Intermittent High Idle - 90 Q45

[Q451990]

I have experienced this a handful of times with both this Q and Q1... typically in colder weather. The engine will either not "idle down" after warm up, or will just start idling high randomly. Once the problem starts, sometimes turning the car off and then restarting it will cure it - or in some instances I have to disconnect the battery for a few seconds... which always cures the problem - typically for several months or more.

It finally acted up today at a time that when I was able to get the laptop to connect with the Consult cable. My theory in the past had been that the ECU was getting a bad signal from the ECU temperature sensor - but that was showing fine... somewhere around 178F.

The ECU was calling for 75% IAC opening when the idle was high at 2K. After disconnecting the battery and reconnecting it - the idle returned completely to normal... with an opening at 10%. The only difference I noticed with the Consult software was that while it was idling high the TPS closed switch was not showing as on, but it did show as on after I reset everything by disconnecting the battery for a few seconds - but the TPS voltages were essentially the same at all times.

As a side note, I'm convinced that it's an electronic control issue, because unplugging the IAC will drop the idle to normal - so I think I can rule out a vacuum leak, FIC issue, or linkage problem.

So is this just a glitch in the ECU - or is it responding to a sensor that I'm either not monitoring, or just don't know what to look for?

Heath

[Q45tech]

Follow FSM to test hot wax warm up idler system.

Is TB housing and plate clean as brand new, same with IAC internal passages and piston and chamber?

[elwesso]

Have you tried giving it a couple good revs and letting it die down. Mine does the high idle thing you describe and whenever i give it a quick rev to about 3 grand it lets down.

[Q451990]

Follow FSM to test hot wax warm up idler system.

Is TB housing and plate clean as brand new, same with IAC internal passages and piston and chamber?

I'm convinced that it's not the fast idle cam on the TB, since unplugging the IAC valve drops the idle speed. Wouldn't that, with my 75% reading from the Consult cable, and the fact that I can "reset" everything by disconnecting the battery for a couple of seconds rule out the wax cam?

TB housing, plate, IAC, EGR, plenum, runners, all cleaned to spotless less than 2k ago when I replaced the injectors and everything under the plenum.

Heath

[Q451990]

Have you tried giving it a couple good revs and letting it die down. Mine does the high idle thing you describe and whenever i give it a quick rev to about 3 grand it lets down.

Tried it several times... didn't work.

Heath

[Infinitiguy19]

Sorry if this sounds um, But I will go for it:

Could it be a problem with the new injectors?

Possibly ohm them to see what they are at.

Maybe even a BG Fuel Rail flush.

See if Deachworks will check them again.

[Q451990]

Could it be a problem with the new injectors?

I don't think so... the problem happened on occasion with the old injectors, and even with my first Q. At some point I will re-read the area of the FSM that shows what inputs feed the ECU to tell it to raise the idle via the IAC valve, since it's obviously being told by the ECU (75% reading with Consult software) and the fact that the idle drops with the IAC is unplugged...

Heath

[Infinitiguy19]

Good to know that its not the injectors fault.

But I hope you get this problem resolved.

[greg_atlanta]

I had a similar problem back in Feb 2000 shortly after I bought my Q. It happened when trans was in park when engine was warm (right before I turned the car off).

Dealer fixed it without replacing any parts, invoice just said "adjusted idle." Never happened again after that even though idle was adjusted several times by T3 (currently turned all the way up).

[maxnix]

Sure there are no invisible (unseen) split vacuum hoses?

[Infinitiguy19]

I had a similar problem back in Feb 2000 shortly after I bought my Q. It happened when trans was in park when engine was warm (right before I turned the car off).

Dealer fixed it without replacing any parts, invoice just said "adjusted idle." Never happened again after that even though idle was adjusted several times by T3 (currently turned all the way up).

So they never solved the problem, just taped it up?

[greg_atlanta]

Problem was solved, never happened again. Not sure what they did since there were no new parts involved.