Idle Hunting -- The problem continues!!
 

My 84 has had an "idle hunting" problem. Based upon "conventional wisdom," I've replaced the idle control valve, the O2 sensor, and the cylinder head temp sensor. Yet, the problem persists. :(

Does anyone have any ideas on what could be wrong? Help would be appreciated.

Scott

Scott,

Sounds like you have replaced the key items I would check.

One additional item you may wish to check is the mixture level. I had some unstable idle issues and engine surging when my mixture was set to rich. I leaned the mixture (with the use of a CO meter) and my idle issues went away. This may not be your problem, but might be worth a shot.

Bruce
'84 Carrera
'77 911 (Euro spec)

With ignition timing and CIS system functioning properly the engine RPM will hunt when the mixture is not set properly. I would suggest adjusting the mixture to spec using one of two methods using an Co2 sensor up the tail pipe or adjust using a dwell meter on the O2 test jack. I used a Dwell meter to adjust my mixture and was able to eliminate the idle hunt and pass the Calfornia smog check at the DMV test station. My car was selected to be tested at the DMV test station and not just the local smog check shop. I believe this was required due to the age of the car and possibility of being a gross polluter.

The car runs great -- very strong, and the gas mileage is good. It also passed the California smog test when I registered it. If the idle is too rich and I adjust it, what will that do to performance and smog issues?

Thanks for the help, guys.

Scott

Scott,

I would think leaning the mixture would reduce emissions, but I don't need smog testing on my car, so I do not speak from experience.

Bruce

Scott,

Had a similar problem when I got my car back in December.

Started it up one morning and it was fine for a few seconds and then it would start hunting.

The hunting would get progressively worse, rpms would go slightly higher and lower with each cycle, and it would eventually die.

Had all of the same stuff checked and it turned out to be the DME had gone bad.

My wrench swapped it and all was fine.

The problem "seemed" to come out of no where but in retrospect the cars driveability was poor before the hunting started. Might have been a sign of trouble but as I'd only had the car 2 weeks I had no prior experience with how it behaved.

You might try to find someone and swap out your DME as a test.

Good Luck

Scott

When you say the DME, are you referring to the DME relay? If so, mine was replaced within the last 3000 miles.

Thanks!

I'd suggest checking for unmetered air leaks. My '84 [rip] had a serious hunt issue. I found that a few of the intake manifold nuts were finger-tight, which later proved to be due to gasket deterioration. I pulled the injection system off the engine and replaced all the gaskets, had the injectors refurbed while I was there, and all was well, afterward.

regards,

mark hald

Scott

Not the relay but the brain itself.

Scott

Drive down to your local shop and have them swap one (DME) from a car that runs great. If yours then runs great-problem solved, buy a new or rebuilt one. Or it could be an air leak issue.

My 84 is driving me nuts :mad:

Wroummm.....rrrrrr.....wroummm......rrrrrr....wrou mmm....rrrrrrrr......wroumm.......rrrrrr etc.etc.

Mechanic tried to fiddle with a contact at the side of the throttle housing where the gas cable attaches - made things worse. :(

If your car has cruise control, you might want to check the cable at the throttle to the left of the idle adjustment screw. If it is adjusted slightly too tight (i.e. no slack at all), then it could cause the throttle to not fully close, which results in a "surge" effect- 600rpm up to 1100rpm down to 600 rpm and so on. The cable end has a black plastic adjustment, similar to a bicycle's handbrake, and can be adjusted by hand- clockwise loosens the cable to provide slack.

Doug: My problem is intermittent. It usually happens at the end of a spirited drive, and usually when I'm above sea level. I'm guessing the mixture advice will cure the problem, but will the cruise control cable cause an intermittent issue?

Thanks for your help.

How's your idle speed? I've read here that lowering it sometimes solves the problem. I've also read that vaccuum leaks can cause it, and a mechanic told me to slowly spray carb cleaner on the hose joints; when the car stalls, then you just found the leak. Dunno if it works.

BTW, I'm having the same intermittent problem. My idle is at 1000 rpm and I think that may be part of the problem.

Are you sure it doesn't do it below sea level sometimes? :D
-Wiseass

Wiseass, er, Chris:

Here in Los Angeles, most of my driving is at sea level or within 1,000 feet of sea level. When I get up in the hills, however, and the car is warm, I have the problem. Based on what I'm hearing, it sounds more and more like a mixture issue.

Scott

I wonder if the barometric pressure thingy beside the DME could be disconnected or not working?
-The Poster Formerly Know As Wiseass :D

Well, if the idle hunts when the air is thinner, such as at altitude, then your car is running richer up there - so perhaps you need to adjust your idle mixture at the air flow meter. This is adjustable via something like a long 3 mm allen wrench up under a plugged hole in the air flow meter. The aluminum plug needs to be removed to access the adjustment screw. Best way to remove it is to drill two small holes in the plug and insert a pair of needle nose pliers into the holes and pull it out. Without complex air/fuel measuring equipment, disconnect your O2 sensor and measure the voltage fluctuation off the sensor with a DVM. Properly calibrated, it should fluctuate evenly between 0.2 to 0.8 volts at running temperature. Turn the screw clockwise to richen, counterclockwise to lean. You basically are adjusting the amount of air bypassing the measuring flap in the air flow meter to set the idle mixture. Attached is a pic of the plug location on the meter.

http://forums.pelicanparts.com/uploads/afmplug.jpg

Here's a typical voltage output of a healthy Bosch sensor:

http://forums.pelicanparts.com/uploa...or voltage.gif

Check the Altitude Correction switch under the driver's seat. Changes the injection pulse width at altitudes > 3300 feet. Without this compensation the car will run richer as Steve mentioned.
Good luck, (and post the findings!)

Steve
Where do you read the voltage from? Will an ordinary digital multi-meter do the trick?

Regards

David