intermittent fast idle, 85 240 GLT

The coolant temp sensor supplies the signal for regulation of the idle air control motor to the engine control unit. A faulty sensor or connection or wiring harness problem can cause the control unit to open the IAC all the way.

Bob

It's possible your wiring harness is causing this. Since you have to replace/repair it anyway...

Thanks, Bob!

Had read about the coolant temp sensor in the constant idle system section of Bentley. However, the troubleshooting section only discusses low idle, not fast. Thanks for shedding light on this problem. Will be able to test the sensor this weekend after I install the new wiring harness.

Mike

Having just had a wiring harness problem myself, I would definitly start there. Harness rot will cause all sorts of weird random electrical faults.

What year is your 245? Are you talking about the thermal valve that's usually blue plastic that's screwed into the head and buried under the intake manifold?

How often does the high idle occur? I've had a periodic fast idle for a few seconds ever since I bought my 240 turbo new. These days, it only happens once every six months or so. It has never been satisfactorily diagnosed to this day. The dealer, (Keyes Motors in Van Nuys, Ca.) wanted to do the shotgun approach several years ago but I declined...of course it was after the warranty period, Duhhh.

Thanks for all the great responses! The fast idle has happened four times. Each time for at least a couple of minutes, always while driving. Was hoping to replace the wiring harness today, but things didn't work out. Hopefully, tomorrow as my alternator is acting like it wants to jump on the "list of things to fix". The alternator gauge needle hangs just above the red on the low end of the scale for a few minutes until the car warms up, then it jumps to the high end of the meter. Belts look fine, although I'll replace them just in case. Hoping this is also related to the wiring harness (crossing fingers), but I doubt it. Started searching for advice on the alternator in the event I can rebuild or clean something to fix it. After repairing my wiper motor (thanks to Dave Shannon's great site), I'm hoping I can repair my alternator, too. I'm not really holding my breath though.

1985. Yes, the thermal valve is exactly as you've described. Replaced a number of vacuum lines today including both that connect to this valve. No fast idle today, but I did notice that while the car's cold, it'll idle smooth, then the turbo boost will slowly rise while the engine rpm's drop and the idle becomes rough. Then, after about one second, the boost drops back down and the idle rises while the engine returns to a smoother idle. Just starting to look this up in my Bentley manual.

The alternator problem sounds like the beginning of brush failure - as it warms up the brushes begin to make contact. The devil of brush failure is that the warning light doesn't come on because it depends on current through the brushes.

The brushes are part of the regulator block, which cost me $35 US the last time I bought one. You can do a band-aid fix by removing the regulator assembly from the alternator (be sure to disconnect the battery first!) and bending the attachment posts for the brushes so the brushes extend a smidgen farther. It will buy you a few weeks and confirm the problem.

Mike

That thermal valve is part of a system that provides cold engine acceleration enrichment. It should pass vacuum when the engine is cold, and not when the engine is hot. The bellows device that it's connected to on the firewall should close its electrical contacts when vacuum drops suddenly (as during sudden accelleration). Those contacts should be open during normal conditions. This temporarily grounds a pin on the Lambda computer, which enriches the mixture.

Make good sense. Last experience I had with a voltage regulator was in high school on my '59 Ford pickup. Was pretty certain things had changed since then, just wasn't sure how. Was going to try the band-aid fix this weekend, but I came down with the flu. Will just order a new regulator from FCP Groton, install it and go from there.

Thanks again. Mike