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I've got a very stubborn jeep that just won't stay running. I've been fighting it for a year now. And since has improved. But I've been just uses sing and I need a way to solidly diagnose what's going on with it. It has a 2.5l 4cyl Tbi. The issues I'm having is most defiantly effected by the weather and climate and time of day. But the issue seems to mostly fluctuate with time of day. 1. So in the morning at like 5:25am I start it to warm up before I leave for work and it'll start and run just fine till it got to be about 100 degrees then it'll idle down to like 300 rpm and stall ( unless i floor it and hold the gas then it would run fine and do it again. 2. some days it floods so I have to give it gas to get it to start and it'll be fine for the rest of the day. 3. some days (seemingly more humid) it'll start fine or it'll do the first problem mentioned but as I give it gas ( like to go up a hill or some thing) I lose power and then it kicks in again till l let off the gas then give it gas it'll repeat. 4. It kind of goes along with issue 3 it hesitates or "fish biting" effect with acceleration 

I've replaced 

fuel pressure regulator, ignition coil and module, spark plugs and wires, distributer cap and roter, timing chain. Last year i did have issues with the injector but since has been fixed. And in the fall the map sensor went bad and almost left me stranded. I did some digging in the Internet and tested the egr and it wont hold vacuum so that's going to be replaced here soon but I don't think that's the issue but it could be playing a part in it 

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Sinisterwillys1940...Based upon what you describe, I would test the MAP sensor (even if new or replaced).  The EGR, if the valve is unseating or stuck in the open or semi-open position, would cause the symptoms you describe.  

If the EGR valve won't hold vacuum, the EGR diaphragm is defective.  If the EGR stem and plunger are stuck either partially open or unseated, you would have a variety of vacuum issues.  

The EGR is a substantial "vacuum leak" if slightly open or unseated at an idle.  The valve should be closed at idle, and it opens just as you crack open the throttle.  EGR is ported vacuum:  highest just off-idle and at or near zero with the throttle valve shut.

With protection from heat (wear a glove), here's a simple test of the EGR:  Engine idling, squeeze or lift up on the EGR diaphragm to open the EGR.  Note the change in engine idle stability, typically the engine will begin running roughly.  If unseating the EGR plunger does not effect engine idle at all, the valve was already open (partially or more).  The valve should be seated completely at an engine idle.

Moses

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Thank you for the info Moses. I'm sorry I didn't reply right away. It didn't change engine idle very much and I went a head and replaced the egr. It's defiantly made a huge improvement in the way it runs. But now it idles high (1300-1500 rpm) and back fires or desils when I shut it down and hesitates once in a while. It hasn't tried stalling yet but I'm keeping my eyes on it. How do I test the map sensor? 

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Sinisterwillys1940...Read through the forum discussion below...Your TPS and idle controls are likely way out of adjustment from the attempt to make this engine idle properly with a defective EGR valve.  If you adjust the TPS, idle speed and air control to factory specifications, the idle should stabilize and the dieseling will stop...See the "2 Dec 2013" part of the discussion, there's a PDF with info on making adjustments...Moses

Edited by Moses Ludel

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 I did forget to mention that I replaced the idle control cilinoid this fall. I forgot all about that. The tps was replaced by the last owner and its probably out of adjustment. But the same stalling issue still remains. I was driving it today and it acted up. But the idle is way smoother and it's running cooler... When it's running right. It does seem like weather has a huge effect on it. Thanks for the info Moses it's been very helpful 

Edited by Sinisterwillys1940

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Pursue the TPS adjustment and follow the factory steps outlined in our discussions...The TPS setting and throttle valve angle are critical to performance.  I believe you'll cure these problems!

Moses

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I have used factory workshop manuals for over fifty years now...I always seek out a shop manual for the vehicle in question...I have four bookcases filled with shop manuals, and I refer to them regularly...You can often find factory shop manuals on eBay or at used book sources.

If the Jeep is a keeper, I strongly recommend finding an official Jeep shop manual for it.  The MAP test and all other details can be found there...

Let me know if you still need the MAP test information.

Moses

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This is "newish" but still a TBI system, right?  Not a transplant MPI (1991-up) motor with multi-point injection?

Here are the details on the MAP sensor for a 1987-90 Jeep 2.5L TBI four:

 Jeep 2.5L TBI MAP Sensor Test.pdf

I've included the cover illustration for your book search.  These manuals have everything you need to know about troubleshooting, component tests and settings for the 2.5L TBI—and all other Jeep engines produced this model year (also applies to the range of 1987-90 Jeep models).  A factory service manual from this period would be very helpful and authoritative.  This is the level manual that I depend upon, and they are nearly always reliable and accurate.

From 1991-up, the service books break down by model each year.  XJ Cherokee and YJ Wrangler pair up, the Grand Cherokee (1992-up) and 1991 Grand Wagoneer have their own books.  Later, the XJ Cherokee and TJ Wrangler books go separate ways.  Try eBay or used book sources...I've chased down a factory workshop manual for every vehicle we've owned...I've purchased a new book or CD for the newer vehicles.  These books are now available in CD version for later model vehicles, and this has driven down the cost substantially.

Moses

Jeep 2.5L TBI MAP Sensor Test.pdf

Edited by Moses Ludel

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Single port injection. 1990 2.5 jeep tbi. Thanks again Moses this is very informative. I did get around to adjusting the tps and it seemed to defiantly run smoother and it has great take off. But I'm keeping an eye on weather to see if there's an change. The idle is still high but I haven't adjusted the idle control cilinoid yet. Ill test the map sensor after all the adjustments. Thanks again!!! 

Edited by Sinisterwillys1940

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I adjusted the isa and broke the head off the bolt. Then I got a new one and its adjusted now to spec. The idle is still high but it's running better. It was trying to stall again today while I was driving to get the new Isa 

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SW 1940,

I have a 1991 YJ 2.5 MPI. When I started it in the AM it would fire right up and idle fine for about a minute then idle down till it would almost stall. Traced it to a bad O2 sensor in the exhaust. Seems the ECM does not look for a signal from the O2 sensor at start up but shortly that after it does, and no signal means bad/no idle. 

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im having a similar issue, i just replaced my IAC on my 1990 2.5l yj and now i have i high idle that starts at 2000 and gets up to 2500. There is no sputtering, just runs really high. only thing that i changed was the IAc. Any help would be really appreciated as this is really got me stuck

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Hi, ventriel...The IAC must be adjusted properly.  Below are two PDFs from a forum string where I covered the factory ISA adjustment steps:

"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."

Try this first.  Let us know if you cure your fast idle situation.

Moses

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Try turning the steering wheel from lock to lock and see if that brings the idle down. I'm not sure what it means but when mine used to do it before I finally got the adjustment right (the scanner wasn't giving the right rpm value). 

 

Keep us posted!

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Moses, i tried both, i cleaned the map too. why would this problem come up when all i touched was the IAC? i had perfect mainifold vacuum before i replaced the IAC, i have checked since and no surging when i spray starter fluid on and around the vacuum lines.

Sinisterwillys1940, it does make a little difference but i think that is because it puts the engine under stress using the powersteering

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ventriel...In the information I shared (two PDFs above), there is a subtle message about full extension of the ISA during the installation and other needs to prevent damaging the ISA motor.  Make sure it's not damaged...Read through both PDFs, that's the most comprehensive detail Mopar provided on troubleshooting the ISA motor.

This kind of high speed idle requires two things:  fuel and air.  The closed throttle switch, ISA motor and throttle position sensor would be areas of concern.  If you have ruled out an EGR leak, closed throttle switch and a defect or adjustment need at the ISA motor, check the TPS voltage at this point.  Here are several forum topic threads on your troubles: 

 

 

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Found the issue, it was a faulty ISA motor. the gears inside were shredded, thank you far all your help it has a better idle then it did at first now thanks to your steps

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Yea, ventriel!  Good work...Pleased that you found the issue and cured it.  Others will benefit from your discovery of the shredded gears...Welcome to the forums!

Moses  

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