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Hard Starting/Rough Idle
There's an interesting tip in one of the Audi service bulletins that
worked wonders for my '89 200T. Apparently there is a bank of relays
that controls the cold starting/idling/black magic mechanisms that keep
the engine running. These relays are grounded by a single brown wire
which attaches to a bolt just in front of the idle stabilization valve.
The grounding wire actually terminates in a spade lug which is in
direct metal-to-metal contact with the top of the bolt, but is insulated
from below by a piece of painted (black) metal. So, the grounding path
is through the bolt and into the intake manifold.
The problem is that Audi used Loctite on this bolt, which can eventually
lead to intermittent or no grounding at certain engine temperatures. This
could explain why you have problems idling when the engine is hot, as well
as why you can cure the problem by tapping on the idle stabilization valve,
since the grounding bolt is right in front of it. The recommended fix is
to remove the bolt and remove all the Loctite from both the bolt and the
hole it screws into. The bolt on my Audi was completely coated with Loctite,
and removing this improved both the starting and idling significantly.
I don't really know how you can clean out the hole, except to screw the
bolt in and out a couple of times and clean it each time.
Hope this works for you.