95 4.0 low idle, otherwise fine - North American Grand Cherokee Association
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Old 04-16-2017, 06:07 PM   #1
turo187
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Join Date: Apr 2017
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Default 95 4.0 low idle, otherwise fine

Hello everyone.
94 4.0 which idles very low.
The high idle is actually where the normal idle should be (about 700 rpm) and the normal idle is just above the first line over the 0. My guess is about 300 rpm.

It doesn't have a vibration except for being just slightly rough for being so low, and if you hold the throttle so it runs about the normal idle speed, its very smooth.

It doesn't stall, no hesitation, just a low idle.

The IAC has been cleaned with no change. New plugs, wires, distributor cap & rotor.
The ignition system maintenance restored the power, but never changed the idle.
Cooling system has been replaced, except the heater core.
Fuel pump is 3 years old as well as the filter. Original fuel pump failed completely.
The only time the check engine light has been on was recently when the exhaust broke recently from a plugged cat. Even that went away the next time the car was started. (very recent actually) Only code was for low O2 sensor reading.

We have had this Jeep for about 3 years, and the problem has never changed. Never goes up or down, not better or worse.
The jeep has plenty of power.
I will say the Jeep PO neglected it/didn't know how to care for it
Just stupid stuff, like transmission cooler lines cut and connected together to bypass the cooler, transmission fluid was changed by PO, and lately it has been leaking a lot around the pan. I just changed that gasket, and when I had the pan off, I noticed old rubber gasket stuck to the transmission and it had a cork gasket on it.
Just other issues along with that, but nothing too bad to fix up.

It does have the typical ZJ issues of the era. Speedometer, door harness (was poorly previously repaired), death wobble, etc but one step at a time until its all working.

I'm assuming the instrument cluster uses the same temperature sensor as the ECM, which does match to my infrared thermometer. I will have to check the operating temperature again as I just finished the cooling system, but even when it is running around 200F, it still has the low idle.
Unplugging the IAC, and the maf (which I don't think is the correct term for this part) resulted in no change except the maf causing a check engine light.
I've plugged each vacuum line at the manifold to see if there was a vacuum leak causing it, and it still didn't change.
Actually the idle become normal with a fairly large vacuum leak.
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