Idle Problems
 

Having some problems with my engine “stumbling” when it returns to idle ('86 3.2 Carrera, 60K miles).
Here’s what happens: Say the car is in neutral and idling. I rev the engine to 3000 rpms and then release the throttle. The rpms start heading down to idle, but just before it reaches idle, the rmps go to 0 and then quickly recover to idle speed. It feels like the ignition switch has been quickly turned off and on. The car never actually stalls, but it really feels like it wants to. Anyone run into this before?
I read in the Bentley manual that the EFI actually cuts off fuel when the car in decelerating, but I’m not sure this is related.
Side note: It also seems like my idle speed is a little too low. The book says 800rpm and I’m at about 500-600rpms.

I have the same thing happen but I think mine's more related to when I'm running my AC and the compressor's causing the pull... it acts the same way though... drops straight to 0 and 'barely' recovers. My only solution thus far is to adjust the idle speed to compensate.

Check your idle air controller (IAC). Also check your that your idle position switch is showing connectivity when the throttle is closed.

There are threads ad nauseum to search on those topics within this bulletin board.

Steve Wong's 911chips.com site probably has a troubleshooting procedure. I believe he does have a procedure for setting idle speed.

Good luck,

Doug

Search on ICV (Idle Control Valve), rough idle, etc. Lot's of info in the archives...

Thanks, I wasn't quite sure what to search for. I'll go through the procedure in the Bentley book to set the idle switch position and check the ICV.

When you search, look for posts on cleaning your ICV, and making sure the "plunger" is moving freely. Another thing to look for is a vacuum leak. Good luck, you'll get there!

Ok, so I take the ICV out and check that it is vibrating when the ignition switch is in the ON position (it does), I clean the valve to make sure it is moving when shaken (it is), and check the resistance between the inner and outer terminals (20 and 40 ohms per the spec). I’m guessing the ICV is ok. I then took a look at the idle switch and full throttle switch. Both switches work exactly as they should with continuity at the appropriate times.
So the idle switch and ICV are working fine by all accounts, but the idle is still around 500rpms. Anyone have any suggestions where to look next. O2 sensor maybe? Vacuum leak?

Most vacuum leaks cause rough idle all the time and not just when the rpms are coming back down. O2 sensor may be the problem. Easy to check by setting the idle with the ICV jumpered off and then disconnecting the O2 to see if that fixes it. Make sure that your idle is adjusted properly by jumping the ICV.

The idle is low but fairly steady, so I'm inclined to think that it is not a vacuum leak, but I'll shoot some starter fluid around and make sure.

I'm not sure what you are saying here (I'm pretty dense at times - like now - so forgive me). How do you jumper off the ICV and set the idle? If I could set the idle without the ICV, why would I need an ICV in the first place? Is the ICV only used to set the idle mixture (not the idle speed) by reading the O2 sensor and modulating the air flow through the bypass?

The idle mixture might be too rich.

Asked out of ignorance, not expertice - doesn't the O2 sensor look after that?

Some other ideas...

http://forums.pelicanparts.com/porsche-911-technical-forum/351177-3-2-idle-stalls-throttle-lift.html

I had the exact same symptoms on my 1987 (idle dropping & stalling when I went to nuetral at a stop light). It turned out to be a bad ignition coil. Part replaced & the idle has not acted up since.

Did cleaning the ICV get rid of the near-stalling problem when you blip the throttle? Now just low rpm @ idle?

That's very interesting. I would have never considered the coil. The O2 sensor was replaced not too long ago, so I'm thinking it's Ok. The coil is so cheap it doesn't make sense to not replace it. Off to the part store.

After taking the ICV off, checking all the switches (idle, full throttle) and resetting the throttle plate stop, I really did not see any difference. I DO however have a new appreciation for how important it is to have a magnetic pickup tool. The top of the engine is a black sucking hole for dropped parts...

I would not rule this problem out, but it is very very unlikely to be the cause.

Hi. It's been a couple days since anyone has responded here. I was just curious... Was it the coil and/or did replacing the coil help anything at all?

The 02 sensor has its limitations if the mixture is way rich which is the reason why the mixture specs are very specific.

stlrj - thanks for the answer.

I still have $20 that says the coil ain't the problem....

Idle switch procedure is here:

http://forums.pelicanparts.com/porsche-911-technical-forum/280886-idle-adjust.html