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Low boost pressure
One permanent recommendation when investigating low boost pressure:
BEFORE YOU DO _ANYTHING_ ELSE - take off the turbo - > intercooler hose at the
intercooler end, and check for metal fragments and/or the impellor retaining
nut. One of the turbo's failure modes is to for this to drop off - the vanes
then slide up and down the shaft, grinding themselves against the outer
housing. The turbo still delivers _some_ boost, and the MB/RR servo system
adjusts the wastegate to spin the turbo ever harder to try and maintain what
boost there is. I hate to say this, but it's usually 0.3 to 0.4 bar.
The metal filings that result migrate through the intercooler and up into the
inlet manifold. Where they go from there I leave to your nightmares.
It's not a very common way for turbos to fail - I know of two, one of which
was my ur-quattro. But the consequences make it worth the first check.
Otherwise, the next thing to check is the "magnetic valve" that switches the
upper chamber of the wastegate between inlet manifold and turbo inlet pressure
to fine-tune the boost. You can use the ECU to test this part and its
controls in situ on most MBs and 20vs. If you need to know how, let me know
and I'll look up the ECU sequence.
Committee Member, UK Audi [ur-]quattro Owners Club