Dans993C4Cab

Ok gang, a little help please.
I'm the 2nd owner of the car which is coming up on 100k miles. I've had the car for almost 4 years and it's always had a rough idle. Even to the point that on occasion it stalls as I slow down for a stop light.
However... If I've got the a/c on the car idles fine and never stalls. Any ideas on why? Everyone just recommends getting the top end done. I'd love to not spend the $6.5k to do that if I don't have too!! All ideas will be greatly appreciated!
Dan

vjd3

Probably a tiny vacuum leak somewhere ... my car idles just a little bit low without the AC on, and I've never been able to chase it. It doesn't come close to stalling (and I have the RS clutch and lightweight flywheel which makes the car easier to stall and it never has) but it shakes a tiny bit -- unless the AC is on, then it idles like a champ. (When the ECU senses that the AC is on, it bumps the idle a little to compensate, and that's just enough to smooth it out.) You might clean and try adjusting the idle stabilization valve to see if that helps (ISV).

I've also been told that this is just an inherent a characteristic of some 993s (and this is not the first 993 I've owned that has done the same thing), and it can be fixed with a custom ECU that just bumps it up 100 rpms or so (TechArt, Rennsport, etc.), but I can tell you definitively that my car shook a little at idle before I did the top end, and it did it after the top end, too, so I don't think that will cure you.

kjr914

First thing you should do, as vjd3 suggests:
http://www.pcarworkshop.com/index.ph...-_ISV_Cleaning

Last thing you should do, is rebuild the motor to fix an idle problem.

Stealth 993

If your service is all up to date, it's either a vacuum leak (vacuum lines pop off easily) a dirty/bad ISV ( idle valve), or a failing flywheel sensor. The MAF might also need a cleaning, but it should throw a code, it may not hurt to see if there are any stored codes that are not throwing a CEL.

Body Roll

There are a few threads that attribute a failing dual-mass flywheel to rough idle. I don't quite understand what happens to the DMF to cause this- maybe someone can chime in with an explanation.

Mike J

You can also adjust the ISV to get a slightly higher idle...I did this due to my RS pulley setup in order to help the alternator at idle..

Cheers,

Mike

Dans993C4Cab

Many thanks to all!!! I really appreciate the advice. Wanna guess what I'll be up to this weekend?
Thanks again guys. You Rock!

vincer77

I found this in a search...

I may be experiencing this. While shifting, my car tends to bog down. At first it was mostly when cold, but now it is doing it more often. I've cleaned the ISV and will clean the MAF next and check rotors/caps etc... I am also getting a rattling sound when I shut the engine off - I read this is also a symptom of failing DMF.

roland_klk

Hi Mike,

may I ask how you adjusted the ISV of a 993 to get a slightly higher idle?

Thanks Roland