diagrams.  Usually these are PDF attachments that you can open.

As for tachometer readings, I would trust the Snap-On scanner.  Be aware that the tachometers were different for 2.5L fours and six-cylinder engines.  Did you install a 2.5L (four-cylinder) tachometer?  If your diagnostic scanner or another tool has an induction pickup that clamps over the #1 spark plug lead and provides a tachometer signal, you can get the most accurate rpm reading that way. 

The reading you get on the Snap-On scan tool is simply information/data from an ECU/ECM/PCM.  An rpm signal is usually data  from the crankshaft position sensor.  The only way the information could be inaccurate would be a problem with the CPS (sensor) or the ECU.  Does it sound like the engine is idling at the speed displayed on the scan tool?

Let us know the TBI test port fuel pressure reading...

Moses

Thanks a lot Moses for your kind, complete and fast reply. 

The catalytic converter is not clogged yet. I replaced a couple of month ago to rule out that theory. I have routed and checked the vacuum lines several times. I believe they are ok at this point. 

I will explore the fuel pressure theory. It sound very solid. Either the pressure regulator, the fuel pump or the return line. That's uncharted territory for me. Where is the pressure regulator? and how it can be adjusted? Do you know about any tool to check the pressure?

The new tach is for the 2.5 L TBI. 

Thanks again Moses, have a great weekend!

[Moses Ludel]

Julio...You're welcome.  I thought you needed to focus on the pressure regulator.  The unit is at the throttle body.  There is some adjustment possible with the balance spring/diaphragm design.  The diaphragm and mounting gasket can weaken, leak or be misfitted.  This is a source of fuel leakage, poor mileage and enriched exhaust for emissions tests. 

We have discussed the fuel pressure regulator here at the forum many times, so you will find details by simply using the search keyword(s) "pressure regulator", "2.5L TBI", etc.  Here is just one example below.  Note that I also mention the throttle position sensor voltage, which I would highly recommend checking.  The TPS can also create an enrichment issue or emissions test failure.  The TPS voltage and test procedure are described in detail within the factory workshop manual (FSM):

I have also shared a trick that's useful for determining the spray pattern of the TBI regulator and the injector.  You move the air intake horn aside from the top of the TBI unit.  Using your induction timing light, hook the pickup to #1 spark plug lead, and run the engine at an idle while shining the timing light at the TBI unit's throat.  The injector relies on TDC for #1 cylinder as a signal.  This gets picked up with the crankshaft position sensor and flywheel position. 

You will see the spray pattern of the fuel plus any leakage, bad flow or fuel dripping that shouldn't be there.  I did this with GM TBI as a dealership tech in the eighties.  A decade ago, I photographed the injector pulses on a Corvette Cross-Fire Injection (1982-84) 5.7L V8 engine for Corvette Magazine to illustrate a normal spray pattern.  The timing light was the light source in a dark room...It's a very cool and quick diagnostic method.

As for tools, I also have written to that subject in our forum exchanges about the 2.5L TBI system.  There is a plug at the TBI test port where you hook up a fuel pressure gauge.  You're looking for 14-15 psi at this test port.  This is a quick and easy test if you have the right gauge and fittings/adapter for the TBI port thread.  The port adapter fitting was a Chrysler tool.  You can improvise with the correct thread pitch fittings and a common fuel pressure gauge that reads in this range (0-30 psi would be fine, higher will work, you need an accurate 14-15 psi).  

There are CD versions of the factory shop manual (FSM) available for 1987-90 YJ Wranglers.  Specifications, tools and test procedures are covered.  You can get a CD or even a download (PDF format manual) at eBay.  This would be a sensible investment.  I rely on genuine FSM print copies in my library.  Actually, the CD or downloads are much easier to navigate or copy/print out pages for work in your shop.  I highly recommend that you get a copy if you plan to work on this Jeep® much.  Make sure it's a copy of the actual FSM.

Here is an example.  Bishko uses the factory FSM, and this is a CD version.  There are often download copies available for less, so shop a bit.  This is the manual that you want, it's discounted but still higher priced than in recent years.  (It could be that popularity is driving up demand.  Shop and find out.)  1987-90 YJ Wrangler manuals will work.  My print copy set is 1989 and works well for these model years.  If you find a 1989-90 FSM for less cost, it will work:

https://www.ebay.com/itm/152798258131

Let us know how this works out, Julio...

Moses

 

[Julio]

22 hours ago, Moses Ludel said:

Julio...You're welcome.  I thought you needed to focus on the pressure regulator.  The unit is at the throttle body.  There is some adjustment possible with the balance spring/diaphragm design.  The diaphragm and mounting gasket can weaken, leak or be misfitted.  This is a source of fuel leakage, poor mileage and enriched exhaust for emissions tests. 

We have discussed the fuel pressure regulator here at the forum many times, so you will find details by simply using the search keyword(s) "pressure regulator", "2.5L TBI", etc.  Here is just one example below.  Note that I also mention the throttle position sensor voltage, which I would highly recommend checking.  The TPS can also create an enrichment issue or emissions test failure.  The TPS voltage and test procedure are described in detail within the factory workshop manual (FSM):

I have also shared a trick that's useful for determining the spray pattern of the TBI regulator and the injector.  You move the air intake horn aside from the top of the TBI unit.  Using your induction timing light, hook the pickup to #1 spark plug lead, and run the engine at an idle while shining the timing light at the TBI unit's throat.  The injector relies on TDC for #1 cylinder as a signal.  This gets picked up with the crankshaft position sensor and flywheel position. 

You will see the spray pattern of the fuel plus any leakage, bad flow or fuel dripping that shouldn't be there.  I did this with GM TBI as a dealership tech in the eighties.  A decade ago, I photographed the injector pulses on a Corvette Cross-Fire Injection (1982-84) 5.7L V8 engine for Corvette Magazine to illustrate a normal spray pattern.  The timing light was the light source in a dark room...It's a very cool and quick diagnostic method.

As for tools, I also have written to that subject in our forum exchanges about the 2.5L TBI system.  There is a plug at the TBI test port where you hook up a fuel pressure gauge.  You're looking for 14-15 psi at this test port.  This is a quick and easy test if you have the right gauge and fittings/adapter for the TBI port thread.  The port adapter fitting was a Chrysler tool.  You can improvise with the correct thread pitch fittings and a common fuel pressure gauge that reads in this range (0-30 psi would be fine, higher will work, you need an accurate 14-15 psi).  

There are CD versions of the factory shop manual (FSM) available for 1987-90 YJ Wranglers.  Specifications, tools and test procedures are covered.  You can get a CD or even a download (PDF format manual) at eBay.  This would be a sensible investment.  I rely on genuine FSM print copies in my library.  Actually, the CD or downloads are much easier to navigate or copy/print out pages for work in your shop.  I highly recommend that you get a copy if you plan to work on this Jeep® much.  Make sure it's a copy of the actual FSM.

Here is an example.  Bishko uses the factory FSM, and this is a CD version.  There are often download copies available for less, so shop a bit.  This is the manual that you want, it's discounted but still higher priced than in recent years.  (It could be that popularity is driving up demand.  Shop and find out.)  1987-90 YJ Wrangler manuals will work.  My print copy set is 1989 and works well for these model years.  If you find a 1989-90 FSM for less cost, it will work:

https://www.ebay.com/itm/152798258131

Let us know how this works out, Julio...

Moses

 

Thanks again Moses. I remember the pressure regulator because I replaced it when I bought a kit to rebuild/refresh the TBI. All the gaskets and rings where there and that triangular shaped membrane was the pressure regulator. 

Do I need a special gaude to check the fuel pressure or a 40 USD tool from amazon is enough?  How is that adjustment performed on the pressure regulator? I do recall something like a sloted screw in the midle. 

[Moses Ludel]

Julio...As long as you use the correct fitting(s) to attach the pressure gauge to the test port, any fuel pressure gauge of decent quality will work.  Fittings should be available at NAPA (Weatherhead/Eaton) and other automotive or even hardware sources. We're not talking about very high pressure, so the fittings can be brass.  Distinguish the port thread size first...Once you know the thread size, you can put together a fitting.

Let us know what you find...

Moses

[Julio]

On 11/28/2022 at 10:23 PM, Julio said:

Thanks again Moses. I remember the pressure regulator because I replaced it when I bought a kit to rebuild/refresh the TBI. All the gaskets and rings where there and that triangular shaped membrane was the pressure regulator. 

Do I need a special gaude to check the fuel pressure or a 40 USD tool from amazon is enough?  How is that adjustment performed on the pressure regulator? I do recall something like a sloted screw in the midle. 

Hello Moses. I finally received the pressure test kit. Unfortunately, none of the fitting included fit into the "pressure test port plug". I had to remove the fuel supply tube and attach it there, hence, I was not able to test it with the engine running. Nevertheless, the readings  of the fuel pump where normal. 

 

Without removing the TBI I jumped into the other possibility, the “fuel pressure regulator”. I noticed the illustration you sent me depict a “pressure regulator screw”. Mine in the contrary seems to have a flushed or slim bottom. It there any protective cap underneath there? Thanks again!

[Moses Ludel]

Julio...Were you able to identify the thread size of the pressure test port plug?  Take the plug and your pressure gauge to a NAPA store with a good assortment of fittings.  You should be able to put together the needed fittings.  If you are successful, please let us know the thread size and type of threads at the port (female) end.  

Chrysler offered a TBI "Diagnostic Connector" for testing the TBI fuel pressure.  This connector (shown below) uses a Schrader valve and common EFI Schrader valve connection fitting.  (EFI fuel rail pressure testing kits usually attach to a Schrader test valve at the fuel rail.  For those on a budget, Harbor Freight has affordable EFI pressure testing kits like this one that will fit most applications:  https://www.harborfreight.com/basic-fuel-injection-service-kit-64939.html.)  The Chrysler/Jeep adapter to a Schrader valve is Mopar part number 83501572. 

I'm certain this adapter is only available used (maybe eBay) or as an NOS part (again, try eBay).  There is likely an aftermarket equivalent if you can determine the TBI port thread size.  A Schrader type fitting would enable quick testing at any time.  AMC and Chrysler should have made this adapter a permanent part of the TBI assembly.  This is a common test method for EFI/MPI fuel rails.  You can see how the connector replaces the TBI test plug in this illustration:

 

This is an illustration of the Mopar 83501572 adapter (or aftermarket equivalent) that works with an EFI Schrader valve test kit.  This illustration is listed in the Mopar parts catalog for 1988 Jeep 2.5L YJ and XJ engines and used on other applications of the AMC/Chrysler/Jeep 2.5L TBI engines.

Aftermarket Schrader valve adapters and EFI service kits show up at Amazon and elsewhere.  Make sure the Schrader is rated for fuel use.  If you can find an adapter/fitting that will fit the TBI test port thread, you will need a coupler to connect the adapter to your gauge.

Otherwise, the pressure tester that you purchased can be used by installing a temporary brass or steel "T" fitting between the fuel supply line and the TBI unit's fuel inlet fitting.  The fuel would flow from the fuel supply line to the TBI unit through the  brass or steel "T";  the third port of the "T" goes to your gauge.  You may need the "T" plus a few extra fittings to create this "adapter".  You can even set up this temporary "T" at the engine side of the fuel filter if that's easier or preferred.  The results should be close enough.

If the return line to the fuel tank is not obstructed, you should get the same pressure reading at this "T" as you would get at the test port.  Why?  Because the TBI regulator lowers the available fuel pump pressure by continuously flowing excess fuel to the fuel tank via the return line.  The pressure reading at the "T" should be in the 14-15 psi range when the fuel pump and engine are running.   

You mention not having an adjustment screw on the regulator's base.  Illustrations in the 1989 FSM show a recessed screw that may not be readily visible.  If you have a small mirror, see whether you have a screw that is flush or inset at the base of the bowl.  If not, as you suggest, the Mopar parts catalog shows a replacement regulator that has a preset spring tension and a ball holding the spring in place.  It looks like the illustration below and does not have the threaded adjuster screw shown in the FSMs.  The spring and ball set a fixed adjustment on the fuel pressure regulator.  There may be aftermarket versions that do not use the ball and have the smooth bowl you describe...That spring is inside the bowl or there is no lower (smaller) spring at all?  In the design below, the small spring either sets a fixed pressure or acts as a damper—or both:

 

As long as you're getting the 14-15 psi at the adapter "T" between the fuel supply line and TBI unit (or at the engine side of the fuel filter), you should be good.  If pressure is as high as the fuel pump pressure, or well above the 14-15 psi, check the return line flow into the fuel tank for an obstruction.  Once normal TBI fuel pressure is confirmed or reset, I would check the TPS (throttle position sensor) setting. 

The TPS is adjustable and should be adjusted to the voltage settings found in the FSM.  Many owners and technicians mess around with the TPS to get a stable or desired idle.  The TPS is like a rheostat switch and must have the right voltage settings and readings for each angle of the throttle valve.  Often, adjusting the TPS to the right voltage outputs at the specified throttle valve angles will reduce or eliminate an over-rich or a lean fuel mixture. 

I would also check vacuum line routing to the EGR valve and test the EGR valve function directly, using a hand vacuum pump attached directly to the EGR valve with the engine idling.  The EGR diaphragm can be tested for a leak with the hand pump and gauge.  There should be a distinct effect on the idle when the EGR valve opens and closes. 

If the EGR diaphragm will not hold vacuum, replace the valve.  If the EGR is original and not responding, but it does hold vacuum, I would remove the valve and either clean its exhaust ports or replace the unit.  I clean an EGR by soaking the metal base and valve plunger in carburetor cleaner then removing carbon.  (A soda blaster might do a reasonable job on light carbon or scale.)  Do not allow carburetor cleaner to get into or near the diaphragm.  Carburetor cleaner will warp or destroy a diaphragm.

Let us know your findings...

Moses   

 

[Julio]

Thanks a lot Moses!!! After a couple of months and various thread sizes error on the purchase of the adaptor I finally got the correct one. Your guess was absolutely right. I was not able to test it with the "T" method. The fuel pressure is higher than it should be: between 18-20 PSi. 

I will check the return line. In case the fuel pump is incorrect and wrongly replaced in the course of the las 30 years. Do you know the correct part number? Thanks again.

[Moses Ludel]

Julio...Yes, this is an unusual thread size for a fitting.  The port is normally used with the Mopar pressure gauge kit, which is not a common thread.

So, good, you're getting somewhere with the fuel system diagnosis and enrichment issue.  Here are the original 2.5L TBI part numbers from Mopar.  A factory NOS pump could be difficult to find, this is just for reference:

83502994  PUMP PACKAGE, Fuel - TBI 2.5L Eng. with 14.5 gallon fuel tank
33000184  FILTER, Fuel - 2.5L Eng.

83502995  PUMP PACKAGE, Fuel - 2.5L TBI Eng. - U.S., Canada and ECE with 20 gallon fuel tank
33000184  FILTER, Fuel - 2.5L Eng. - U.S., Canada and ECE

For aftermarket fuel pumps of the correct pressure, below is the popular Crown Automotive replacement pump.  The Crown 83502995 is a replacement for both the 83502994 and 83502995 Mopar pumps.   As you know, the main thing is to get the correct (lower pressure EFI) pump for the 2.5L TBI system. 

This is just the fuel pump and not the entire fuel module with pump, gauge, etc.  The 33000184 filter is not the frame rail filter.  This is the filter at the bottom of the fuel pump inside the fuel tank.  It is not included with the Crown fuel pump kit:

https://www.crownautomotive.net/Fuel-Pump-Kit/83502995.html

https://www.summitracing.com/parts/cwa-83502995  [an additional source for the Crown pump]

Before investing in a new pump, make sure there are no restrictions on the return line to the tank.  Also, determine whether you can adjust (lower) the fuel pressure at the regulator.  You could not find an adjustment screw.  Let me know if that's still the case.  There are other ways to lower the pressure.  18-20 psi is definitely too high and could be causing the rich running condition.

Moses

[Julio]

Thanks again Moses for your kind and fast reply. I don t remember the fuel capacity of my tank. Is there any way of knowing it by means of the VIN number. (2J4FY29E0KJxxxxx). 

I did find the Crown pump 83502995 but it is only the puel pump. I think the best would be to change te whole fuel sending unit. There is another item on eBay that seems to be compatible: "Fuel Level Sending Unit For 1987-1990 Jeep Wrangler 2.5L 4 Cyl 1989 1988 Crown" link: https://www.ebay.com/itm/295255257536 manufacturer part number does not match those you gave me: 83502990. 

Also, as you mentioned. I will check the return line for obstructions before buying it.

 

Thanks again!!!

[Moses Ludel]

Julio...The VIN does not indicate the fuel tank type on your YJ...I looked at the eBay link, that module is listed for a "15-gallon" tank.  There is also another Crown offering that has a few more parts:  https://www.ebay.com/itm/392125484356.

As for fit of the Crown module, a reviewer notes that his fuel pump/gauge module has a square (rectangular) top, not the round top shown in these eBay illustrations.  18 years ago, I replaced the 14.5-gallon OEM tank in my son-in-law's 1987 2.5L Wrangler with a 20-gallon tank.  Both tanks had square (rectangular) module flanges held down by a set of threaded bolts or screws.  The 14.5 and 20 gallon tanks are distinctly different widths/shapes and easy to spot.  (See my photos below.)  You should be able to look at your tank and identify which type you have in your YJ:

Above is the original/OEM 1987 YJ Wrangler 2.5L 14.5-gallon tank.  Note that the left and right sides are square and even with the skid plate's width.  The tank was poly plastic (HDPE) and had a loose tailpipe scorch that the previous owner neglected.

 This is the 20-gallon replacement tank (used) that fits the 1987-95 YJ Wrangler.  The new fuel pump/gauge module is an OEM Mopar replacement unit for the 20-gallon 1987-90 YJ Wrangler with 2.5L TBI engine (the lower pressure pump).  Note the width and shape of the tank.  It is HDPE "poly plastic" with a rectangular flange at the top of the pump/gauge module.

This is the top of the OEM 14.5-gallon tank.  I'm unhooking hoses and about to remove the square/rectangular flange hold down plate that secures the fuel pump/gauge module to the top of the tank.

Here, the original fuel pump/gauge module comes out of the 14.5-gallon tank.  Note the flange is square/rectangular as the eBay reviewer stated—not round.  The thick flange gasket is rectangular.

This is the 20-gallon tank from a recycling yard.  It is HDPE (plastic) with a rectangular fuel pump/gauge module flange.  As I recall, this tank was 1991-95 era but looks just like the 1987-95 20-gallon plastic tank listed at eBay:

https://www.ebay.com/itm/283933653422

Morris 4x4 Center sells both the MTS and Crown 20-gallon replacement tanks.  Each has the rectangular flange:

https://www.morris4x4center.com/gas-tank-20-gallon-polyethylene-plastic-52018287.html

https://www.morris4x4center.com/crown-fuel-tank-and-skid-plate-master-kit-20-gallon.html

So, it looks like these "Crown" fuel pump/gauge modules have a round flange and will not fit your YJ if the flange is rectangular like these examples.  There are aftermarket metal fuel tanks with round module flanges that use a lock ring.  So does the XJ Cherokee.  These pumps may fit that kind of tank...Avoid any fuel pump listed for a 1991-95 YJ Wrangler.  It will be an MPI 2.5L or 4.0L pump with a high pressure rating.  Although these pumps fit up to the tank, they have way too much pressure for a 2.5L TBI system.

I would get beneath your Jeep with a small inspection/extension mirror and see if you can view the top of the tank.  Determine the width and shape of your tank (14.5 gallon versus 20 gallon).  Inspect the fuel pump/gauge module's flange shape.  The rectangular flange has screws or bolts securing it.  The round flange types have a lock ring.

A complete fuel pump/gauge module can get expensive, the reason that many just replace the pump.  Unless the fuel gauge is not accurate, replacing the pump is usually enough.

Let us know what you have for a tank and the flange type...

Moses

[Julio]

Hello Moses,

 

Finally I ended up buying both and installing both. I was not able to obtain 13 PSI with none of them. Not the 18-20 I was getting originaly but slightly above 16. From what I have learned in this process it seems it is not fuel pressure related.

When I crank the engine and it first start it, the RPM are normal for the first 5 seconds. Then somehow a fixed 1000 rpms are commanded. You can tell from the iddle motor that is spinning in order to adjust that iddle. 

In the process of replacing the fuel pump and the fuel sender unit my fuel gauge droped to zero regardless of the fuel quantity. 

Just an observation, both pumps are louder than the one I had before. 

Thanks again and please let me know your thoughts on the matter. 

[Moses Ludel]

Julio...First, regarding the fuel gauge level, check your ground at the fuel pump module and fuel tank.  Then check the gauge sender signal to your gauge for ohms resistance.  If you have power to your gauges, the fuel gauge needs to receive an ohms-resistance signal from the fuel gauge sender.  The ohms-resistance should be 0 ohms when the fuel tank is empty (gauge float in lowest position);  44 ohms when the tank is half full (float raised halfway); and 88 ohms when the tank is full (float in full up position). 

Here is a rough PDF of the troubleshooting for the gauge.  I scanned it from my 1989 Jeep FSM.  This is as far as I can scan without breaking the book binding.  For more details, pick up a CD or print copy of the FSM for your YJ Wrangler:

1989 YJ Fuel Gauge and Sender Troubleshooting.pdf

Since the gauge worked before, either the sender's float is not rising (stuck, binding, an open in the wiring or the float is defective) or the electrical connection from the fuel module to the gauge harness has an "open" (not making proper contact in the connector).  If troubleshooting from the PDF does not solve the issue, this new gauge sender is likely defective.

As for your consistent idle speed rise to 1,000 rpm after start-up, the PDF below is coverage of the idle switch and the ISA motor.  The step up in idle speed could be an idle switch issue or ISA motor related.  Again, the FSM for your Jeep will outline these steps clearly and walk you through how to troubleshoot and stage the ISA motor, the idle stop and the idle step-up switch if so equipped: 

1989 YJ 2.5L ISA Motor and Idle Switch.pdf

If you have A/C or power steering, or both, the FSM clarifies how the A/C and power steering idle step-up speed works.  On that note, make certain that the Power Steering idle step-up speed is working properly.  The 2.5L models are designed to have the engine speed step up when you apply the power steering at an idle.  The reason is that these engines don't have the idle torque to handle the load created by turning the steering wheel and placing a power steering pump load on the idling engine.  Without the idle speed step up, the engine would stall.

This circuit works through the A/C circuit, which also steps up engine idle speed when the A/C clutch engages.  In simple terms, when the power steering signals a high enough pressure (pressure range listed in the PDF below) with the engine idling, the ECU will kick up the idle speed (at the ISA motor) to keep the engine from stalling.  If the pressure switch signal or P/S pump output pressure is too high, this would bump up the idle speed when the P/S pump starts putting out pressure (which could be the 5-second lag).  The A/C compressor clutch engaging would also be a signal to raise the engine idle speed.

A problem with the engine idle speed stepping up could be too much pressure at the P/S switch or a defective power steering pressure switch.  Even without turning the steering wheel, the idle speed might kick up.  Turning the steering wheel with the vehicle stationary will raise the P/S pressure.  A steering gear internal pressure issue could also do this.  So could a power steering pump that puts out too much pressure at an idle.

I would put a hydraulic pressure gauge on the port where the power steering switch threads into the system.  See what the actual P/S pressure is at an idle with and without turning the steering wheel.  If pressure is above normal with the steering wheel stationary, that would cause the idle speed to kick up.  If pressure is not above normal, I would suspect that the pressure switch is defective or there is a wiring issue in either the A/C circuit or the P/S pressure switch circuit.

A quick test would be to simply disconnect the wiring connector at the P/S pressure switch and see whether that stops the idle from stepping up 5-seconds into running.  That 5-seconds might be how long it takes the P/S fluid to boost pressure at the pressure switch.  Causes of high pressure at this port could be a steering gear issue or even a P/S pump that puts out too much pressure at low engine speeds.  Here are the FSM comments about the P/S switch function and the pressures.  The switch is inline near the power steering pump:

Your Jeep is now older technology, as you know.  I depend on my 1989 Jeep FSM set (Engine, Chassis & Body volume plus the Electrical volume) for troubleshooting the 1987-90 2.5L Jeep TBI systems.  I would not attempt to work on any of the 1984-90 AMC/Jeep vehicles with the 2.5L TBI engine without an FSM.  Do you have copies or CDs that cover your Jeep?  Any 1987-90 Jeep FSM set would work for your Wrangler.  1984-90 XJ Cherokees use this same TBI system.  There were minor changes over the years, so 1987-90 would be better coverage. 

I just found this at eBay for any 1987-90 YJ Wrangler owner wanting to work on his or her vehicle.  This is essentially the same as my '89 Jeep print FSM.  Bishko apparently combines both volumes into one CD-ROM or download.  If so, for $30, this would be easier navigation than a print manual and have all the answers an owner needs:  https://www.ebay.com/itm/132408471190.

 

Moses