Cold Start Low Idle. Any Ideas? - JeepForum.com

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post #1 of 15 Old 04-23-2016, 05:10 PM Thread Starter
TheManFrom1998
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Cold Start Low Idle. Any Ideas?

*SOLVED*

I have a 1998 ZG/ZJ Jeep Grand Cherokee 4.0. This started out only on cold starts, but now seems to be on every start up even when warm. On start it will run perfectly for around a minute, then the revs will drop to about 100-200 and it will feel like it will stall. Once you push the accelerator down, it will hesitate and chug forward for a few seconds then everything is back to normal. It will not happen again until the next start. This happens even when the car is stationary and in Park, as well as in Drive.

I have swapped out the TPS thinking it might be the problem, but it was not.

Any ideas would be greatly appreciated.

Thank-you.

Jason.


Last edited by TheManFrom1998; 06-06-2016 at 06:26 AM. Reason: Solved the problem.
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post #2 of 15 Old 04-24-2016, 10:00 PM Thread Starter
TheManFrom1998
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I cleaned the IAC and Throttle Body today, but the problem remains. Will continue to investigate.
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post #3 of 15 Old 04-25-2016, 05:27 AM
jrobeson18
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I think I'm having the same problem, but mine does it randomly, usually cold starts
drive good for 50 ft then bogged down makes a clunking noise and then its back to normal.

Sound familiar/similar?
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post #4 of 15 Old 04-25-2016, 07:50 AM
puubear98
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When you say you cleaned the IAC do you mean the IAT? Intake air temperature sensor? Did you run a test on it?! To me, that would seem to be the first place to look.
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post #5 of 15 Old 04-25-2016, 11:57 AM
kg6mov
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Intake air temp sensor is not used at that stage of the PCM. He means the IAC, Idle Air Controller.


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post #6 of 15 Old 04-25-2016, 01:58 PM
ROBOTYPHOON
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I had the exact same issue and it gradually got worse over the course of a six month period; went from the rare, low rpm start and random rpm drops to misfiring and frequent stalls that weren't immediately recoverable and I had to let my ZJ sit for a period of fifteen minutes otherwise the engine would crank until my battery died.

Fortunately, before I ended up replacing everything and anything that didn't actually need to be replaced, I swapped my PCM out for one I salvaged out of a junkyard off a ZJ the same year and model as mine. I was surprised and at the same time I wasn't; it solved all the issues and I didn't even have to get it reprogrammed or anything. Just plugged it in.

I'm not saying your PCM is failing, I'm just saying that mine was. If you know someone with a 96-98 4.0 you could swap with, it'd be worth a test.

The PCM on the left was dying, the one on the right ran flawlessly; you could hardly tell by looking at them.

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post #7 of 15 Old 04-26-2016, 03:31 AM Thread Starter
TheManFrom1998
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Quote:
Originally Posted by jrobeson18 View Post
I think I'm having the same problem, but mine does it randomly, usually cold starts
drive good for 50 ft then bogged down makes a clunking noise and then its back to normal.

Sound familiar/similar?
It is pretty much exactly the same as what you have said.

Quote:
Originally Posted by InfiniteREA View Post
I had the exact same issue and it gradually got worse over the course of a six month period; went from the rare, low rpm start and random rpm drops to misfiring and frequent stalls that weren't immediately recoverable and I had to let my ZJ sit for a period of fifteen minutes otherwise the engine would crank until my battery died.

I'm not saying your PCM is failing, I'm just saying that mine was. If you know someone with a 96-98 4.0 you could swap with, it'd be worth a test.
I do have two PCM's sitting in my cupboard as I learned a few years ago from this forum that they are likely to fail. ZJ/ZG Jeep Grand Cherokees are rare to find in the wrecking yards in Australia, in almost 4 years I have only seen 4 in my local yards and if they had a PCM, I took it. I have not tested either of my spare two. Do you have to take it to the dealer to get it programmed or can you just plug it in?

Thanks again.
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post #8 of 15 Old 04-26-2016, 09:25 AM
ROBOTYPHOON
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Quote:
Originally Posted by TheManFrom1998 View Post
Do you have to take it to the dealer to get it programmed or can you just plug it in?
I was able to plug mine right in, no programming necessary. The stickers on the top with the module codes and those number sequences didn't even match 100% on the PCM's, old vs new and everything still worked out flawlessly.

I don't know if I got lucky or same year ZJ's don't need reprogramming and it's just a $$$ scam. Either the security system doesn't care that the VIN number doesn't match; or the system isn't functioning properly. As long as your PCM's are 96-98's I'd go ahead and test'em out. Worst comes to worst your Jeep just won't start up with the foreign PCM, in which case you might need to get it programmed.

Just make sure you disconnect the negative battery cable before unplugging anything from the PCM; if you don't you could damage it.
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post #9 of 15 Old 04-26-2016, 11:33 AM
Para32
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I had the same exact problem and cleaning the IAC solved it. At first I took the air cleaner off and sprayed carb cleaner in at it from a port in the side of the throttle body. It wasnt until a removed it and claened it that it worked. Numerous posts on this procedure.
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post #10 of 15 Old 04-30-2016, 05:36 AM Thread Starter
TheManFrom1998
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I swapped out the PCM today and so far it has not shown any sign of the rough idle, but too early to tell. I will let you know after a few days of start ups and driving. But yeah, it seems you can just plug and play the PCM...I did not have to get it flashed at all.
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post #11 of 15 Old 05-11-2016, 09:01 PM
ROBOTYPHOON
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Quote:
Originally Posted by TheManFrom1998 View Post
I will let you know after a few days of start ups and driving.
Glad to hear you were able to swap no hassle; how's it been running?
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post #12 of 15 Old 05-11-2016, 09:07 PM
DPayne1
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Get a Radiator Insulator from Kale Auto. They've got all sorts of hard to find parts that make easy fixes.

http://kalecoauto.com/index.php?main...dxGmleIo4nk463

Hello, I'm Dan. I'm kinda the unofficial forum idiot.
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post #13 of 15 Old 05-13-2016, 11:54 PM Thread Starter
TheManFrom1998
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Quote:
Originally Posted by InfiniteREA View Post
Glad to hear you were able to swap no hassle; how's it been running?
Hey. Its been in for a few weeks and have taken it on some drives, initially I thought it had solved the issue...but it came back...such as intermittent problems do.

Could it be an injector? Or fuel pump issue?
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post #14 of 15 Old 05-23-2016, 03:05 PM
ROBOTYPHOON
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Quote:
Originally Posted by TheManFrom1998 View Post
Hey. Its been in for a few weeks and have taken it on some drives, initially I thought it had solved the issue...but it came back...such as intermittent problems do.

Could it be an injector? Or fuel pump issue?
That's not good news! I suppose it's possible it could be an injector/fuel pump issue.. but what I would check next, is your ignition coil. Make sure there's no cracks in it and it's not damaged; I'd go as far as to take it out completely to make sure there's no damage on the back side.

I noticed that was something you haven't mentioned checking; it could definitely be causing these issues.. hopefully it is because it's an easy/inexpensive fix. Otherwise I'm not sure.
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post #15 of 15 Old 06-06-2016, 06:29 AM Thread Starter
TheManFrom1998
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I solved the issue. It turned out to be the upstream 02 Sensor. It was failing and making the engine run lean at start up. Only problem is here is Australia it is difficult to source Mopar replacement parts...So I had to use a Walker USA brand. I hope it is suitable, I know the general feeling here is Mopar only.

Thanks everybody for your input, at least I know the my spare PCM's work...that is always a good thing.
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