Original message
posted by JAZ on Sat Jun 17 03:06 UTC 2006
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I had been having some troubles with the idle speed going high after it warmed, up, around 1200-1300 rpm, but not always.

It would start normally, run at regular idle and then sometimes runup in idle. If I shut the car down and restarted it, it cleared sometimes. It usually took many cranks to get it going. .....

Since then I have done some tests on the IAC and put an O-scope on the signal. I took it appart and cleaned it, everything looked good. It pretty much stays open all the time, but perhaps someone on can tell me if what I see as diagnostics makes sense.

I have an image at http://home.comcast.net/~jaznm/ that shows what I have found.

JAZ I think i found the problem of the high idle after warm up.I have gone through everything.Resetting base line idle,cleaning ICU unit and throttle body and still had the same problem.So I adjusted the throtle plate idle screw by backing off the screw that has the lock nut 1/2 a turn. Now no more high idle.Idle stays at 900 rpm when cold and after warm up. Hope this works for you. Charlie

I have the same problem with my 85 240 ti and haven't found the problem.I tried a new air control valve and it didn't change . At start up it idles fine but after worm up coming back from a ride the idle is around 1200 rpm.If I sit there parked for about 45 sec. to 1 min. the idle will start to drop back down to 900 rpm.I will try some other things and will let you know if I solve it.Let me know if you find anything. Charlie

Tried a new Idle air control valve tonight, thinking the original has 255K on it. The result was no change. This valve appears to be open all the time as well. I suspected the old valve since I could not get it to jump open or closed reliably when toggling voltage to it.

There is still something happening,I measure the coolant sensor to the ECU tonight and read 300 ohms at running temp, which appears to be okay. If I close down the hose from the IAC, removing flow around the throttle plate and the RPMs came down.

I also brought a propane torch, unlit around the intakes and throttle valve so if there was any vaccum leaks the RPM should have spiked up a bit and it did not.

Any thoughts.

Have you tested the throttle position switch for being closed at idle? Not just the "click" off idle, but test for continuity from ECU pin 3 to ground. That ground is made at the fue rail or throttle bracket (not sure about '84).

If the ECU doesn't detect throttle closed, it may default to a higher RPM setting on the IAC valve.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

I can't address your scope findings but the IAC valve should be cycling somewhere between open and closed constantly to regulate the idle. If it is mostly open then that might explain your high idle and indicate either the valve itself or the ECU is the problem. Dan

My 83 244Ti was idling extremely higher (1800-2k) and the wiring harness was the culprit. Moving the exposed wires on the harness connector that goes along the passenger side firewall, actually reduced the idle. Since I was replacing the harness and had the intake manifold pulled, I heard just cleaning the idle control valve (rather than spend 300), with aerosol brake cleaner to make sure the armature inside moved freely. My car only has 122k so I'm not sure what the typical failure point is for the ICV. Just my two cents worth guys...