Reading 02 sensor right?!
 

This car has been running poorly ever since I bought it. It's an 87 "S." Vacuum level is low I think, at 14 in Hg, but I've done the seals, lines, gasket, etc. As I understand it, that should make the car run rich. But the car runs kind of hot, the plugs have barely any ash on them, so I'm thinking it's running lean for some reason (the plug on the AFM adjustment screw has been pulled, so someone has been ... er ... screwing with it before me). It runs like crap with the 02 sensor connected (which I replaced last year), but better with it disconnected.

When I attached the digital voltmeter to the prong on the 02 sensor lead, unplugged, and I set the meter to the 2000 mv scale, it read 3. I was thinking the mixture was WAY too lean, so I started richening the mixture. I got it up to around 140 mv, but that's as far as I can richen it (screw stops turning). Shouldn't I be getting a reading of 500 to 600 for a correct mixture on the 2000 mv scale? Or am I confused? I've had the AFM off the car to test it and re-track it, and it looked like it was okay. Am I doing this all wrong or what?!

The 'S' has additional sensors to keep it from knocking. There is a hall effect sensor on the cam gear and two knock sensors (specialized microphones -- yes your engine has ears) on the head. Should this circuit fail the timing is backed off about 6 degress. Shouldn't lead to a lean conndition.

The bypass screw on the AFS (Porsche calls it an Air Flow Sensor) only affects the idle mixture setting. Screw it in all the way, back it out till the engine runs a bit rough then screw it in a quarter tun.

Lean conditions are from temp sensors too. Check the engine temp sensor as well as the one in the AFS.