Warm engine idle fluctuation

[Phil Payne]

> > 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.

I don't know the 90Q, but it's CIS-E - so doesn't it use a separate
ISV controller?

In any event, lots of things can affect idle.  Battery voltage
fluctuation springs to mind - a good old dial-type voltmeter might be
fun to watch.

There's an ISV controller test page on my web site, if it's the same
sort of beast the MB ur-quattro uses.

--
  Phil Payne
  http://www.isham-research.com/quattro
  +44 7785 302 803
  +49 173 6242039