high idle

And by high idle, I'm talking 2,000 rpm's. Its a '99 4.0 classic with 141k miles. Sometimes when I start it up, it will idle between 1,500 and 2,000 rmp's for a minute then drop to 800 or so. It will also do it when I put it into park or neutral after driving for bit. It doesn't always happen though. Something that ALWAYS happen is when my AC is on, it idle's at about 1,200 rpm's which i expect to be higher, but not that high. When I'm braking and coming to a stop, the rmp's will jump up to almost 2,000 rmp's lurching me forward, where I have almost rearended someone. I had the I had the IAC replaced at about 110K miles. Any idea's?

First, pull the IAC and intake tube, then clean the throttle body with a TB safe aerosol cleaner. Clean the IAC port with the spray cleaner and cotton swabs, and clean the IAC pintle CAREFULLY with the spray, a toothbrush, and swabs--the IAC can be broken, so be careful with it.

Check the throttle plate's range of operation--closed to WOT, cycle it a few times to see if it is hanging up and not closing.

Put it all back together and fire it up. Post back with results.

X2 on cleaning the IAC. Also check vacuum with a tester (about $20 from most parts houses), should be 15-20 hg inches at warm idle. Any lower and you have at least one leak

joe_peters said:

.... and clean the IAC pintle CAREFULLY with the spray, a toothbrush, and swabs--the IAC can be broken, so be careful with it.

Click to expand...

easy cleaning method:

- fill the center cup in a spray paint cap with acetone
- rest IAC, pintle down, on the edge of the center cup
- go clean everything else
- when ready to install it, it'll be spotless and won't have been manipulated from original position.

Well before I had the chance to clean the IAC, I had starting problems and had to replace the CPS. For two days now I have had no issues with the high idle. Seems strange to me that a new CPS would fix this. This weekend I'm going to go ahead and clean the IAC anyway just in case. Could the CPS have been the problem?

The IAC is a stepper motor that responds to extend and retract signals from the PCM. The PCM (not the Renix ECU) stores information on how the engine has been running, and uses that the next time the engine is started. If you disconnect the battery and drain the PCM memory it has to re-learn. All of that said, the CPS is a signal generator to let the PCM know the physical position of the crankshaft. I would not expect that to cause the PCM to "lose its mind", but honestly it could have been related.