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mtjeep82

1990 Jeep YJ Wrangler 2.5L Choking Down and Starting Problems

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We have a 1990 Jeep Wrangler YJ, 2.5, Throttle Body, 5 speed.  Sometime when we shift gears and accelerate it chokes down, like it isn't getting enough gas.  We have tried cleaning the Throttle Body, the crankshaft speed sensor, changed fuel filter, cleaned air cleaner and ran sea foam through it.  It still does it.  Also, sometimes, when you start it, it dies then it starts really hard, lots of cranking, it is almost like it floods.  Any suggestions? Thoughts?  

 

 

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mtjeep82...Welcome to the forums!  Please check out the lengthy discussion on tuning and troubleshooting the 2.5L TBI system, the information is available here in the Wrangler forum and also at the magazine (www.4WDmechanix.com).  Please take the time to review the wealth of discussion that has taken place, others have similar issues and troubles.

 

When you share that your engine's behavior is like "flooding", confirm whether the issue is too much or too little available fuel.  A quick check of the spark plugs would be sensible.  Look for wet or sooty carbon fouling.  If the plugs do not show this kind of rich mixture when the troubles occur, I would check the fuel pressure at the regulator (pressure port testing).  This requires a fuel pressure testing tool (gauge and hoses with correct fittings). 

 

We discuss this at length in the 2.5L trouble discussion here at the forums.  If your pressure at the test port is beyond 15 PSI, there is too much pressure and possibly excessive fuel flow.  If pressure is normal and the fuel flow is obviously erratic (starving for fuel) or excessive (fouling/enriched spark plugs), the injector could be clogged or at fault.

 

Since you've done the common, simpler repairs and maintenance, you need to start your troubleshooting by confirming whether you're getting adequate fuel or too much fuel.  From there, the discussion at the forums covers a broad range of repairs.  I cover 2.5L troubleshooting as a whole at the magazine, enter "2.5L" (without quotes) in the search box at the magazine home page or the search box at any other landing page of the magazine.

 

Let us know what your spark plugs and pressures turn up...Read through the forums and magazine coverage/discussion of the 2.5L TBI troubles.  Here to help further.

 

Moses

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We checked the fuel pressure and it stayed right at 16 lbs at startup, idle and high rpm.  The spark plugs aren't sooty or wet.  Drove it up the hill and it ran fine as long as you accelerate slowly when you step on it a bit more (like starting on a hill) it chokes down again, then goes to idling fast.  

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Just had a discussion about the idle air actuator.  Here's the post and reply: http://forums.4wdmechanix.com/topic/626-jeep-wrangler-yj-25-tbi-low-stable-speed-issue/.  Your symptoms could involve the idle air actuator, though I would try other possibilities first.

 

I would extract trouble codes from the ECU if any have been stored.  This is early OBD and not as useful or thorough as OBD-II.  There still are functions and device checks.  The oxygen sensor would be a concern.  Vacuum related devices, too.

 

Have you actually tested the fuel flow to the regulator for volume?  Pressure is not enough, you could have a clogged screen at the fuel pump pickup in the tank that's restricting actual fuel volume flow.  This could create all the symptoms you describe.  This is not an ultra-high pressure pump, and a clogged pickup screen would thwart flow.  Put a "T" on your pressure regulator test hoses and run the extra (open) leg into a safe pan or can away from any heat source.  Note the volume of fuel that pumps through, not just the pressure.

 

Moses

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OK, we have installed a throttle body re-build kit, gaskets and pressure regulator, seals. Thoroughly cleaned the throttle body.  We pressure tested fuel input, stayed at 16 lbs during key on, engine start, idle and high idle. Checked return flow, it has a good flow through all stages, key on, engine start, idle and high idle. Also checked the fuel return line to the tank. Replaced plugs. Checked injector for spray pattern, seems to be good with no hesitation. Tested the throttle position sensor and it is in the recommended range. Jeep still chokes down when shifting.  Sometimes if you ease on accelerator when shifting it picks up and runs sometimes it chokes down.  At times, when starting, if you don't hold down the accelerator just right it dies and then does not want to start again.  Any other thoughts? When just driving down the road normal it usually runs well. Sometimes it runs well while shifting, when it doesn't it is very frustrating. Especially when it decides not to start.  It is very sporadic and temperamental. Thank you for any further ideas.  We really appreciate it.  We think we get it running good and then the next day it starts all over again.

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Hi, mtjeep82!  So, you're making headway, the symptoms have narrowed to stumbling between shifts and the start-up problem/return to idle quirk.  Some thoughts...

 

Your fuel pressure is actually slightly on the high side (normal is 14-15 PS), but this is unlikely to cause the symptoms you describe.  You might readjust the pressure unless you suspect test gauge error...Given what you have done (plenty and good approaches!) and the remaining symptoms, I would adjust the Idle Speed Actuator Motor to factory spec.  This adjustment is often out of specification because someone tries to set the idle speed, mistaking this adjustment for an idle speed stop—it's not.  (There is no idle speed screw, that function is ECU controlled.)  An out of adjustment ISA motor would create the symptoms you're experiencing.

 

Here is the factory ISA motor adjustment procedure.  Use great care when running the engine speed up.  Stay away from the engine's cooling fan!  (One way to eliminate risk of fan injury during this adjustment would be to temporarily remove the serpentine drive belt.): 

 

2.5L TBI ISA Motor Adjustment.pdf

 

Read these additional details on Closed Throttle (Idle) Switch testing in conjunction with the ISA motor and adjustments before performing ISA motor adjustment and tests:

 

Closed Throttle Idle Switch & ISA Testing.pdf

 

If this is not the cure, I'd look to the EGR valve or solenoid, or the MAP sensor, issues related to vacuum and vacuum changes.  Be certain you have strong manifold vacuum and no vacuum leaks.  A sticky EGR or out of spec MAP sensor could create issues under deceleration—like when shifting...One step at a time.  You need to take calculated steps.  There will be a clear solution to this specific and symptomatic problem...The ISA motor adjustment would be my first step.

 

Moses

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Our "bargain jeep" is proving to be  challenging! :)  We tested the sensors and hope we have found the problem.  The MAT sensor does read less than 1000.  It reads 0.00.  :(  So we are looking to replace that - only to find out it isn't available?!  Any thoughts to where might we find a replacement?  Thank you for all you help.  Hopefully we'll get this issue taken care of, then this winter we can take care of the issue we knew about, the transmission!  :rolleyes:

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mtJeep82...MAT or MAP, or any other relevant sensor, could be at stake here.  Manifold temp or any of the vacuum related sensors are worth pursuing...I'm still leaning toward something that affects performance as vacuum or throttle position changes—especially the throttle valve returning to idle position.

 

If you're referring to the intake air temp sensor (IAT), here is the OEM Mopar part number:  33002382 SENSOR, Intake Air Temperature (Quantity 1). 

 

I did some research and came up with this source.  Could be NOS, it's pricy at $53:

 

http://www.apdty.com/item.wws?sku=33002382&itempk=20718496&mfr=MOPAR%20PARTS&weight=0.234

 

To broaden your search, the part number also pops up for "1984-92 Dodge Monaco".

 

Frankly, I'd visit your local Pick 'N Pull resource.  These devices either work or not.  A used one that reads correctly (take your ohmmeter to test the part before pulling) would suffice. 

 

Moses

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Well we have replaced the Manifold Air Temperature Sensor.  It seemed to run better but still choked down during shifting and idled high.  So we bought a new Idle Air Control and installed it. It starts and runs at 3,500 rpm and doesn't idle down.  Tested the plug and getting 5 volts to the retract side (with key on engine off) and ground tests good but the controller doesn't move.  We did the jumper wire test on the test ports and are getting 12.5 volts to the plug on both the retract and extend. The idle air control doesn't move at all. It is also showing 5 volts to the wide open throttle switch port with the key on. So sorry to keep bothering you... but we sure appreciate all the help.

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Hi, mtjeep82...We're here to help if possible.  So, is the IAC sticking?  Or is the signal and switching wrong?  Do you believe the ECU is defective?  Try disconnecting the ECU plug and reconnecting.  Check each of the grounds.  There is an engine to body ground that is especially troublesome and creates resistance.  Ground circuits on 12VDC are as important as the "+" side.  Check resistance at the ground points.

 

Did you try adjusting the ISA as per the PDF instructions I furnished on August 29th (above)?  This is a new symptom and sounds like the ISA could need adjustment...Try that before delving deeper.

 

Moses

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Good morning, Moses.  Last night we pulled ECU and unplugged and plugged it back in and the IAC still doesn't seem to do anything.  Do you think it is possible that we ruined something when I "jumpered" it across in the test ports to test it since we don't have access to a exerciser test tool.  Somewhere we read we could do that.  Now we are afraid we messed up something.

 

 We are getting power to the plug that you plug into the IAC but it doesn't extend or retract.  Could the ECU bad? Can we set the IAC (ISA) without the tool? We did use your instructions and it did extend but didn't retract, so we bought a new IAC and installed it and it isn't doing anything now.  Maybe we got a bad one.

 

Thank you again.  :)

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mtjeep82...Does sound isolated to the rebuilt ISA.  Isolate and try testing the ISA without any wiring harness connected.  See whether the unit will extend and retract.  Could be defective or sticking.  Rebuilding is often an "art".  Maybe isolating and actuating the ISA repeatedly will do the trick.

 

If this does not do it, trace the signal from the ECU to the ISA.  Make sure the available voltage is there to retract the ISA.  If necessary, reinstall the original ISA and perform these tests to see if it will retract properly.  That's the best testing short of changing the ECU or other parts.  I would rather see you pinpoint the issue than continue replacing parts.  Parts replacement gets expensive, and you need a clear sense that the problem will be solved. 

 

A quality volt-ohm meter is the best test tool.  A more expensive Fluke-type meter can even do non-destruction insulation resistance and shorts to grounds testing of individual wires and circuits.

 

Once the ISA clearly does work properly, if the symptom still persists, consider the EGR valve.  It can be tested and cleaned short of replacing it.  I remove these valves and soak/clean the metal section only in carburetor cleaner.  (Do not soak the diaphragm in carburetor cleaner, it will be ruined.)  Make sure the EGR valve seats and seals properly and that the plunger/diaphragm moves freely.   Be sure EGR vacuum applies when it should.

 

Moses

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Hi Moses, we are back!

 

We were able to find a exerciser tool on ebay, so we bought that.  IAC seems to work fine with the test tool. Tested the voltage at the ECU and the IAC test port and the plugin for the IAC.  The voltage with engine running and cold was 9.65 at all three places.  Voltage as the engine warmed up it went up to 9.85 on the retract side.  On the extend side it started out at 0.08, cold.  As it warmed up it went up to 0.10.  As the engine warmed up the voltage went up slowly.  It stayed the same at all three test ports. The IAC doesn't move while hooked up to the jeep. With the tester it worked fine.  Do you think the ECU bad? If we took the ECU in to a Jeep dealer would they test it? We did test the grounds, cleaned the contacts and they seem to be ok.

 

Thanks, yet again!

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There is a brisk aftermarket with exchange ECU units.  Some members at our Jeep Wrangler forum have had good luck with the common rebuilt units, one did not.  Regardless, if the bench test of the IAC is positive, the only other cause of trouble would be heat or resistance issues.  Resistance in wire circuits can be measured end-to-end from terminal junctions.  An insulation resistance test is possible with a tool like the Fluke 1587 diagnostic meter, which offers "non-destructive" insulation resistance testing.  Checking for insulation resistance and shorts to ground is often sensible when you have a difficult troubleshooting prospect.

 

There is an outfit at Idaho (http://modulemaster.com/rebuilds/cars-trucks/) that rebuilds modules including engine controllers.  They can service OE modules and often for far less cost than either new or "rebuilt" units.  I had them rebuild a Kelsey-Hayes ABS module for an S10 truck when GM wanted $800 for the part.  Module Master rebuilt the unit to "better than OE" standards, pinpointing the actual weak areas and repairing or upgrading the device.  Cost was $135 with a quick turnaround. 

 

I would contact Module Master and share your findings.  Ask if rebuilding the module/ECU makes sense given your trouble symptoms.  They should be frank.  If you do use their service, the process uses the OE unit and restores it, which can be an asset when compared to "exchange" units where your OE unit is gone forever, often leaving doubts about whether the rebuilt module is an exact match.

 

Let us know what Module Master recommends here...and the outcome.

 

Moses

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Guess they can't repair Jeep ECU's  :(    "I am sorry, but we do not sell ECU's, and we cannot repair the Jeep models.

Mari
Module Master / Circuit Solutions"


 

Probably for the best as the ECU that is in this jeep isn't original.  We will look into the resistance testing and see what we can find there.  Thanks!

 

 

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mtjeep82...Are you suspecting trouble at the replacement ECU?  If you need to match up ECU part numbers, let me know.  We'd begin with your year and emissions package from the factory decal or VIN's first seven digits.  Then move to the part number on that ECU...

 

Moses

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Hi Moses,  We got the ECU repaired.  Got the idle set.  New plugs and plug wires.  It seems to be running!  (knock on wood.)  Next step is a new transmission but we are happy it is running as well as it is.  Thank you so much for all your help!

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We are looking to purchase a rebuilt (or used)AX 5 Transmission for our 1990 YJ jeep wrangler.  Any suggestions as to where to purchase from?

Edited by mtjeep82

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mtjeep82...Considering the vintage of a 1990 AX5, a used unit could be risky. AX5 was popular behind Jeep 2.5L fours through the '90s.  The issue is your hydraulic internal clutch release bearing.  This design ended after 1993.  

You could update, as many have, to a later AX5 with the newer design bellhousing, an external hydraulic clutch release slave plus all of the changeover clutch linkage pieces.  Or you could install a later AX5 transmission into your 1990 YJ Jeep Wrangler by swapping your current front bearing retainer (the internal clutch slave type) into the later AX5.

As for  a rebuilt transmission source, there are shops that can rebuild your unit.  MIT at El Cajon, California comes to mind, I'm not sure what part of the country you call home...Parts are available if you want to tackle the rebuild yourself.  My AX15 transmission how-to rebuild video at Vimeo On Demand would provide some insight into the AX5 although there are differences between these two transmission designs and their parts.  Watch the trailer, and decide whether personally rebuilding your transmission makes sensible.

Moses

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We are finishing up rebuilding our ax-5 transmission and are wondering what you think is the best oil we should put in it?  We see many different opinions on this.  The jeep will just be a light use, no long hard trips or excursions. Thanks

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mtjeep82...We've discussed lube for the AX15 and AX5 from many angles...You'll find this particular quote of mine directly answers your question:  

"The original Mopar gear oil should cross over to Valvoline's current MTF #811095 gear lube.  This emphasis on oil types in these Aisin transmissions has been largely exaggerated, BlueFlu.  I ran the official Mopar AX5/AX15 oil for years, until it was no longer available.  Toyota's equivalent transmissions use a recommended API GL4 or GL5 in 75W-90 or 80W-90 viscosity.  This readily available Valvoline oil should work well.  Redline and Amsoil offer alternative synthetics under these ratings.  What I would avoid is the "motor oil" equivalents suggested now by Mopar, as this is typically not an oil for gear mechanisms that require EP (Extreme Pressure) lubes.  That's my view...Here is Valvoline's viewpoint and part number cross-references:

Valvoline Transmission Lube Catalog.pdf"

From a cost and compatibility standpoint, the Valvoline MT lube described is closest to the OEM Mopar original lube for the AX15/AX5.  I am not a fan of using 10W-30 motor oil in any manual transmission, which some suggest is the current Mopar recommendation for the AX15/AX5.  

I did use the genuine AX15/AX5 Mopar lube years ago when it was available.  Today, I would use Valvoline's MT...Download the Valvoline PDF catalog.

Moses

Edited by Moses Ludel

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