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rojgc 08-25-2012 05:38 PM

96ZJ 4.0L Low Idle
I have a ZJ 1996 Jeep Grand Cherokee Laredo 4.0L.

It idles to low (gauge has 2 marks between 0 and 1 and it sits slightly below the top mark while idling). I have to come to a stop gradually, letting the transmission go into 1st gear or I am afraid the engine might stall. Other then that it seems to run fine.

The only codes that show up on the odometer are 12 and 55.

I have disconnected the battery, removed the throttle body, removed the mass airflow sensor from the throttle body, removed the idle air control from the throttle body idle air control housing, removed the throttle body idle air control housing from the throttle body, removed the throttle body air control plate, cleaned the throttle body, cleaned the throttle body idle air control housing, cleaned the idle air control, cleaned the throttle body air control plate, reassembled all parts back onto the throttle body, reattached the throttle body back onto the intake manifold using a new gasket.

There was carbon buildup, on the throttle body below the air control plate, on the underneath side of the air control plate, on the throttle body idle air control housing, and on the idle air control itself. All carbon buildup was removed (the carbon buildup on the idle air control pintle was removed very carefully).

I still have the same problem!

Do you think this could be a bad IAC (idle air control) even though no codes appear on the odometer display? How do you test the IAC? Can you remove the IAC, while keeping it connected, and then turn the ignition key half way to determine if the IAC will move the pintle?

What about the mass air sensor? Can it be cleaned? Is there a way to test it?

P.S. Will a scan tool show additional codes even when the odometer doesn't show any codes?


rcorn79135 08-25-2012 11:44 PM

Hi, Ron.

I've had a bunch of Jeep 4.0L engines and currently have three 4.0L Grand Cherokees. On two of them that had the very low idle problem (and also on one first-generation Sebring convertible with the same problem), it turned out to be the throttle position sensor. Around $20 for the Standard Motor Products "T" series one at and I think I paid around $45 for an Echlin at NAPA so they aren't terribly expensive (for the 4.0 Jeep anyway - not so with the Sebring vert). It's always a good idea to check that all connectors are firmly plugged in/together, too. Here's how my manual says you can check the TPS with a digital multimeter instead of having the dealer use their scan tool:
The TPS can be tested with a digital voltmeter.
The center terminal of the TPS is the output terminal.
With the ignition key in the ON position, check the
TPS output voltage at the center terminal wire of the
connector. Check this at idle (throttle plate closed)
and at wide open throttle (WOT). At idle, TPS output
voltage should must be greater than 200 millivolts.
At wide open throttle, TPS output voltage must be
less than 4.8 volts. The output voltage should
increase gradually as the throttle plate is slowly
opened from idle to WOT.

Good luck with finding it.

rcorn79135 08-25-2012 11:52 PM

Forgot to mention that if you do the test outlined in my post above (which would require piercing the insulation of the wires to read voltage since the connector would still be plugged into the TPS), make sure to use something to seal the wire's insulation to keep out moisture which can cause corrosion of the wiring within the insulation over time and make your life miserable.

And, if all else fails, there's always having the dealer use their mega-dollar DRB scan tool/programmer or a shop that has one of the Snap-On or other programmers capable of delivering software updates. Sometimes it's a software/programming problem - or just a connector at the PCM needing to be re-seated.

Again, good luck.

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