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oil filler-cap effect on idle--how much?

In message <v01530501b034b79cf1d7@[]> pjrose@servtech.com (Phil/Judy Rose) writes:

> I know the importance of minimizing any intake manifold leakage and thus
> had  replaced the dipstick's O-ring when I noticed that dipstick removal
> had no effect on idle. Now the idle sags momentarily when the dipstick is
> pulled. But how about the filler cap? When I remove it, the engine stumbles
> seriously, but seems to quickly recover--no stalling-out. Is this the
> expected behavior if the system is otherwise tight? I suppose if I'm
> loosening the cap somewhat slowly, the ISV may be able to compensate? '89
> 100

You have a vacuum leak that has been compensated for (by a lazy dealer) using 
the mixture adjustment.
What is supposed to happen is that the breathers from the engine should be 
clamped off while the mixture is set, then they should be re-attached and it 
should make no difference.  If it _does_ make a difference, find the leak.
If the mixture is set with the breathers attached, leaks will be compensated 
for - but only at idle.  At other throttle settings, the mixture will be wrong 
and/or the OXS controls will have to fight the metering head.
The way to find these leaks is with an unlit (!) propane torch.  Wave it over 
the hoses, etc., until the engine's note changes.  There's your leak.
Some leaks are easy and cheap to fix.  Some are expensive enough (e.g., 
crankcase rear oil seal) to scrap a marginal car.

 Phil Payne
 Committee Member, UK Audi [ur-]quattro Owners Club