Uncontrollable, Super High Idle

I’ve a '88 Jeep Comanche Pickup, 4-speed manual with just over 250,000 miles. I’ve had 3 garages look at as it’s developed a problem with it’s idle speed; it uncontrollable high. They’ve replaced the TPS switch, checked all the sensors (okay), no vacuum leaks, no kink in throttle cable–no one can find the problem. Any ideas?

Well there MUST be a vacuum leak for the idle to be so high. Somehow or another, extra air MUST be getting into the motor. Somethings to check:

  • Any cracked vacuum lines? (These can be hard to see sometimes, look all around each line)
  • Is the Idle Air control motor working correctly (or is it stuck open)?
  • Is the throttle blade stuck open with an object?
  • Possible that a PCV valve (if so equipped) is leaking extra air to the intake system

How high exactly? If no tach, does it seem higher than the high idle when you first start it up in the morning? Does it idle lower then get faster as it warms up?

Daughter’s 97 Taurus high idle. Tried everything until one day while working the throttle lever near the t.p. sensor and i.a.c. valve, we were trying to see if there was an obstruction keeping it from closing all the way.
What we found we would never have gone looking for initialy.
…The trottle butterfly was worn oval on just one side from being sprung aginst one side of the intake plennum and , even when completely closed, there was a crescent moon shaped space allowing constant excess airflow and high rpm at idle.

How high? Has anyone checked the ignition timing?

88 Jeep 4.0L High Rev Start Condition Fix
After a long and tormenting troubleshoot for a High Rev condition at start I found the problem !!!

The Knock Sensor on the 1988 Jeep 4.0L has a flimsy metal foil that wraps the Sensor wires for electrical shielding. Due to deterioration over time for a variety of reasons this foil does not protect from induced EMF from adjacent wiring and as such gives erroneous electrical data to ECU. This can be corrected by replacing the failed shielding with a braided shielding that has a wire attached to ground (preferrably at both ends of the shielding, can be accomplished by soldering wire onto the end of braid with ring termination at end of wire that is screwed into ground locations). TA-DA, no more problems and a long term fix for an oldie but goodie !!
Posted by: 88comanche4l