Need Help! Stumped. -
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post #1 of 9 Old 04-11-2011, 03:29 PM Thread Starter
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1993 ZJ 
Join Date: Mar 2011
Location: Davison
Posts: 16
Need Help! Stumped.

I have a 93 jeep grand cherokee with the 4.0. The jeep for the most part drives great when it is running. I have noticed it does have a rough idle when stopped and transmission is in drive. If the jeep is put in park or is in motion their seems to be no problem.

But if i leave the jeep parked overnight with out driving it for 5 or more hours it will run very rough, spit sputter, and seems to be a backfire in exhaust if i try to give it gas. I have to wait till the jeep is at operating temp. so it can be drivin. I can then usually drive the jeep all day with no problems. I'm hopeing somone may have had this problem and knows how to fix it.

I HAVE CHANGED the fallowing: Fuel pump, Fuel filter, Idle air control valve, MAP sensor, Air/Temp flow sensor, coolant temp sensor, oxygen sensor, Plugs twice ( they seem to run a little hot), plug wires, cap, rotor, igniton coil. brand new battery. All the vacuum lines look good, canister not cracked.

I thought to change the TPS but i get NO codes at all, and the sensor tests good with the volt meter, with throttle shut and open.

Its not a security issue, the alarm is disarmed. Also the PCM has been changed before i owned the jeep, its not a factory PCM which leads me to think thet the previous owner has changed it.

What would make the jeep spit and and almost die out after sitting overnight? could mostiure be getting somwhere? i have no idea what to check next. it seems to me if it was anything electrical i would be getting a code. And i am not getting any codes what so ever. Please help if any ides.

if it is a fuel presure regulator, wouldnt the jeep run bad the whole time?

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post #2 of 9 Old 04-11-2011, 03:35 PM Thread Starter
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1993 ZJ 
Join Date: Mar 2011
Location: Davison
Posts: 16
Ive also put a crank shaft postion sensor, still no fix. Still runs bad in morn ing until jeep is at operating temp. Then i can dive the jeep all day unless it sits for more than 5 hours then i have to wait till it is operating temp.
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post #3 of 9 Old 04-11-2011, 05:00 PM
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Try taking all the relays that are in the PDC that will interchange and swapping them around. Clean the legs.

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post #4 of 9 Old 04-11-2011, 05:09 PM
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1997 ZJ 
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Location: San Antonio
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Have you tried any injector cleaners? Thats about the only thing not mentioned on your list that I can think of. I would think it would run bad all the time if an injector was clogged. I don't know.
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post #5 of 9 Old 04-12-2011, 01:10 AM
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1993 ZJ 
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i was looking on another thread on older fuel pumps the check valve starts to wear out and it loses fuel pressure when it sits for too long and takes time to get back to full pressure, maybe check fuel pressure after it sits for a while should be at 35-39psi im told

93 zj laredo 4.0 with k & n intake 2 inch bb skyjacker shocks and 31 bfg at
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post #6 of 9 Old 04-12-2011, 04:03 AM
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Had similar issues with my work van, not a jeep but neverthless, there was an ambient air temp sensor behind the grill somewhere that was reporting -30F engine world run extremely rich and load up at idle. Circuit was not open or shorted so no codes were generated, a mechanic and alot of head scratching but eventually fixed it, -30 outside air! This just happened last week, we've had some wild temp swings here in ohio, 80 one day 35 the next and thats what we think finally did it in.
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post #7 of 9 Old 04-12-2011, 09:05 AM Thread Starter
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1993 ZJ 
Join Date: Mar 2011
Location: Davison
Posts: 16
I have tried seafoam to try to clean injectors, but i believe if i had a leaking injector the problem would clear up in 2 min or so. Im not to sure about the ambient air temp sensor, i will have to look into that. And also i will check the check valve. Thanks guys i will let you know what i find.
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post #8 of 9 Old 04-12-2011, 09:14 AM
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What's going on with the egr valve? What's intake vacuum reading when running rough?
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post #9 of 9 Old 04-13-2011, 11:29 AM Thread Starter
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1993 ZJ 
Join Date: Mar 2011
Location: Davison
Posts: 16
there is no EGR valve, in 93 i guess its all ran with vacum. The jeep has a vent tube that runs from fuel tank, to canister, and vacum off the to manifold and such. All the lines look good, and so does the canister.
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backfire , fuel , rough idle

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