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1996 Grand Cherokee Rough Idle and poor MPG


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ffemt5530
New User

Oct 4, 2009, 3:38 PM

Post #1 of 9 (8556 views)
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  post locked   1996 Grand Cherokee Rough Idle and poor MPG  

1996 Jeep Grand Cherokee Laredo 4.0 (160,500 miles)
-What I’ve replaced thus far-
STARTER (TWICE)
IGNITION COIL (TWICE)
BOTH OXYGEN SENSORS
CATALYTIC CONVERTER
MUFFLER AND TAIL PIPE
CRANKSHAFT POSITION SENSOR
CLEANED THROTTLE BODY
CLEANED IDLE AIR CONTROL VALVE
AIR FILTER
FUEL FILTER
RAN LUCAS THREW ENGINE SEVERAL TIMES
RAN OCTANE BOOST THROUGH ENGINE SEVERAL TIMES
SPARK PLUGS
WIRES
CAP
FUEL PUMP RELAY
STARTER RELAY
BATTERY
REPLACED TWO SCREWS ON PCM (READ THAT THIS REMEDY WORKED FOR SOME, BUT NOT FOR ME)
HAD JEEP CHECKED FOR VACUUM LEAK, NONE REPORTED
CHECKED PCV VALVE AND CLEANED IT (ALL LOOKED GOOD)
CHECKED MANIFOLD FOR LEAKS, NONE REPORTED
HAD FUEL PRESSURE CHECKED-REPORTED IN NORMAL RANGE

*HAD JEEP CHECKED BY FIVE DIFFERENT MECHANICS (ONE WAS EVEN THE DEALERSHIP) AND HAD IT HOOKED UP TO MANY DIFFERENT DIAGNOSTIC TESTING EQUIPMENT-ALL SAID THAT THERE WAS NOTHING WRONG WITH MY JEEP.


*At this time I have no check engine light.


*The issue is that my jeep will run great, but when I take it on long distance trips (1-2 hours) it starts to act up. The problem is that approximately an hour (usually) into the trip I will come to a stop light and have to stop and my jeep will idle at abnormal rpms (between 100 and 200). My jeep never stalls, however. When I give it gas the rpms increase just like normal, or if I shift it into neutral the rpms will shoot up to normal ranges as well. Also, I am experiencing terrible gas mileage and I can smell the exhaust inside of my car. Finally, I am experiencing somewhat hard shifting while at higher speeds and sometimes when I’m in a lower gear and the engine is working to downshift to maintain a certain speed at which I’m traveling. This has been a ten month annoyance of mine and all the mechanics I have been to at this point have failed. I am looking for the answer to my problem, but any help is appreciated. Thanks so much!


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Oct 4, 2009, 3:49 PM

Post #2 of 9 (8546 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

I think this engine uses a timing chain. They can be erratic and fool techs.


Check for lash in that chain. 160K is enough even for real chains vs known maint for the rubber ones,

T
_________________________________________
Long retired now


ffemt5530
New User

Oct 4, 2009, 4:10 PM

Post #3 of 9 (8541 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

The contents of your response is an entirely new concept to me as I have never heard of this or had this mentioned to me by any of the mechanics I've been to. That being said, could you be gracious enough to tell me what that is, where it's located, and how I would go about checking to see if what you have recommended is bad?

I have read A LOT of different forums relating to my problem and all of them indicate that it could be a bad EGR, IAC, Map Sensor, or Throttle Position Sensor. I am just wondering if you are dismissing any of those as the answer to my problem.

Sorry if I sound like I'm prying, it's just that I have been spending a lot of time and money on trying to fix this problem and I'm fed up. I'm really placing a lot of hope in this particular forum to end my many months of anger and frustration.

Thanks again!


ffemt5530
New User

Oct 4, 2009, 4:10 PM

Post #4 of 9 (8540 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

Also, would a bad chain cause me to have poor gas mileage?

Thanks


Sidom
Veteran / Moderator
Sidom profile image

Oct 4, 2009, 4:48 PM

Post #5 of 9 (8532 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

A loose timing chain could cause some of your problems & would definitely affect fuel economy. With your setup there is 2 ways to check it. One would be to put a breaker bar on the crank & move the crank back & forth while looking at the rotor in the dist, noting how long it takes to go from one direction to the other. There is no real spec for movement, it come down to experience. The 2nd which is a little more accurate is to hook up a lab scope to the cam & crank sensors & check the sync of the patterns while running, noting for movement while reving the engine. Any thing more than 10º out indicates something is going on.

From what you've said I would probably start off with making sure there in no exhaust leaks. You should never be getting exhaust gases into the cabin. Those have had problems with their exhaust manifolds & an exhaust leak would affect your gas mileage, it would lean out the exhaust gases making the comp compensate by adding more fuel. a look at the fuel trims would help confirm this.

As far as the low idle, I would be looking at the IAC pretty close. Those are a dual coil IAC and in the past I've had problems with one coil going bad & not setting a code, also since it is electrical, the failure was only happen when hot & under load.........






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ffemt5530
New User

Oct 4, 2009, 6:36 PM

Post #6 of 9 (8524 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

Thanks again for responding. I have checked for leaks in the exhaust manifold. I was told to spray some type of fluid (I used WD 40 and Throttle Body Cleaner) around the exhaust manifold while it was running. I found no leaks. Everything from the catalytic converter (including the cat) is brand new-just put on approximately one month ago. Just a note-the exhaust smell in my cabin only appeared whenever I was coming to a stop, and it was only a faint amount unless I opened the window. It is a smell of unburned gas because my engine is running either too lean or too rich. Would a faulty IAC give me poor gas mileage, and is changing a timing chain a simple task?

Thanks


re-tired
Veteran / Moderator
re-tired profile image

Oct 10, 2009, 11:10 PM

Post #7 of 9 (8509 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

A very high percentage of jeeps were indeed fixed with the pcm screws , Go back and try backing out those screws a couple of turns .If no change ,go to a salvage yard and get a pcm harness and swap it out. One other thing ,look down at front of eng in the opening in front of #1 exhaust tube and see if the harness is stuck to it.


LIFE'S SHORT GO FISH


mink88
New User

May 20, 2011, 7:49 PM

Post #8 of 9 (7374 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

same problems but it does it right off,im about to pay a bum to drive into a local jeep dealers show room ,e
i have replaced all three sensors on the throtle intake,starter,batery,pcm,relays,fuses,cut the cat out put new one in,new o2 sensors, no wires crosed or out of place,replaced cap rotar,wires,plugs,

on subject off subject had to pull two relays nothing on at all but massive drain on battery,second i cut the cat out because of massive carbon build up, the night i cut it out replaced with welded pipe an o2 sensors it got 18 to 22 mpg home 30mile drive, next day went back to running like crap with new added problem you can be drive fine bad mpg and it sputers if you mash the gas does nothing but about stals out let of the gas goes back to okay idle then it goes back to almost stalling to very loud back fireing, with thick black carbon smoke cloud.


(This post was edited by mink88 on May 20, 2011, 8:04 PM)


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

May 20, 2011, 7:59 PM

Post #9 of 9 (7370 views)
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  post locked   Re: 1996 Grand Cherokee Rough Idle and poor MPG [In reply to]  

This is a 2 year old thread. If you have a question, please start a new thread.




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