Audi 5000 idle problems

[Tom Christiansen]

Folks,

I seem to have a little trouble getting my 1987 Audi 5000 Quattro to run at
a stable idle while in closed loop. The idle is fine (stable 750 rpm)
during engine warm-up, but when the engine is warm, the idle creeps up to
1400-1600 rpm and oscillates between the two values. If I shut off the
engine and re-start it (should sound familiar to most MS Windows users :-))
it runs great for about a minute, then the idle starts oscillating between
1400 and 1600 rpm.

It all started when the car failed the emission test on Friday. It was
emitting too high HC in idle. So I cleaned the idle stabilizer valve and
the differential pressure actuator. The car seemed to run a bit better
after that treatment. Then, I wanted to check the mixture by measuring the
voltage on the O2 sensor. So I put a voltmeter on it. The voltage jumped
around between 0.1V and 0.7V. I figured, this is rather useless, and with
the engine running, I disconnected the O2 sensor from the green wire
leading to the computer. 15-20 seconds later, the idle crept up to the
1400-1600 rpm.

I've connected everything now. The O2 sensor still gives a signal jumping
between about 0.1 and 0.7V (actually I think the voltage is negative
compared to ground - yes I did check the polarity on the meter) and it does
respond with a sudden drop in voltage to a boost of starting fluid in the
air intake. The ISV operates when 12V is applied to it. The current to the
diff pressure actuator is around 10mA, fluctuating with the rpm. The vacuum
hoses look good and I hear no leaks.

What's going wrong here? Did I fry the computer or the O2 sensor by
connecting/disconnecting them with the engine running?


Thanks,

	Tom