Porsche Enthusiasts Club Forum

Driving my car his week I think its not running quite as it should. Its barely there but it seems to drop its revs a little at idle from about 700 to 500 where it stumbles just fractionally and then comes up and is fine. At normal speeds it seems there is just the barest hesitancy under load.
AFM do you think?
Having read clarks garage it looks ok to get to and clean the tracks.

Check ignition first they are very sensitive to plugs. Also check for air leaks especially around the ICV.

Sounds a little like it could also be the throttle position switch, either failing or has moved out of adjustment.. This will cause the DME to not see that the car is at the idle position, and will cause the rpm to drop down bellow ideal and then rise back to idle speed when letting off the throttle, especially if the car has not just been revved but actually had the revs above 2000 rpm for say 30 seconds or so.

The problem is that a lot of 944 turbo's need their throttle bodies rebuilding, which causes boost pressure, with the associated oil mist on non catch can cars, to get into the throttle position switch, which then causes the fine oil mist to contaminate the sweeper part of the sensor/switch, as well as the microswitches.

Also, if anyone has ever messed with the switch, or the throttle stop, the throttle body will use the throttle position switch as its idle stop, which means whenever you return to idle, there is force which will push on the microswitch, buggering it, but also will move the whole component out of position.

It sounds like it needs just a 30 min job of re-adjusting the switch and then re-setting the throttle bypass screw with the associated proceedure on the electrics to put the DME into adaption mode...

The other thing it could be is a dirty throttle butterfly, which will mean that you do not have the usual amount (since idle adaption was last performed) of bypass air getting past the now sealed with gunk throttle butterfly...

If you were keeping the car, I would probably recommend a intercooler clean out, throttle body rebuild, new throttle position switch and the car re-set-up. Well worth while doing.

One last thing to check, is how air tight your dump valve is... pull off the vacuum hose, put your lips around the nipple and suck, while sucking, place the end of your tongue against the opening on the nipple, at which point, if you stop sucking, the nipple should keep hold of your tongue under vacuum... Sounds a bit kinky! But its one of the best ways of testing it if you do not have a mightyVac or other vacuum hand pump (that sounds kinky too! lol)

A bad bypass valve (AKA dump valve, will cause the same problem as you describe, as will the throttle bits)

thanks both

Mine seems to have a similar problem, it only does it when the car is cold and coming to idle after moving.


I wonder if doing the same would cure my problem.

On the many old aircooled ive worked on that sort of this is often related to a small air leak

Mine has recently been checked and all secured with cable ties. I have noticed its only seemed to start since I recently fitted the the large spring that was missing from the throttle body.

Scott, if yours started after refitting the throttle spring, it is probably your throttle position switch needs to be re-adjusted and your idle adaption re-set (flash way of setting up the baseline idle speed)

Mine seems much happier with a proper, new dme relay.
I had forgotten that I'd changed mine for a spare when the first expired and the spare was the old 944 part.
So it now has a new 993 part.