BrianG

I have noted 3 occasions in the 4 months I have has the GT, a strange phenomenon of VERY HIGH speed idle. This has occured only within 5 minutes of intitial start-up. I normally allow the car to idle after start-up for about one minute before rolling away.

What happens is that upon clutch disengagement, the engine races up to 3000 rpm. It remains there for a minute or 3... not longer. It falls back to normal idle speed spontaneously. No amount of "tapping" the gas pedal makes it drop. There is no change to the feel of the gas pedal. It does not matter what the vehicle speed is. I have rolled to a stop to see if there is a change. There isn't. I have continiued to drive... no change but a pretty brisk city-driving speed! I have turned off the ignition and restarted... no change.... it re-lights at 3000 rpm.

When it finally drops back to normal idle speed, all functions seem normal.

Cold idle speed is slightly elevated from warm idle speed.... not nearly this fast.

I notice (hear) a slight "thrumming" in the idle speed of this GT that is not present in the S4. This is not observable in tach needle motion.

Otherwise the engine starts easily, and runs strong and smooth.

Randy V

I had the same problem on my GTS awhile ago.

I disconnected the battery for several minutes to reset the brain.

That eliminated the intermittent high idle for me.

Other causes of high idle are vacuum leaks and/or the idle stabilizer valve.

AO

Big vacuum leak (not one of those tiny lines) can cause this. Listen for some hissing or take a can of starting fluid and, with the car running, spray it here and there and see if the idle increases.

Vlocity

I take it that you are positive that you don't have a frayed throttle cable hanging up......

Ken

Jim M.

My GTS did that on occasion, until I lubed the throttle cable. Hasn't repeated yet.

Jim Mayzurk
93 GTS 5-spd

BrianG

The "at rest" throttle position is unaltered during this event.

I am hunting a comb-flap actuator leak that I can actually hear. Perhaps these things are related.

TheoJ

Brian,
you would need a serious leak to cause this 3000 rpm. You can take the vacuum suppy to the interior off at the brake booster, and plug it. Then you'll know for sure.
The 3000rpm idle should however also happen when the engine is warm if it is a big vacuum leak. I've seen similar 2000 rpm when the little crank vent hose at the oil filler was leaking badly. Not really easy to hear but visible when not deep under the intake.

Like Andrew said: reset the ecu. If it idles fine at beginning and soon goes back to 3000 rpm it is a vacuum leak in that area somewhere.
Theo

the flyin' scotsman

Brian............as well as disconnecting the battery for a LH reset I'd also apply the WD40 trick to the ISV and inspect all cables and mechanism that are on the throttle.

Perhaps there some dirt on the spline where all those cables attach..............easy job to remove, clean, lube and reassemble.

I would think if there was a massive vacuum leak to cause the problem it would be there all the time.

BrianG

I don't think it is likely a vacuum leak since the occurrance is so rare, and so specific to "time since starting"..... unless there is a warm-up function that is vacuum actuated, that I don't know about.

I rather suspect the ISV..........

....and while we are in the topic of ISV..... the GT moves at low speed VERY differently than the S4. I know to expect a difference due to the differences between direct mechanical coupling and viscous coupling, but it is almost impossible to idle the GT along without it developing a driveline "buck", and having to declutch to settle the oscillation. Is this normal for a GT, or is this an indication of ISV disfunction?

AO

Brian,

If you're running a performance clutch then it might buck a bit, but a stock clutch should allow for a very smooth take offs. Mine only shudders in reverse and that's with a Spec Stage 3+ clutch... very grippy.

BrianG

Andrew it's not the take-off. It's idling around (or just off idle) as in in a parking lot trying to roll along a 2 mph. The GT set's to bucking like a bronco across the driveline play.

Rob Edwards

I just re-did the intake (twice!) as PM and to fix a high idle. I now have a steady ~800 rpm idle, but I definitely get come slight bucking when coasting (foot off the gas) in first gear below 1500 rpm. I guess GT's just like the whip......

the flyin' scotsman

Perhaps a nicely maintained S4 is the cure.

marton

That does sound like a minor vacuum leak or other reason for incorrect fuel/air mixture.

Marton

John Speake

Brian
For the low speed, foot off condition that you and Rob mention, you could try experimenting with the fuel cut off restore rpm parameter with your SharkTuner.....