911 SC Idle Trouble

[schmidter91]

Yesterday I was working in the engine compartment of my 1982 Porsche 911SC, fixing the plastic connector in which the oxygen sensor plugs in to as it had cracked over the years. I was afraid this crack was causing the oxygen sensor to make poor contact when the engine was hot, causing the engine to stumble on acceleration. The plastic connector was beyond repair, so a new jumper was fabricated. I started the car and drove it only to discover an idle problem.

The problem seems to worsen as the car warms. If the engine is revving at 2,000rpm and you take your foot off the accelerator completely, it starts hunting up and down for idle before finally stalling (see attached video). If the clutch is pushed in at 800rpms, the car seems to settle at that idle speed, although, starting off it tries to die and search for an rpm range. When I disconnect the oxygen sensor from the plug, the idle does the same thing. The car runs perfectly in every other rpm range, in fact, it seems to run better than before I replaced the cracked plug.

http://www.youtube.com/watch?v=9DHq-O-Jtqo
In the video, I revved the engine up to 2,000rpm and took my foot off the gas pedal completely for the remainder of the video.

What baffles me is that the idled perfectly before I replaced the oxygen sensor connector. Nothing else was changed. Thanks for the help!

Ryan

[longhornchris04]

Mine did the same thing when the mixture was just a bit too rich. If I stayed in gear until ~1000 rpm the idle would hold but if I opened the clutch above 2k the needle would fall to ~500 then oscillate until I touched the throttle.

My CIS is pre-lambda so I can't tell you if thats a contributing factor. I do know that the lambda system is designed to run slightly rich when in open loop - warm-up, >30% throttle, and O2 sensor out of range. Perhaps you don't have the O2 sensor hooked up.

Since I don't have a CO meter, I tested by trial and error. I found a good empty road, got the engine warm, ran down and dropped the clutch from ~3k rpms. If the idle didn't catch, I'd pull over, put another 1/8th turn in and try again. This was after adjusting the idle and mix so the engine didn't hunt when warmed up.

Check all your connections again. Odds are your mix is too rich and the lamba was masking it until you futzed with the O2 sensor and now it isn't sending a signal.

Also, your acceleration stumble could be caused by the over-rich mix. The lamba systems goes open-loop as soon as you go past ~30% throttle.

[T77911S]

the surging or hunting means it is too rich.

check the connector to the WUR and make sure it has 12v on it and it is connected.
otherwise, you may have done something else to make it run rich. did you fix any air leaks or replace any vac lines?

check the fuse for the control relay, 16 or 18, cant remember

[labdag]

was experiencing the same. changed o2sensor, a bit improvement but not much. Might have leaks around Injectors/sleeves.

[labdag]

[Bob Kontak]

Are you absolutely sure the O2 sensor is connected to the new set up? Do you have a dwell meter to test the functioning of the Lambda system? Do you have a multi meter to see if the O2 sensor is putting out voltage?