Extremely High Idle on 93 SL2

You've done all this work and replaced a bunch of stuff, but have you read the codes from the PCM to see if it indicates what is wrong? Autozone will do this for free, or you can do it yourself with a paperclip with the instructions here:

to the OBD1 diagnostic code section. I would suspect your IAC valve because it controls the idle. But read those codes first to see if the PCM indicates anything else.

Lane [ lane (at) evilplastic.com ]

It sounds like you know more than the average person that comes through here but just to make sure you do know your 93 has "two" ECT sensors right? One for the dash gage and one for the ECM. The one to the ECM would be the important one.

The speed sensors wont affect idle. Have you cleaned/checked the EGR valve?

In my previous reply, I forgot to respond to the part about 2 ECTs. The temp gage in the instrument cluster has been working correctly (at least it appears to) all along. The ECT I replaced was the one with 2 leads on it. I haven't messed with the single lead one above it. Thanks again. Terry

I caved in and took the car to my mechanic. He has been able to solve every problem on every car I have taken to him - until now. He saw no codes to indicate why the idle is so high (up to 3000 RPM with clutch pedal depressed). All the sensors appear to be functioning correctly. The IAC reports being where it is commanded to be. He tells me that given enough time at ($1 per minute) he can find and fix the problem. His inclination is a problem in the Powertrain Control Module. Any thought on this. Also, if this is the route to go, how do I get the right module. I see some on Ebay where the seller says the numbers must match. Is this true or do the two cars just have to have the same engine, transmission, and perhaps some other items in common (for example ABS vs no ABS). Since this is a rather long thread, here is what has been done so far - 1) IAC replaced; 2) TPS checked and found good; 3) ECTS and connector replaced; 4) O2 sensor checked and found good; 5) throttle body removed and thorougly cleaned with throttle body cleaner and tooth brush; 6) PCV valve and hose replaced (hose was brittle); 7) Powertrain control module removed and replaced (having been in the computer industry, I have seen a lot of problems cured by removing and reseating connectors); 8) Vehicle Speed Sensor checked and found good; 9) EGR valve replaced. I am pulling my hair out. Any ideas besides changing out the PCM? Is it worthwhile ohming out the sensor leads? Last clue (for what it is worth) - if I warm the engine up, bring the car to a stop and wait for the engine speed to drop to

800-900 RPM and remove the IAC connector, the idle speed stays low on a test drive under conditions that would normally cause the problem leading me to believe the high idle speed is due to the computer commanding the IAC to open the pintle valve.

Thanks!

Terry

Given what has already been replaced, I have two additional thoughts on possible causes of high idle: a defective Coolant Temperature Sensor (CTS) or a possible vacuum hose or intake manifold leak.

Bob