911 Rod

86 Carrera hunting for idle.
Used to do it occasionally. Now all of the time.
I just dropped $250 on an ICV and it didn't fix it.
If I let the clutch out to bring the revs down, or shut it off and restart, it's fine.
If I leave the oil cap off it's fine. (ask me how I know this)
I don't feel any kinks in the line from the oil tank to the breather.
This is really frustrating.

nineoneone

unplug the o2 sensor, it makes mine run funny too. My friend and I pondered this repeatedly for sometime, He drives a 87 928, and we both looked at each other knowing that the 02 should not and does not effect idle (wrong) proven fact, can go right now and plug it in and the idle acts funny. don't understand it. good luck with it. try it and see. mine smoothed right out.

911 Rod

Don't you need the o2 sensor?

scottb

My 84 did the exact same thing and, like you, I dropped a chunk of change on a new ICV. I also replaced the O2 sensor, but that didn't help either.

The problem was the mixture, and once my mehcanic adjusted the mixture, the hunting stopped.

Problem solved.

Scott

911 Rod

I know the mixture adjustment you are refiring to.
In 86 they changed to a different type of fuel injection that does not have the mixture adjustment ****.

Peter Zimmermann

Rod: Incorrect! The idle mixture can be set the same way, by using an allen wrench in the air flow meter. However, a specific set of instructions must be followed to do the idle speed/CO adjustment. Hunting is usually indicative of a rich mixture, most often caused by an incorrect setting or a defective AFM. I know a shop who specializes in 911s - you wouldn't believe the stack of old air flow meters that they've collected!
Pete

911 Rod

Thanks Peter. So this isn't something I can do?
The part that puzzles me is that it doesn't do it when the engine is cold, when the cap is off, if I shut it off and restart it, or if I let the clutch slip a little bit to bring the idle down. I'm thinking it has to be mechanical. no?

Peter Zimmermann

Rod: I think that if the hunting was mechanical the idle would just speed up until it was comfortable and stay there, much like squeezing the gas pedal lightly in order to bring the revs up. Hunting has always been a symptom of rich running, and that's been consistent with MFI, CIS, Motronic, even carbs. I mention the AFM because it's a good place to start, not necessarily fix, your car's problem. I've seen a large number of AFMs that could not be adjusted any leaner than 2.0 - 2.5 % CO, and, of course, that won't cut it (the spec for USA, Canada and Japan is 0.6 - 1.0 % CO). You need to get hooked up to a CO machine and test the car before you spend more money!
Pete

911 Rod

Thanks again Peter.
it could be difficult finding someone that specializes up here in the great white north. Might have to go to the stealership.
I figure the amount of money I have saved over the years doing everything myself, this little trial and error is not to hard to take.
My understanding is that the ICV usually is the problem.
Any idea why I can make it stop by doing the things I mentioned?
Thanks
Rod

Peter Zimmermann

No idea - for me bandaids always clouded the diagnosis process, the less that I get confused, the better off I am! Let us know what the test shows...
Pete

JCP911S

OK... just a suggestion, but my 84 used to hunt at idle...then I replaced the Cat with a bypass tube, and it doesn't do that anymore... maybe your Cat is all gooped up, or maybe it is just a coincidence???

911 Rod

I have a bypass pipe in now.
My Bentley is stating the O2 sensor controls richness at idle.
My next move is to unplug it and see.
Peter.
I know what you are saying about take it in.
If I was close to you, I would bring it to your shop.
But here, I don't have trust in any shops that do EFI. Carbs yes.
I really want to do as much trouble shooting as possible and at least eliminate the obvious.
Replacing the ICV did make a difference. It does not do it all of time now, the revs don't go as high, and it stops sometimes after 15 sec.
I'm going to check the electrical connection to the ICV as well as the hoses. Again.
Thanks
Rod

rnln

peter is right again, I think.
I do have the idle hunting once in a while and I also do have back fire once in a while.
I used to have a carbs car and everytime I adjust it too rich, I got pop and also hunting.

Peter,
Would smog shop be able to test my CO?

911 Rod

I get the backfiring under de-acceleration all of the time too.

Peter Zimmermann

rnln: A smog shop can test your CO, unfortunately what they test for is different than a tuneup setting. When you tune the car the CO is set pre-catalyst, with certain items disabled. When the smog shop tests the car it's done at the tailpipe with all systems functional - two different things. The theory is that when the car is tuned using Porsche's required specs, it will: (1) run at its optimum, and (2) pass a smog test. You can't use a smog shop's info to confirm that the car is tuned properly because on board pollution control systems can mask/compensate for a variety of issues (incorrect CO, a few too many HC, etc.).
Pete