Warm engine idle fluctuation

[Huw Powell]

> My 1988 90Q idle is about 1200 RPM when fully warmed up. It then starts to
> fluctuate between 1220 and 750 every two seconds. It only remains at 750 for
> an instant. I've changed the idle stabilization valve and squirted WD40
> around vacuum lines with no success. I have been told this could be the O2
> sensor or air control potentiometer.

whatever it is has to be capable of generating a cyclical problem,
rather than a fixed one.

could very well be the OXS.
unlikely to be the potentiometer, since it will pretty much be sitting
still at idle.
could be a flaky idle switch, at least that is easy to test.

as you point out, bypassing the ISV settles it down (when warm) so it is
involving the ECU at some point.

try it with the OXS unplugged, and also with the idle/WOT switches
unplugged and report back...

--
Huw Powell

http://www.humanspeakers.com/audi/

http://www.humanthoughts.org/