Questions and Answers

Mustang High Idle Q. Hi, I have an 1989 Mustang LX, 302 (5.0 liter), multi-point fuel injection, 5 speed, 100,000 miles, P/S, P/B, air, cruise.

I have found that at times it idles between 2000 and 3000 rpm. It usually happens when I'm on the highway. I notice it when I let off the gas, and the car won't slow. I step on the clutch, and the engine speed rises to 2000 to 3000 rpm. If I restart the engine (step on clutch, ignition off, wait 4-5 seconds, ignition on, release clutch), the engine usually settles into a nice idle, and I can carry on. Sometimes I may need to restart 3 or 4 times, although this is rare. Usually, one restart will do it. After restart, the car sometimes will re-enter the high idle immediately or within a few minutes.

I have seen it happen one time with my car stopped, transmission in gear, clutch disengaged. (I was waiting for traffic to clear while backing out of my driveway). The engine was idling nicely, and then in the span of about 5 to 10 seconds, the idle increased to 3000 rpm.

I have only noticed this when the engine is at normal operating temperature. I believe it also occurs only in high humidity. One time I was driving on a 1 1/2 hour trip. For the first one hour on the highway, the car ran perfectly (I had to stop several times at intersections, and the car always idled nicely, and the car also slowed when I let off the gas). For the next 1/2 hour I was driving in very thick fog, and the high idle problem dramatically showed itself.

I have spoken with several people, dealership mechanics, and checked several web sites, but nobody has any ideas.

So far, I have:
- read the codes from the engine controller (no complaints, errors, etc).
- cleaned the idle air bypass valve
- replaced the throttle position sensor

Wild speculation on my part:
1) Atmospheric pressure sensor
An incorrect reading causes the controller to open the idle air bypass valve too much? (I don't even know if valve opening is a function of ambient pressure - I assumed valve opening is a function of engine speed when the throttle position sensor indicates closed throttle, implying the controller has a measurement of engine speed, further implying that the controller should complain (flash the check engine light) when the engine is idling at 3000 rpm with my foot off the gas pedal!).

2) Wiring problem
Oxidation on connector contacts?

The check engine light never comes on when the problem occurs. This causes me to think that the controller is reading acceptable values on all of the sensor inputs.

Any ideas? I'm stumped!

Thanks,
Tony

A. There is a device called the Hot Idle Compensator that could be bad. It is not computer controlled so the computer will have no idea if it is good or bad.

The hot idle compensator cools the engine during hot idle operation. The compensator may be integral or mounted separately. When open, the compensator bleeds air into the manifold, leaning out the air/fuel mixture at idle. This increased air intake causes an increase in engine idle speed, resulting in cooling of the engine. Temperature rise on the bi-metal sensor lifts the normally closed valve and opens the air passage. A valve that does not fully close will cause high idle speed and excessive exhaust emission.

WHile the engine may not actually be hot, over time the bi-metallic sensor does go out of spec and may open too soon, causing the problem you describe.

Information provided courtesy of ALLDATA

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