Re: high idle

[paul.heneghan@bbc.co.uk (Paul Heneghan)]

From: Mark Zvolanek <mzvolane@sydney.ipex.com.au>

>As the engine warms up, around 10 to 15 minutes from initial start, the
>revs increase from 800 to 1000 RPM. The increase happens in stages not
>all at once. Would anyone know what might be causing that.
>I read in recent posting low voltage can cause high revs, how is that?
>What should be the correct voltage supplied to battery from alternator,
>and how much should it drop when for example lights are switched on etc.

Ignore the low-voltage idle speed boost.  I doubt very much that in the 
first 10 minutes your battery voltage is reducing!

There are two main things that control the idle speed:
 - the ISV (Idle Stabiliser Valve - cylindrical valve at back of head))
 - the TBS (Throttle Bypass Screw - brass screw in manifold near throttle)

If the TBS is out too far, then when the engine is cold, it might idle at 
the right speed due to the ISV, but when the engine warms up, the revs will 
rise, and even if the ISV is fully closed because of high revs, the TBS will 
be letting too much air through.

Try disconnecting the electrical conection of the ISV.  If it makes no 
difference, then either it's stuck (often happens - try washing in carb 
cleaner) or else the TBS is open too much (also often happens - the o-ring 
perishes and doesn't grip anymore).  Try screwing in the TSB a little.

Correct procedure in Bentley.

Paul
paul.heneghan@bbc.co.uk
1984 Audi 80 quattro
1989 Audi 100 Avant

p.s. my guess is that it sounds as if somebody increased the idle with the 
TBS to compensate for a non-functioning ISV.