When the vehicle is idling in DRIVE or REVERSE my RPM's fluctuate in intervals every three to four seconds

Tiny
JPUCKETT
  • MEMBER
  • 1996 JEEP GRAND CHEROKEE
  • 4.0L
  • V6
  • 4WD
  • AUTOMATIC
  • 175,000 MILES
Good day,

My vehicle has about 175,000 miles on it. I have driven the car for the last 20,000 miles and years and I have had no issues with it. It sat parked for about seven months while I was deployed overseas and drove perfectly fine for the three weeks I was back. I then left for another three weeks overseas again and had it parked on a hill. When I got back from these three weeks about six days ago the car has had several issues.

To start, most of the oil had drained out of the car and was on the pavement. I immediately refilled it and have not had any leaks since, so I am not sure if this is really part of any issue that persists.

Secondly, when the vehicle is idling in DR or R, my RPM's are starting to fluctuate in intervals every three to four seconds. It will dip down to maybe one hundred or so and then pop back up to normal. When it does this, the entire front end of the car starts to shake and shudder, and everyone can feel in vibrating inside the car. The car has stalled twice while sitting at red lights. This stops completely if I put the car in park or if I apply any pressure to the gas pedal. It happens when the car is cold or warmed up as well.

I originally suspected it was an issue with my IAC so I pulled it off and cleaned it real well, and then let it dry overnight. After putting it back in and firing her up, I got the same issue. I then concluded it must have just been bad, so I bought a new one, but same issue persists. I have been recommended to get new plugs and wires in there but some of the shops around here think it will fix it and some do not, and I would rather not shell out $200.00 or so for a tune up that may not be needed. I do have fuel injector cleaner in the tank right now and about half a tank of gas left but it has not made much of a difference yet.

Thanks for reading!
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Wednesday, September 28th, 2016 AT 10:08 AM

12 Replies

Tiny
AL514
  • EXPERT
A fluctuating idle like that may be a vacuum leak, somewhere false air can be getting into the intake or other place causing the Jeep to hunt for an idle under a load such as being in gear. Start by checking for any broken vacuum lines or any place air might be getting in after the throttle plates. As the car has been sitting off and on while you are away, gaskets can start hardening up and not sealing correctly. As for your oil leak, if the car was sitting with the front end facing down hill, oil could have leaked out the front main seal, or rear main if sitting the opposite. Either way you have an oil leak.
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Wednesday, September 28th, 2016 AT 11:56 AM
Tiny
AL514
  • EXPERT
The Jeep is definitely searching for an idle under a load. The computer is trying to compensate for whatever the problem is by opening and closing the IAC and just cannot find an idle that will stay within range.
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Wednesday, September 28th, 2016 AT 12:00 PM
Tiny
JPUCKETT
  • MEMBER
Okay, when I get home from work today and she cools down I will take a look around and look for any broken hoses. If I do not find any issues there how would I go about checking the gaskets? I will do a bit of research myself as well.

It was parked with the front of the car higher up on the hill so I would say it would be leaking from the rear seal then. Thanks very much for the assistance. If I get anything figured out I will report back what I find.
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Wednesday, September 28th, 2016 AT 12:12 PM
Tiny
AL514
  • EXPERT
There are a couple checks for vacuum leaks, one is putting a vacuum gauge inline going straight to the intake manifold. The other is a bit more dangerous and I do not recommend it, us techs do it cause we have done it many times, but it can cause a fire if you are not very careful. Vacuum leaks will also cause a hissing sound sometimes and you can track it down that way. Hopefully this will help you a bit. Let us know what you find and we will help from there.
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Wednesday, September 28th, 2016 AT 1:14 PM
Tiny
AL514
  • EXPERT
Also, do you have any way to check the fuel pressure? Just to make sure the fuel filter is not clogged and the pump is functioning correctly.
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Wednesday, September 28th, 2016 AT 1:18 PM
Tiny
JPUCKETT
  • MEMBER
I have done a visual inspection of all of the vacuum hoses that I could find and I did not see anything cracked anywhere. I verified that I do get AC in all vents and turning AC on or off at full power does not seem to cause any fluctuation changes either. I do not have a vacuum gauge up here either, nor do I have a way of checking my fuel pressure. The previous owner suggested that my fuel pump may be bad as well. I just moved up here so I have access to no tools at the moment.

I spoke to a local shop and they invited me to bring it in so they could see if they could diagnose the issue free of charge. So, if I cannot find anything in the morning while she is cold I will bring it over there after work to see if they can find something.

I do not have any check engine lights, but the previous owner told me they had the light come on for a catalytic converter once. They simply disconnected the battery to reboot the computer and it never came back on so they disregarded it. Could this be related?

Thanks for the help so far and I will log back in after I get to work to report if I find anything else in the morning.
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Wednesday, September 28th, 2016 AT 9:34 PM
Tiny
CARADIODOC
  • EXPERT
Hi guys. AL514, send me a private message if you cannot figure out how to do it.

At any time right before this started, was the battery disconnected or ran dead? The vacuum leak is by far the most common cause of what you described, but if power was lost to the Engine Computer, "minimum throttle" will have to be relearned before the computer will know when it has to be in control of idle speed. When that relearn has not taken place, usually idle speed will just be too low all the time, but on some models the computer will still try to make adjustments. The fact idle speed is changing proves the idle speed motor is working and the air passage is not blocked by carbon.
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Thursday, September 29th, 2016 AT 12:20 AM
Tiny
JPUCKETT
  • MEMBER
Battery was disconnected for the seven month period, and six to seven times during 2015 for one to two month long periods as I was out to sea each time. The battery ended up failing on me right before the 7 month deployment and I ended up having to buy a brand new one right before leaving. How would I go about having it re learn the correct idle if that is indeed what is wrong?

Thanks.
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Thursday, September 29th, 2016 AT 1:25 PM
Tiny
AL514
  • EXPERT
https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Here is an article on checking for vacuum leaks. But the PCM will relearn sensor data and drivability conditions on its own just by being driven, this is what happens when we find something like a bad sensor and it is replaced. We would clear the diagnostic code for the bad sensor and over time the PCM will relearn the parameters of the new sensor and adjust the air/fuel ratio accordingly.
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Thursday, September 29th, 2016 AT 2:40 PM
Tiny
RENEE
  • ADMIN
Great teamwork AL514 and CARADIODOC!

Renee
Admin
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Thursday, September 29th, 2016 AT 3:42 PM
Tiny
JPUCKETT
  • MEMBER
Alright,

So I wasn't able to find anything myself in the end and took her to the shop that offered to diagnose her for free.

They came back and said that I have a loose distributor. They're looking for a replacement right now but the factory manufactured one was discontinued some 2-3 years ago. He hasn't found one that he likes yet so he is going to continue looking over the weekend and that he would call me back Monday.

It seems plausible to me that it could be the distributor, what do you guys think? From what I see online it is intended not to move while spinning but he stated mine is loose and rumbling around. After looking around online a bit it seems like hes most likely correct.

I'd just like to thank you two again for being so helpful and informative. I'm learning as I go here!
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Friday, September 30th, 2016 AT 9:02 PM
Tiny
CARADIODOC
  • EXPERT
What he is referring to is the bushings inside are worn and the shaft is wobbling around. That affects the relationship between the pickup assembly and the part that triggers it, and that means the ignition timing is bouncing around. That has been a common problem on some models, and erratic idle speed will result. You typically would also feel light surging at highway speeds.
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Friday, September 30th, 2016 AT 9:09 PM

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