96' 5.2 starting to run rough and stall at low speeds - JeepForum.com
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post #1 of 35 Old 06-30-2017, 04:41 PM Thread Starter
ScottyGunn
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96' 5.2 starting to run rough and stall at low speeds

I have talked to mechanics, searched the web and this forum. There are more ideas than one could imagine.

It started a couple months ago and is getting worse. It will stall after driving for a while and then letting off the gas to slow down, or idling at a light.

Mine has no EGR Valve. Suggestions are clean TPS or MAF, but what else could be doing it?

There are no codes or check engine lights.

Any ideas? Thanks


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post #2 of 35 Old 06-30-2017, 04:58 PM
LouC
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Is it doing this all the time, or just when warming up, or after a part warm start? Because if it's doing it during a part warm start, then it could be a lazy upstream 02 sensor, we just had this on our '98 ZJ 4.0 and after running it with the scanner hooked up I could see that the sensor was slow to respond right after it transitioned from open loop to closed loop operation (in open loop the ECM does not use data from the upstream 02 sensor to adjust fuel/air ratios, but after it transitions, it does). It would run rough for about 1-2 min, then after smoothing out it was fine, till it cooled off enough for it to go back into open loop operation. A new Mopar upstream 02 sensor fixed it.
But if it does not match this pattern and it does it all the time, it could be other causes. I like to hook up my Actron OBD-II scanner and watch 02 sensor operation and fuel trims when trying to figure out odd problems like that....

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post #3 of 35 Old 06-30-2017, 05:17 PM Thread Starter
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It seems to be only after warming up, it will sputter if I'm still moving slowly or stall if I stop. I have an OBD2 with live readings, but I don't know what to look for.

The mechanic at work thought EGR, but mine doesn't have one.

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post #4 of 35 Old 06-30-2017, 06:29 PM Thread Starter
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Could it be the Idle Air Control? I only seem to notice the rough running when I'm idling or slowing to idle.

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post #5 of 35 Old 06-30-2017, 06:47 PM
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Could be they get carboned up and you can remove it and clean both the throtyle body and the passage where the pintel fits in the throttle body.....

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post #6 of 35 Old 06-30-2017, 06:48 PM
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Clean the throttle body really well, and then go clean the grounds while you're at it. That would be a good start.


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post #7 of 35 Old 06-30-2017, 06:58 PM Thread Starter
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Ok, will try those tomorrow. I just tightened all the vacuum hose connections, cleaned out the crack case breather and PCV. It ran great at speed and slow and idle after warming up. But, that doesn't mean it's fixed. The problem is quite erratic.

Thanks for the input!

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post #8 of 35 Old 06-30-2017, 08:30 PM
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Don't mistreat the IAC.
https://www.jeepforum.com/forum/f13/h.../topics/756934


When my TPS went out it would chug/stall at stoplights and intermittently totally fail when I wanted to pass at highway speed.
That was dangerous.
But you need to do the basics first before dish out coin for sensors. And only use Mopar sensors. Not the parts store junk. That's hit or miss and it can get expensive quickly.
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post #9 of 35 Old 07-01-2017, 09:56 AM Thread Starter
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The IAC seems to be the ticket. I will update later after it has time to prove out.

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post #10 of 35 Old 07-01-2017, 03:05 PM Thread Starter
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Well, I spoke too soon. I ate lunch and then went to move the ZJ out of my garage and it did it again at start up. grrrrrr

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post #11 of 35 Old 07-04-2017, 06:46 AM Thread Starter
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Quote:
Originally Posted by kg6mov View Post
Clean the throttle body really well, and then go clean the grounds while you're at it. That would be a good start.
Which grounds do you mean? There a A LOT of them

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post #12 of 35 Old 07-04-2017, 06:48 AM Thread Starter
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Quote:
Originally Posted by PolkaPower View Post
Don't mistreat the IAC.
https://www.jeepforum.com/forum/f13/h.../topics/756934


When my TPS went out it would chug/stall at stoplights and intermittently totally fail when I wanted to pass at highway speed.
That was dangerous.
But you need to do the basics first before dish out coin for sensors. And only use Mopar sensors. Not the parts store junk. That's hit or miss and it can get expensive quickly.
As of yet, I have not had a problem while at high speeds, over 25 or so, unless I have let off the gas to coast. It only seems to happen when the RPM goes down.

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post #13 of 35 Old 07-04-2017, 08:28 AM Thread Starter
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Quote:
Originally Posted by LouC View Post
Is it doing this all the time, or just when warming up, or after a part warm start? Because if it's doing it during a part warm start, then it could be a lazy upstream 02 sensor, we just had this on our '98 ZJ 4.0 and after running it with the scanner hooked up I could see that the sensor was slow to respond right after it transitioned from open loop to closed loop operation (in open loop the ECM does not use data from the upstream 02 sensor to adjust fuel/air ratios, but after it transitions, it does). It would run rough for about 1-2 min, then after smoothing out it was fine, till it cooled off enough for it to go back into open loop operation. A new Mopar upstream 02 sensor fixed it.
But if it does not match this pattern and it does it all the time, it could be other causes. I like to hook up my Actron OBD-II scanner and watch 02 sensor operation and fuel trims when trying to figure out odd problems like that....
I actually just got the Code reader on it doing a live reading when it started acting up!

The jeep was sitting and warming up, had not been on the road yet. After 3-5 minutes it started acting up and the O2 sensors started going nuts. #1 was pretty steady prior to the rough run and during rough would drop all the way to .000 and the #2 seemed to go just opposite the #1, going up to .800-.900.

This rough idle wnet on for about 2-3 minutes and then settled down to normal again.

The fuel info was on another screen so I didn't watch that during rough.

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post #14 of 35 Old 07-04-2017, 09:12 AM
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Could be your upstream O2 sensor then.
Normally when you start from cold it is in open loop operation and the O2 sensors do not regulate fuel mix. They will both read 1.0v on the scanner then. As it warms up they will switch to clised loop where the upstream one will flip from rich (more than .5) to lean (less than .5). The flipping back and forth will be reflected in short term fuel trim which will vary opposite from the sensor reading. The downstream sensor will not vary much. What I saw with ours is that the upstream one will get stuck at a rich reading and then the short term fuel trim would go excessively lean to compensate (-33%) that caused the rough running and near stall.
I changed the upstream O2 sensor and it has run great since.
You could try UN plugging the upstream sensor (this will set a check engine light but you can erase it with the scanner) and see if it does not do it. This has the effect of keeping it in open loop operation not using the upstream sensor to adjust fuel mix. The down stream one (#2) monitors cat efficiency and does not vary that much.

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post #15 of 35 Old 07-04-2017, 09:22 AM
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If you do wind up replacing the upstream I advise getting an OE one if you can. Trick to getting it out:
Spray penetrating oil on the gasket for the old sensor so it seeps in the threads. Do this several times. Then warm the engine up (does not have to be fully hot but like 1/4 -1/3 the way up from cold on the gauge). Then carefully turn it out. Make sure the wrench you use fits tightly. Be aware the exhaust pipe will be HOT!

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