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MB engine overboost.

In message <9711278832.AA883263067@genchip.uunet.ca> "Mike Hopton" writes:

>  All the symptoms of a sticking waste gate? Very high boost followed by
>  fuel pump shut down and then everything is back to normal.

No, we don't think so.  As far as I know, the only professional who's
ever experienced one 'live' was Martin at BR Motorsport, who said he'd
never felt anything like it.  If it was something like a sticky
wastegate, we'd see it on other models - especially MC-2/1Bs.

The problem running round in my mind is: Where is all the fuel coming
from?  Other experiments are starting to suggest that an MB is close
to the fuel system's limits when only gently tweaked - how is the
fuel for this massive surge being supplied?

The only idea that seems to make any sense so far is that the cold start
valve is being triggered excessively.  The MAC12 logic (like its close
relatives) provides for triggering the cold start valve if the engine is
accelerated hard when cold, where 'cold' is indeterminate.  We haven't
investigated this condition much, because none of us likes thrashing a
cold engine.

The reason the cold start valve makes so much sense is that it's
independently fed by the fuel pump - it's attached to the main feed
line at the fuel metering head, but it isn't in any way _metered_
by the head, and so it represents additional fuel throughput.  The MB
has a tendency to run lean at full throttle (especially with aged
fuel system components) so dumping fuel into the inlet manifold would
have a proportionately greater effect than one might expect.

 Phil Payne
 Phone: +44 385302803  Fax: +44 1536723021