Rick Veague

My '87 951 (recent acquisition) has been running very lean. Rough idle, hesitates on acceleration, exhaust way to hot. I had a local Porsche shop scope the exhaust, turns out it had been leaned way out. They adjusted the mixture - put it right on the money with the O2 sensor disconnected. Ran a lot better, but still not right. The readings with the O2 sensor in seemed too low, so I went ahead and replaced it (used the Bosch part # and spliced the old pigtail on). No real difference.
With the (new) O2 sensor disconnected, the car runs GREAT! Connected, it hesitates on acceleration, and seems to be running way to lean. If I check the manuals, it appears the O2 inputs go right to the DME.

Any guesses what's going on here? Bad DME (I hope not!) Something else making it run too lean (that disconnecting the O2 sensor compensates for?) The fact that someone leaned it way out really worries me.

TIA

txhedg

why didn't they set the mixture with the O2 connected? I guess you could just run it with the 2 permanently unhooked...except for when you need to pass the emissions test

Anyone know what the downside of just leaving the O2 unhooked is?? Its done on track cars frequently with no apparent problems.

fletch.

How do you "adjust the mixture" on a 944T?

Rick Veague

They set an adjustment on the air flow meter (said it was way off). According to the emissions sticker, the CO setting is measured with the O2 sensor disconnected, which is what they did. They then reconnected it. I didn't know it was adjustable either. Clearly the mixture is controlled by the DME, so the mixture is set indirectly by controlling the inputs.
I'm sure by disconnecting the O2 sensor I'm "fooling" the DME to run richer, most likely masking another problem (assuming the O2 sensor I put in was good).
The only thing I have not measured yet is fuel pressure; don't have the tools to do that (yet).

rv