High Idle 1990 Regal Custom 3.1

[Regal Custom]

I have been chasing a high idle problem and need some help! It began with a sag or hesitation problem then after replacing the PCV valve and hose started having a high idle problem (Error Code 35). The idle will only come down on its own after resetting the ECM by removing the fuse or battery cable. After relearning, it will run fine until the engine is turned off. Upon restarting, the idle goes high to around 2000 rpm and will not come down even when warm. If I remove the fuse and reset the ECM, the idle will come down on the next relearn. Scanning does not indicate any unusual or out of range sensors with one exception. I noticed the BLM is running high, on average 147-150. I do not know if this is related. Also, the IAC steps (or position) is 100 after turning the key off/on. The IAC valve appears to be functioning properly because when the ECM actually gives the IAC a command to lower, the position goes down to 10-20 after a relearn.
Replaced CTS, IAC, PCV valve & hose, upper intake plenum gaskets, various vacuum hoses.

[tomogchi]

That almost sounds like a vacuum leak. The part i dont get, is the resetting the ecm will make it run normal agian; unless the egnine is trying to compensate for the lack of pressure. I'd spray all the vacuum hoses with Carb cleaner or brake Cleaner..... See if you have a leak in your distrobution block, or any various hoses. Does your problem persist even after you put it in drive (and possibly drive it around for a little bit), and put it back in park to idle? and have you tried unplugging your MAF sensor? Then see if your car runs any different then.
Good luck!
Bill

[Regal Custom]

Thank you for the reply. Once it relearns I can drive it all day and it will work like new, as long as I do not shut it off. It will shift fine in all gears. One thought I had was an intermittently bad TCC relay causing the ECM to reset, but I'm not sure this would cause the problem. I forgot to mention that I also replaced the MAP sensor. I will give both suggestions a try and let you know what happens. Thnaks again.

[Regal Custom]

Thank you for your reply. I went back and checked for vacuum leaks. Only found one very minor leak at the distribution block on top of the throttle body. I tried restarting and the idle was high and the Block Learn (BLM) was pegging at 150 again. I removed and plugged the PCV hose that goes from the air filter to the valve cover and the BLM started dropping to 125-130 range. I thought it may have been a faulty PCV valve so I installed another new (DELCO) PCV valve. No luck, the idle would not come down.

When I scanned the ECM, it appears that the IAC will move to the correct position when commanded by the ECM, however it appears the ECM does not give the correct command. For example, the desired idle indicated by the ECM (from the scanner) is 700 rpm but actual idle is above 1500 rpm. After restarting the engine, the command position will not go lower then position 50 (or steps) when it should be about 10 -20 steps. This results in an idle above 1500 rpm. I can't figure out what is causing the ECM to not lower, or "kickdown"
the IAC to the normal idle position of 10-20 steps.

[Regal Custom]

UPDATE - Problem Solved

This one had me going for more then two months. In a fit of rage, I just started unplugging sensors one by one until something changed. Finally when I yanked the Coolant Level Sensor (CLS, the one in the radiator) the idle jumped up, then lowered to ~800. I knew I had found something. I reviewed the scanned data recorded during the test and noticed some funny things happening to the 5 volt reference supply from the ECM. I then disconnected the battery to reset the ECM - with the coolant level sensor disconnected, and reconnected the battery leaving the CLS unplugged. I restarted and let the ECM relearn and that solved the problem. It appears that the CLS was intermittently shorting and causing the ECM to go nuts. I hope the information can help someone else.