Thanks to this board and some advice from people like Wil Ferch, I diagnosed that I may have a bad O2 sensor. The car ran fine, but had a bit of a surging idle when on even throttle between 2500 - 3200 RPMs. It also "hunted" around a bit for an idle point on cold startup. So I unplugged the O2 sensor to see if this made a difference. It did - the car ran smoother. The other interesting thing is that the gas mileage dropped quite a bit with the O2 sensor disconnected - I assume this is because the car runs rich without any feedback from the sensor.

In any event, I ordered a new O2 sensor and installed it last night. The difference in how the cars runs is pretty dramatic. It is smooth at even throttle in the range where it was a bit rough before and idles better on cold startup. Hopefully my gas mileage will improve as well. This is a really easy DIY project, so if any of you have these symptoms give it a shot. By the way, O2 sensors are supposed to be replace every 60K. My car is nearing 110K and I am pretty sure that the O2 sensor had never been replaced.

 

Bill do you still get a little variation in your idle speed. My O2 sensor has been replaced recently too.
And I get a little increase in rpm at idle maybe 50 rpm between 900 and 950.I was told this is the O2 sensor working.

 

Hi Bob - yes, the idle RPMS do move around a bit. It is much smoother than before though. The real difference is at cruising RPMs of say 2500 - 3200.

 

Bill, take the next step. Disconnect the O2 sensor, reset your mixture to 3.5% CO and enjoy. Your car will have better acceleration and won't "hunt" at low-speed part throttle.

 

Paul - are you saying to take the O2 sensor permanently out of the loop? I just bought this new sensor??

 

No bill he means disconnect it when you are tuning the car. After the CO has been properly set reinstall 02 sensor.

ken

 

[quote]Originally posted by ken louie:
<strong>No bill he means disconnect it when you are tuning the car. After the CO has been properly set reinstall 02 sensor.

ken</strong><hr></blockquote>

No, I mean forget the O2 sensor and get better performance (milage will suffer a little, though).
Hopefully you bought the $30 Bosch universal sensor instead of the $130 Porsche rip-off.
The way the lambda system works (on K-Jetronic)is the O2 sensor generates a variable voltage depending on how much O2 is in the exhaust. This signal is conditioned by the lambda computer under the passenger's seat and that, in turn, determines the duty cycle of the frequency valve. The frequency valve varies the fuel pressure in the fuel distributor so the engine is always leaned to 14.7/1, known as the stoichiometric ratio, which lets the cat clean up the exhaust and gives good fuel economy. Lousy for performance, though.
When you disconnect the O2 sensor the system runs open loop and allows you to reset the mixture to something richer with all the benefits I previously mentioned.

 

I agree with Paul about the 3.0- to 3.5% CO settings and disconnecting the O2 sensor, however, if you still have the Cat on the car, be aware that it will clog faster with this mixture. It is best to get a bypass for this setup, also, your car will run cooler without the cat.

Noel
80 911SC

 

[quote]Originally posted by Noel:
<strong>if you still have the Cat on the car, be aware that it will clog faster with this mixture. It is best to get a bypass for this setup, also, your car will run cooler without the cat.

Noel
80 911SC</strong><hr></blockquote>

Absolutely, Noel. The rich mixture will poison the cat. A test pipe works best. Don't believe the bs about pre-mufflers unless you want the car to be real quiet. Going to the test pipe makes the 3.0 engine sound like a Turbo at idle.