Hi Everyone,

I have only recently (about a month ago) purchased my first Porsche a '96 Carrera 4 and have a question about how the car idles. I have searched the forum archives and haven't found an answer to my question yet. When the car is cold and I come to a stop the car settles into a nice steady idle right away. However once it warms up, whenever I come to a stop the idle will oscillate up and down a few hundred rpm for 10 seconds or more before settling down to a steady RPM. I purchased the car from a dealer and therefore have no maintenance history for the car other than what they did. They did do a minor tuneup and did clean the ISV. Since the car only has an issue once it warms up, I assumed that there might be an issue with the Oxygen sensors and just replaced all four but there has been no change in the warm idle behavior. I plan to clean the ISV myself this weekend but in the meantime I wanted to check and see what the normal warm idle behavior of the car should be. Is it normal for the 993 to have a warm idle that takes several seconds to settle or should it settle to a steady RPM within a second or two like most other cars? Thanks.

GC

 

I'm curious as to any answers you may get from someone knowledgeable on the subject. I have a '97 C2 with 57K that came with almost all records from new, a clean PPI (had a top end rebuild 10K miles ago) and a recent complete 60K service from a top rate Porsche shop. I noticed the same thing Saturday AM for a bit, exactly as you describe. The car seems fine as far as I can tell otherwise and runs very strongly.
Experts?

 

A good and cheap place to start is to clean the ISV (Idle Stabilizer Valve). There is a DIY on it. I will try and post a link here in a few minutes. The ISV should be cleaned at each oil service IMHO.


Here ya go: http://pcarworkshop.com/index.php/993_-_ISV_Cleaning

Check out this website for other helpful information. www.pcarworkshop.com

Mike

 

Mike, he said the shop already cleaned the Isv.

based on my turbo's behavior, no that is not normal.

hot or cold, the rpm doesn't fluctuate more than maybe 50rpm.

 

My '95 has always exhibited a rather lumpy idle, but it's quick to settle down, hot or cold.

I'd clean the ISV - correctly.


Andreas

 

That's what I get for not reading the entire post. Sorry for the shoot from the hip link.

Mike

 

Is the idle steadier with the AC switched on (and fan running)?

I've had several 993s, and some of them (including my current one) seem to idle just a little bit on the low side without the AC on.

 

maybe try another/new DME-relay...

 

sounds like a classic lean-surge. Mine does it when cold, but calms as it heats up -- in fact my daughter complained about it this morning as we were backing out of the garage (Her: "make it stop!" Me: "I can't, but it will go away soon.")

Mine, I assume, is due to a small vacuum leak. Short of hunting them all down, it is something I will live with. But mine is minimal as it only happens during warm-up.

 

Thanks for all the suggestions. I tried using the AC to see if it changed anything but unfortunately it did not. I suspected some problem with the Oxygen sensors since it seems to behave well while the fuel injection system is running in open loop mode and warming up the Oxygen sensors. But since replacing the sensors hasn't made any change I'm thinking it might be a vacuum leak but I'm not quite sure I understand why it doesn't affect the engine until it warms up.
I'm curious about the DME relay suggestion. I was under the impression that the relay only controlled the power supply to the engine management computer but did not really affect how the operation of the computer. Am I wrong about this?

GC

 

I cannot think of how a relay would do anything other than power the DME, so I would agree with your assumption.

In terms of vacuum leaks, I cannot explain why the lean-surge occurs when warm or during warm-up but not other times, but that is not uncommon in my limited experience. I assume it is because the computer is correctly compensating during the phase it works correctly but not being able to otherwise. Why can't the computer do it all the time? Probably -- I'm guessing here -- the leak is temperature dependent (as metals expand, causing or curing the leak).

 

Maybe a bad temperture sensor.

 

Sounds like a small vacuum leak. Using a good scan tool, one can watch short-term fuel trims to see if the ECU is adding more fuel to compensate.

The lean surge is most likely to occur after the engine is warm and the head temp sensor has "told" the ECU to return to the normal fuel mixtures. When the engine is cold, it runs richer to prevent stumbling until warm and that masks vacuum leaks and lean running.

 

Ok. But what is going on with mine when it lean surges (slightly) during warm up?

And it stalls every once in a while when I disengage the clutch at about 1.5-2k.

 

Would the Durametric software allow me to watch the fuel trims? I haven't purchased a scan tool yet so any recommendation would be appreciated.

GC