jrp3000

Hi,
I am having intermittent idle issues on my 1999 996 3.4 with 88,xxx miles. Sometimes the car runs fine and normal, but sometimes with the AC on it idles steadily at anywhere 1200-1600 rpms and with the AC off surges between 500 and 800 rpms. Any ideas what can cause this? I plan on disconnecting the battery for a while to reset the ecu, and removing and cleaning the MAF sensor tonight, so that might possibly fix it. Any input is appreciated.

Ahsai

If it's cabled throttle, it's good to clean the idle control valve as well. Need to remove it and power it with a 9v battery and spray with carb cleaner.

ChicagoSpeed996

Yep, usually the IAC is all carboned up. Also clean around the throttle plate.

white out

Clean the MAF while you're in there.

jrp3000

Two weeks ago I ordered a new idle control valve, installed it, cleaned the throttle body, and the MAF sensor. The car ran like brand new for two weeks. This morning it started acting up and is back to its old ways. Any ideas?

Macster

That idle behavior from my Boxster proved to be an AOS.

You can try the remove the oil filler tube cap "test" with the engine idling.

However, the AOS, if it is the AOS, may not be far enough gone to produce excessive vacuum though, so if the cap comes off with no real struggle that does not let the AOS off the hook. Once the cap removed the engine will react. That is normal, as the removal of the cap creates a big intake air leak. Put the cap back on and the engine should recover in practically no time.

When investigating the cause of the symptoms I found the area right behind the throttle body very oily, the area where the hose from the AOS connected to the intake manifold. Also, the throttle body was oily, especially the butterfly valve. It was wet with oil. Well a drop was hanging at the bottom of it.

The significance of the presence of oil didn't dawn on me until later. I cleaned the throttle body and the engine perked up some but the improvement was short lived.

jrp3000

I tried unplugging the MAF and driving the car. It seems to idle a lot better, could this be indicative that the MAF needs to be replaced?

DBJoe996

Did you clean it out with MAF cleaner? And did you clean up the Idle Control Valve and Throttle Body? Before buying a new MAF sensor, clean it. Did you reset the ECU by disconnecting the battery because it was throwing a CEL? Would help to know if you are getting any codes. Just so you know, the MAF sensor does not have much to do with idle and low RPM's. If I remember correctly it starts to come into play at about 1400 rpm. However a fouled one might be sending the DME incorrect information.

Other sources of idle surge have been mentioned. Try removing the oil filler cap while the engine is running as stated and see what happens. Vacuum leaks are a common factor with idle and surging. Places to check, oil filler tube cracks, AOS, vacuum hoses and vacuum canister, intake clamps on TB and plenums for starters.

Ahsai

Is the new idle control valve a genuine Porsche part?

DBJoe996

Now I see. Like Ahsai asked...is the new ICV original Porsche? After that, leaning towards a vacuum leak. Any CEL codes? And another thing to add: I cannot remember my exact symptoms (sorry), cleaned MAF repeatedly, then figured WTH and bought a new Porsche MAF. That fixed it.

jrp3000

The part is Bosch replacement. No CEL until I unplugged the MAF. I will plug my code scanner in and check codes when I get home.

jrp3000

cleared the codes p0102 and p0112 from the MAF being unplugged. Car ran fine all weekend, then Monday morning started acting up again. Im going to rule out AOS temporarily because 1. the car does not die when you remove the oil cap, however the revs do drop, 2. if you put your hand over the oil filler tube with the engine running the suction feels the same as on my buddys 911 that is running well. I ordered a Bosch MAF sensor off amazon last night and it should be here tomorrow.

jrp3000

MAF sensor did not fix the problem. Boxed it back up and sent it back to amazon. Best way to search for vacuum leaks? Common places to look?

5CHN3LL

Time to start taking a close look at all of those vacuum lines. Maybe a smoke test if you have access to a smoke tester (or a cigar - see YouTube for ghetto smoke test instructions)...

sjg1138

Honestly, I found it easier to just replace all the vacuum lines rather than trying to find the leak.
Here are a few good threads:
http://www.renntech.org/forums/topic...-vacuum-lines/
https://rennlist.com/forums/996-foru...m-diagram.html
http://www.renntech.org/forums/topic...-out-of-ideas/
http://www.bimmerfest.com/forums/sho....php?p=4519113