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94 S4 Lose of Boost/Vacuum



In message <Pine.BSI.3.96r.980824003032.18268A-100000@shell1.interlog.com> Dwayne Cosby writes:

> ANY ideas? Could the waste gate or the by pass valve be stuck open?

Before you do _ANYTHING_ else - pull the turbo --> intercooler hose
at the intercooler end and check for metal shavings.

The reason is that a variety of failure modes produce metal splinters,
and these can be driven through the intercooler and right into your
engine.

The probability of finding the fault this way is fairly low - but the
procedure is easy and the potential rewards (saving your engine) are
_HUGE_.  Do it for peace of mind before chasing the problem further.

If you find splinters, check the turbo inlet for foreign bodies,
including the impellor retaining nut - I've seen two come off and
cause this type of problem.  And - if you've found splinters - you
have a _major_ strip and clean job on your hands.  If you find metal
grit in the throttle body (God forbid) it's a top-end strip.

Wastegates don't stick, IMO, as often as mythology would have you
believe.  If the spring breaks, OTOH, there will be no resistance to the
wastegate opening and you'll never see boost.  Take the top off the
wastegate (just six nuts - moderate hand pressure will hold the cap down
while you undo the last nut) and check the spring, its anchoring, and
that the wastegate plunger moves freely.  I don't know about your
specific wastegate, but if the cap on the one fitted to the MB is just
allowed to rest on the spring, the metal to metal (ignore the diaphragm)
gap between the cap and the wastegate body is 24mm.  The MB's spring is
coded with blue paint, and is 75mm in length when relaxed - 3mm wire
diameter and 3 1/2 turns.

Maybe we should tabulate this information for different engines?  It
makes it very easy to spot shims when you can just loosen the cap and
measure the gap.

Are you measuring with a gauge that would also show vacuum - i.e., a
-1 bar to + 3 bar gauge?  Are you actually seeing _zero_ bar, or
negative (relative) pressure?

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
 (The contents of this post will _NOT_ appear in the UK Newsletter.)