Mongo

Re-reading this, do you have any record of O2 sensor replacement? Since it was unplugged, it couldn't hurt to assume it was bad, and replace it during your troubleshooting.

worf928

IIRC, when the OP unplugged the O2 sensor the car died. Ergo, his working O2 sensor is the only thing keeping it running because it's grossly over-fueling.

Mongo

So we know he has a working O2 sensor, and the car's running does not change with the Temp II plugged, or unplugged. If he can swap the LH out with a friend and the problem continues, we then know the LH is not the issue. What about the MAF? Still no clue on the condition of it.

Good point on the fuel damper and regulator vacuum lines. However, when my regulator failed, I had fuel in the vac lines, but it didn't make the car run like his. Then again, every car is different.

davek9

What do the spark plugs tell you, any a different color, maybe its a leaking injector?
Worth a look, may point you in the right direction

Dave

worf928

Every 928 may have a different combination of contributing factors and modes of failure. I don't know if it's thoughtful design or dumb luck but many common failure modes can constructively interfere to keep a 928 more-or-less running. Countless times I've fixed one obvious problem only to have the 928 run worse due to one or two or three other failures that in combination canceled-out the failure mode of the thing I fixed.

My direction to check if the vacuum lines are sucking fuel comes a) from one such experience described above and b) because the OP's description is consistent with gross over-fueling as a primary failure mode and because he's almost ruled out the temp-II sensor, idle/WOT switch, and the 928 dies when the o2 sensor is unplugged. Thus it suggests that the signal from the o2 is being used by the LH to use the maximum possible trim to lean-out the AFR. If everything else is fine unplugging the o2 sensor should not cause the engine to die. If he unplugged the 02 sensor and the behavior didn't change then, yeah, 02 sensor might be dead. It still might be out-of-spec or half-dead, but I very-much doubt changing it will solve the (apparent) over-fueling problem.

On an S4 when presented with a non-obvious failure mode, the very first thing I do are the Hammer tests (or the 90% of them that can be done with a multi-meter for '87-'88) so that I know what's working and what isn't. I don't know if the OP's done all of them or not. One important test would be ISV activation.

If the ISV is fine and the vac lines are dry then I'd swap the LH because that's the next culprit for an over-fueling failure mode. Vac line suckage is less common than LH failure, but it's easier to check-for if you don't have spare 928s and spare LHs lying around like I do.
If an LH swap isn't convenient and the LH in question hasn't been rebuilt, I'd probably get it rebuilt just because. It's not money wasted since all 2.3 LHs will fail.

Louie928

Your problem may be the LH. Send me your LH and I'll check it for you. If it has failed I have John Speake rebuilt LHs available.
My address is:
Ott's Performance
2485 Dry Creek Rd.
Mosier, OR
97040

Mongo

Definitely take Louis' advice. Send your LH in. And, as worf928 said, rebuild.

They all fail; that is a fact.