The continuing Idle weirdness saga...

[Rand Soper]

Well, the idle weirdness in my '88 80q has gotten weirder.  Last week I
replaced the radiator (all copper $225.00 from Discount Import Parts in
Milwaukie, Or)  and replaced both the coolant temp. sender (for the dash
gague) and the coolant temp. sensor (for the ECU).  Both were original to
the car, as are all the other sensors.  My temp gauge now reads correctly
and the idle was better for two days.

When I first bought the car it idled fine but within a day the idle switch
went dead.  I replaced it and all has been well since except that it would
idle,rock steady, at 1000 rpm.  Now two days after replacing the radiator
and the temp sensor it will idle anywhere between 800 and 1800 rpm.  It does
not seem to have any pattern.  When it is fully cold it will idle at about
850 rpm steady as a rock.  As it warms up it starts to idle all over the
place.  Sometimes it will idle very steady but high and at other times it
will start out at 1000 rpm and slowly climb to 1600 or 1800.  And
occasionally it will come back down.

The car starts and runs perfectly otherwise.  It pulls strong all the way to
redline.  I am getting 21'ish mpg in  town and 26'ish on the highway.  I
have cleaned the ISV and it seems to work.  If I put 12v to it it opens all
the way.  If I check the voltage at the ISV at the connector with it
disconnected from the ISV and the engine running it reads 13.8v.  Am I
checking the voltage wrong?  I have read recently that the voltage should be
in the 4v range.   I have checked the two grounds on the intake manifold and
they are clean and tight.  I even cleaned the throttle body tonight.  My
wife's '93 dorF van would get a cruddy buildup on the throttle body that
kept the butterflies from closing correctly and I thought it couldn't hurt.
It didn't help much.

I would suspect a vacuum leak in the ISV vacuum hoses except that it doesn't
idle weird when it is cold.  I am also starting to suspect the O2 sensor.
Once again my wife's old dorF van would run weird when the O2 sensor would
go bad even though it didn't set any codes.  There are no codes on the Audi.

Should I just replace the ISV and the O2 sensor and see what happens?  Are
there any other sensors that may affect idle?

Any help would be greatly apreciated, and the help I have gotten so far on
other things has been invaluable (especially Huw's web site)

Thanks again,

Rand Soper
'88 80q - People are surprised to find out it is almost 16 years old.