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95 ZJ 4.0 low idle, otherwise fine

Discussion in 'Grand Cherokee, Durango, etc' started by snoopdogie187, Apr 24, 2017.

  1. snoopdogie187

    snoopdogie187 Member

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    Hello everyone.
    95 Grand Cherokee, 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 or just under that.

    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. High idle isn't rough either. Does need new motor mounts soon, so not perfect.

    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.
     
  2. ImperialCrown

    Level III Supporter

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    An OBD1 scan tool with the correct Chrysler engine diagnostic connector may offer some diagnosis. Does the 'Target' idle match the 'Actual' idle speed? The PCM should control idle speed through the crank position sensor signal (known RPM).
    The scan tool should also be able to raise or lower idle speed by stepping the AIS motor up or down. This will verify that the PCM can at least control idle speed.
    Introducing a vacuum leak should raise idle speed.
    Your Jeep has a MAP sensor, no MAF.
     
  3. AC TC

    AC TC Well-Known Member

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    Make sure you know the true rpm, not just what the gauge is telling you...
    - The computer knows the Engine speed, if it doesent the Engine wont run..period!
    Maybe the previous owner have been fiddling with the " idle" screw that is used to set the throttlebody base air flow.
    - this can make the idle Circuit overloaded but i doubt it as you have a higher Cold idle.
    Im guessing that its the gauge thats wrong, but this can quickly be verified as IC wrote.
     
  4. snoopdogie187

    snoopdogie187 Member

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    An OBD 1 scan tool, as far as I can find, seems to be out of my price range, especially Chrysler related ones.

    I'm fairly certain the gauge is correct for the tachometer. It defiantly sounds like its running slower than it should be, and that the high idle isn't high at all.
    I will try to verify this with a tomorrow (provided it isn't raining again) with a digital laser tachometer on the crankshaft pulley.
     
  5. ImperialCrown

    Level III Supporter

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    I would certainly not suggest buying a scan tool for this possible one-time use for this low-idle problem. It would be no where near a cost-effective choice. You would either want to borrow one or pay a shop the diagnostic fee for their findings with their OBD1 tool.
    A shop 'Diagnostic Charge' is usually a 0.8-1.0 hr charge for evaluating and estimating a repair like this. You are just basically looking for an answer.
    Is the PCV line clear? Many times these plugged up and would spit oil into the air cleaner box.
    There are 4 wires between the PCM and AIS. All must be unbroken and ungrounded and go straight between the 2 components. Sometimes they can get pinched, cut or rubbed-through.
    The 1993 service manual mentions an AIS 'exerciser' tool 7558. It is described on p. 14-36 here:
    http://oskin.ru/pub/chrysler-dodge/manuals/Service Manuals/1993_ZJ_Jeep_Grand_Cherokee/93ZJ_14.PDF
    I think that later DRBs had this AIS test built-in where you could select various target speeds and make sure that the engine could meet them.
     

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