Idle hunting... why? How to fix?
 

Got '85 3.2 that's starting to hunt at idle. Pulses between 1,100 rpm and 1,200 a few times... then settles down to 1,000. Interested in what causes this. More interested in how to fix whatever's wrong.

EDITS: Only does when engine warmed. O2 sensor replaced about 18 months ago.

On my 81 SC it was a combination of a bad O2 sensor, and fuel mixture. Also, the hunting at idle was continuous after the engine warmed up.

TY Curl. Replaced 02 sense roughly 18 months ago... no guarantee but am thinking that's OK. Can you tell me how you adjusted mixture?

almost certainly the idle air blending valve. Pull it, clean it, clean the contacts on it. Mine was hunting in the same way and the above cured it - not instantly - but over the course of a few months of driving it got better and better to the point where I rarely see it any more.

I should back up and say I was so sure of my diagnosis in my case because It was a on a freshly rebuilt motor with all new o2, sensors etc etc. However if the symptoms exist when the motor is cold you can rule out the 02 as it is being ignored by the DME.

A rich idle mixture will do this. It gets rich, revs up then the DME pulls the idle back down.
After the car warms up and uses the O2 sensor circuit it goes away. This symptom typically shows up when the temps drop down in the fall.

There is quite a bit of documentation on this topic on the board.

Take the ICV off and see if the valve moves when you shake it in your hand. If not, spray some carb cleaner in there to clean it up so it moves freely again. Reinstall and fire up the car. Mine was doing something similar so I did this and the hunting is gone.

TY on air blending valve RMM. Never does when cold... only after warmed up.

Makes sense... rich and DME. My hunting is after engine is warmed up. TY Scott.

LOL... really XNO, just shake it?! Is there any "carefully" with that or just load in with martini and shake?

To keep track of input, am listing suggested causes, recommended fixes, and relative links. Will move this forward as it's updated.

O2 sensor - replace (mine's not that old so will leave it be for time being.)
Air blending valve - remove and clean (if hunting exists when engine is cold... 02 sensor can be ruled out) - I searched for "Air Blending Valve" and did not find fuel related info
Idle mixture too rich - lean out idle mixture - Pelican Technical Article: The Motronic Engine Management System - 911 (1984-89) -
ICV / Idle Control Valve - remove and test / clean - http://forums.pelicanparts.com/porsche-944-turbo-turbo-s/330312-idle-valve.html

When I had a the idle hunting problem it was a vacuum leak. One of the rubber hoses from the oil tank was cracked.

It is astonishing how dead steady the idle is on my car after the total rebuild. Most of that I attribute to the good seal of all the intake and fresh rubber on every connection. two of the intake bolts were just finger tight. How they got loose is a total mystery. I don't think that intake had ever been touched before.

The ICV is a good place to start. I tried to clean mine in the past and had no luck on a long term fix. It is easy to replace so I put a new one on and that fixed it.

I have had my 911 for 19 years and driven it myself for 90,000 miles so a lot of problems have popped up more than once and be fixed. They kinda run together in my brain.

Hey Glen... exactly what "intake bolts" are you referring to?

The ones the hold the intake runners down. There are two on each cylinder sealed with a individual gasket.

LOL. Well, I guess not THAT "recklessly" :) As far as I can ascertain, the ICV has a coil of some kind on the left end that the connector goes into. The valve is coaxial to the coil. So when the system opens and closes the valve by applying voltage to the coil, the valve is actually rotating. If you remove the ICV and look at the air intake part you'll see what I mean.

To be more specific, if you rotate the ICV in your hand like it's a screwdriver and you're quickly turning it, you should see the valve rotate in there. Mine seemed pretty sticky, and there was a good amount of black "soot" in there. So I sprayed some carb cleaner to clean it out. I could then rotate it, and the valve was way more free and "floppy" in there.

So in my mind, the DME was trying to move the ICV during idle, but the ICV was slow to react due to the valve being sticky, which was responsible for my hunting.

The nice thing about checking the ICV is it's free :)

My mechanic adjusted the mixture, so I am not really sure what they did. I did have to recently take to a mechanic to adjust the mixture again, as it was stalling and bucking between 2000-3000 RPM. I watched him - he took off the airbox cover, and used a ratchet at something near the fuel distributor. He also adjusted the idle screw and it ran great. Took him 5 minutes and it passed smog too!

Is there a way to do this without a CO meter?

Use the o2 sensor perhaps?
paging scarceller?

I would bet the Idle Speed Valve, or ISV. Others referred to it as the blending valve above. On a cold engine, the DME tells it to open, adds more air to the engine. as the car warms up, the idle starts to raise, the DME tells the ISV to close some. If it doesn't, the idle gets higher, car gets warmer, until it hits the DME limit of about 1200 rpm. DME cuts the signal, maybe to the injectors, I am not sure. The idle drops, signal is restored, idle increases cause the ISV is still stuck open, repeat.

Could be other things, like a vacuum / intake leak. What is happening is the idle gets too high because of the extra air, and the ISV is closed all the way, so it can't help. Pulsing starts.

Since you said it will eventually stop pulsing, and a vacuum leak would stay leaking, I am pretty sure it is a sticky ISV.

Idle hunting... why? How to fix?
 

Sorry plz ignore


81' 911sc widebody

This is nice but OP has a 3.2 Carrera with an entirely different air/fuel induction system. Sorry, but you guys are muddying up the waters here.

85 3.2 with the factory chip should idle around 800 rpm. If you are idling at 1k and haven't made any adjustments I would put my money on a vacuum leak. Could be as simple as a cracked hose or loose intake runner. Hopefully you don't have to replace intake runner gaskets.

Lots of info if you search 'idle bounce'.

Agree, mostly. I replaced my O2 sensor and that made it better but didn't cure it. I cleaned the idle valve and that almost completely cured it, but it still happens occasionally when the car is hot. Going off the throttle quickly will often cause the symptom where coming off the throttle gently will usually make it settle down.