Too fast idle

[mcconnaughey]

I have a '90 4.0L that starts fine and runs fine as long as the temp outside is above 50F, but when below it will idle around 1500-1800 rpm. This only happens when I am on the highway/ just came off. It does this for a while on the side streets and then idles around 1000 rpm. I tried shuting the engine off and restarting to reset anything, no luck. It will only idle down after 5 to 6 min of off highway use (side streets).

I replaced the Map sensor the other day and the motor temp gauge reads normal (190-200F). I'm not sure if the computer has a seperate temp sensor that is possible faulty or a stuck TPS? The Jeep does have 195K on it.

Any help, as always, will be appreciated.
Thank you,
Alan

[8Mud]

I´ve never seen a Jeep (4.0 XJ) stay that long, at that high of an idle. Have seen some other manufactures vehicles do it though. One was a sticky throttle cable, next time it´s idled up, open the hood, a physically press the throttle closed, just as a test. Another was the gas pedal hanging on the carpeting/floor matt.
Saw a Chev. that would idle that high, was the intake manifold bolts. They´d come loose and caused a major vacume leak.
Don´t believe the idle motor (on the throttle body) will idle the motor up that high, full open (for the idle motor) is only around 1000 RPM or so, from my experience.
Most of the 4.0´s I´ve seen, with a vacume leak would idle up high, then settle in to a lower idle. The lower idle was usually really rough. I was always under the impression the computer, wouldn´t allow an auto tranny motor to idle that high (I could be wrong with my assumpton).
If it seems connected, to the outside temp. the manifold temp. sensor might be worth a look see. The wires often hang on hot parts. Another thing to check is the continuation of the fuel injector cable harness, runs over the front of the motor and hooks up to the 02 sensor, the engine temp. sensor (for the computer) and the knock sensor. On the lower left of the motor. I´ve often seen the wires rub, chaff and cook on the exhaust manifold. 02 sensor and the temp. sensor both have imput for the idle.
Don´t think it´s fuel related, to much fuel will often cause the motor to idle rough and stall.

[4xBob]

My 1990 4.0 engine has had the high idle problem, twice. Both times the TPS was defective- easy to replace, but must be set up (adjusted) correctly on all non-HO engines. You can check the TPS function with a volt-ohm meter (VOM), try a search here for the procedure. 4xBob