High Idle and O2 Sensor in 911SC
 

While doing the rear shocks in my buddy's '83 SC today, I noticed the O2 sensor was disconnected. The car was running great this way. When buttoning the car back up, we reconnected the O2 sensor and then took the car for a test drive to test out the ride with the nice new Bilsteins. Unfortunately the car had a high idle of around 1900 rpm and wouldn't go lower. We unplugged the O2 sensor and the idle fell back to normal.

Any ideas ? My '83 SC has the O2 sensor connected and it runs great. Both our cars have the deceleration valve disconnected and plugged.

My buddy just had his car "adjusted and smogged" by a reputable Porsche mechanic so maybe they disconnected it. But it should be able to idle normally with the O2 sensor connected shouldn't it ?

Anyone else with this situation ?

I don't have an O2 sensor so I don't know how difficult this is to do, but can you try your known good sensor in his car and see what happens? Or just replace his w/ a new one. There are threads on this using, I think, a generic or Ford one that needs to have the wire ends changed.

I'm surprised the car can pass smog in CA w/o an O2 sensor.

I'll bet your friend's got a large vacuum leak and the O2 sensor is richening the mixture to correct for it.

Regards,

Jerry Kroeger

Which year is this? Did I miss that? Anyway, if the mixture is correct and plug the O2 sensor back in that idle will not change, its kinda how I gauge if my CO tester is working. So like the above post stated, pulling it, and adjusting the mixture is just compensating for some other issue.

Next interesting tidbit, if this is an 80' and above your buddy is actually losing power as a result of this. On a dyno with the O2 sensor disconnected the AFR's go crazy in the later cars.

Thanks guys - yes, I know something's not right. The O2 sensor is supposed to be plugged in. I also suspect a vacuum leak. He's going to call his mechanic today and see why they left it unplugged and what's going on. It's not clear if the O2 sensor was plugged in or not when the smog test was run, all we know is that the car was returned with it disconnected and it will only idle at normal idle speed with it disconnected and at 1900 rpm with it connected. BTW, it's an '83.

I also have an '83 just like his, and my O2 sensor is new and plugged in, idles fine and passed smog easily like it is...

On my car, the three pronged aluminum tube on the back side of the airbox chaffed against the fiberglass cover for the on engine oil cooler. I had the same symptoms untill I dropped the motor and replaced the aluminum tube and some cracked vacuum lines to the distributor.

Regards,

Jerry Kroeger