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


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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!

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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

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  • Moses Ludel changed the title to 2.5L TBI Jeep YJ Wrangler Running Rich and Won't Idle
  • 8 months later...

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?

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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 

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  • 5 months later...

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.

9E00A88E-2A4E-4B9A-9219-7D76E5124872.jpeg

0169DE5F-442B-4245-B3B4-2A224CDE4979.jpeg

219922B3-74FC-4DDE-8617-577E5BC03617.jpeg

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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

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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.

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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 

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  • 1 month later...

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. 
 

 

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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

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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

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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 

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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

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  • 1 month later...

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?

 

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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

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20210821_202204 (2).jpg

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  • 3 weeks later...

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.

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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

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