heliflyer

Have tried a search but can't find anything.

Recent issue - when changing down a gear for example from 3rd to 2nd, and using heel and toe - if I blip the throttle it will increase without delay but will then stay at a relatively high level for several seconds so I will end up in 2nd with the rpm remaining at say 3000 if the clutch is depressed. it will stay there for a few seconds while the clutch is held depressed before returning to idle at 900.

could this be a vacuum leak or AOS starting to fail ? No other issues or smoke etc at start up and the car drives really well other than this rpm issue.

any thoughts appreciated.

5CHN3LL

Sorry for my ignorance (my '99 C4 is e-gas) - is your car e-gas or cabled? If cabled, I'd check both the pedal and throttle body for smooth action. Either way, check the throttle body to make sure nothing is gummed up or being interfered with...

RollingArt

I would suspect a vacuum leak.

Do you know an indie mechanic with a smoke tester?

Of course it could always be some crazy electrical glitch!

heliflyer

Thanks guys - car is e-gas and throttle body recently cleaned but that has made no difference.
I have heard that the electronic unit stepper motor on the throttle body sometimes plays up but I would have expected poor response in both directions in that case.

5CHN3LL

Vacuum leaks are always a suspect. How's the idle? Solid, or is it hunting?

sammyf

I've also seen it happen with a dirty fuel filter...

heliflyer

idle hunts at first start when cold but is stable when warm.
After a long drive, idle will sit at 1500 for about 15 seconds when coming to a stop, then reduce to a stable 800.

fuel filter changed 2000 miles ago - but worth a look
Thanks

alpine003

This.

Also your latest description sound like the IAC trying to do it's job based on circumstances.

wwest

There is no "IAC"....

The Porsche engineers have decided the throttle plate control parameters, what you are experiencing may well be SOP.

Blip, downshift,....then CLUTCH...

Unforeseeable action...?

Ahsai

You have a problem right there at idle so maybe focus on diagnosing that first. The spec rpm with ac OFF should be 700 +/- 40 rmp. Your 800 is too high if ac was OFF.

Agree it sounds like a vacuum leak. If you have durametric, you can check the fuel trims trying to diagnose it further.

heliflyer

I'm away for a couple of weeks now but will plug the durametric in on my return and monitor the fuel trims.
Thanks

alpine003

Doh, that's what happens when you have 5 cars.

Was looking under the cowl under my windshield the other day for awhile trying to figure out where the cabin filter element door was on the wife's MDX. Forgot it was in the glovebox. lol

And I always do the left to right key shuffle dance as well quite often too.