user 783240

Hello Gentlemen, I just picked up a 1988 911 3.2 and I noticed a strange problem. At times I get the idle surging or hunting when cold and I also have a higher idle than normal at about 1100 rpm. I read through as many posts about ICV and idle screws but none seem to help. I also checked the service manual and it's making me believe that the ECM or DME? is the culprit.

Things I've done to troubleshoot:

Checked idle switch for click sound and also continuity. I think it is working, but the continuity is very very weak (.2k ohms) when closed, but when I open the throttle there is definitely an open circuit in the switch.

Pulled wiring harness out of idle switch and according to the manual I'm supposed to see the rpm jump 500, but it did not make a difference if it was plugged in or not.

I also made a jumper wire to the female side going to the ECM and that didn't make a difference either. Am I right in thinking it's the ECM unit or DME? Not sure what I should call it. Thanks for any input.

EDIT: Forgot the mention the car has an aftermarket exhaust and just over 100k

dshepp806

How about rechecking the mix and resetting the base idle?

I'd also ensure you've fresh (perfectly functioning) parts in the ignition path(s)....too many things to check WAY before hopping on the DME route (what's a DCM?)

Idle aside, how is she running?

Doyle

user 783240

Sorry, I meant DME in the first post NOT DCM

The car seems to be running great. I have most of the maintenance records and most of the usual wear items have been replaced, including a top end rebuild and clutch.

As far as the mix, is that same thing as the idle screw on the front of the throttle body area? I tried turning it clockwise to get the idle down according to one post I saw, but it doesn't seem to be responding in either direction.

Reiver

On the 3.0 motor, and I suspect they are similar in this, if the CO is off/rich it will hunt at idle when cold or not fully warm. It 'goes away' when at normal temp.
A bad 02 sensor will also make the idle hunt up/down.

user 783240

I just recently got an emissions test and it showed that the CO% was a very low reading at 0.01 at 15 mph and a 0.01 at 25 mph. Does this sound normal? Which way should the CO screw be turned to adjust for rich and lean? Thanks.

Reiver

I'll let another 3.2 guy answer on your mixture as I'm not familiar with your motor.
You are obviously running a stock cat exhaust system.
If you want/need your CO tuned you should do that at a shop while the CO is being read by a proper exhaust analyzing unit. If you are a reg customer they usually do it for free as it just takes a second or two and they'll also be able to tell if your 02 sensor is bad (they adjust the 02 with the sensor unplugged and if it is bad the sensor will make the analyzer gauge jump wildly when pluuged back in) like your idle is hunting now.

user 783240

Thanks, I'll definitely look into that. After searching for possible solutions, I found a thread on pelican where a guy had very similar symptoms and high idle and tried all the normal troubleshooting. Ended up being a fried transistor. I'll report back and see what I can find out.

Shannon123

I may be off base, but there was another link recently where Ed suggested that checking grounds (battery, engine) as they could potentially cause surging / hunting.

rusnak

the idle screw only gets you in the ballpark. In reality, the ICV really controls idle, and it is controlled by the DME. If the ICV is malfunctioning, then the car won't idle correctly.

You can disable the ICV by jumpering the DME socket, which makes the vanes in the ICV stay half open for baseline. A hunting idle sounds like a possible vacuum leak. It's possible that someone set baseline CO super high to "correct" the mixture for the vacuum leak, which would make things worse.

user 783240

Let's assume I test the ICV and get the expected resistance between the ohms and I can see the ICV move with a 9v battery. If I jumper the DME and nothing changes idle does that mean the DME isn't sending the correct signals to the ICV? In other words, if the ICV is functioning properly manually, does that mean the DME is the culprit?

theiceman

sounds to me like you are way down the garden path and have your mind made up it is the dme. Do yourself a huge favour and step back.

I f your ISV were malfunctioning It would hunt when warm to as the vanes are constantly in motion to maintain good idle. Just take a can of brake cleaner or wd-40 and spray it in your ICV with it removed from the engine until it runs out clean. You will be amazed how gunked up they get.
have your base CO checked as that will mess up not only your idle but will cause hunting if it is too rich on start up. Check all your hoses for a vacuum leak as suggested.

rusnak

Another possibility is that the vacuum line to the fuel pressure regulator is faulty, or has fallen off. Or the fuel pressure regulator diaphram is leaking or busted.

A good idea to do a visual/ hand check of the vacuum line to the fp regulator, and a fuel pressure test, which after checking the grounds, is de rigueur for diagnosis.

user 783240

I made some interesting observations. I took the ICV out and cleaned it. Checked the resistance and it checked out with 40 ohms between the outer terminals and 20 ohms between the center and either terminal. The harness was also showing 12v coming from the center pin.

Here comes the interesting part. I put the ICV back in and shorted B and C from the DME socket. I was able to adjust the idle screw clockwise and noticed the rpm lowering. I stopped when the rpm was about 850, but when I took the jumper out of the DME socket, the idle went back up to about 1100.

I guess the next thing is to check the vacuum line to the fuel pressure regulator and get a shop to adjust my CO since it was reading really low during emissions inspection.

Any further suggestions given the new data? Thanks guys.

rusnak

yeah, have them check the fuel pressure. This will tell you if the fuel pressure is too high, which it could be. There might be an obstruction in the return line, for example. When jumper the socket, can you see if the ICV vanes are centered? In other words, connect it back up with the ICV hoses not connected, and look down into the body?

user 783240

I'll have to check if the ICV is centered when I jumper the socket. What would that tell me? Could it be the ICV isn't properly centered or malfunctioned and that's what's causing too much air to come in?