Low boost/lack of power MC2-engine (MC dual knock)

[urq]

... given the statement that you're reasonably certain that there are no
leaks in the intake system and you are seeing no stored fault codes, one
place I'd recommend that you look is the plumbing to the Wastegate Frequency
Valve.  I had a problem on an MC-1 where the line from the WGFV to the
wastegate cap was broken ... Audi put the vacuum line inside a larger
diameter vacuum line, and a stress point developed where the smaller line
exited the larger line.  This one was quite a pain  to track down because it
would work sometimes.

The wastegate spring is weaker on the MC engines to allow for a safety
feature that prohibits max boost when problems are noted by the ECU ... but
it also makes it so that the engine can't make full power unless the spring
gets augmented by pressure applied to the upper chamber of the wastegate via
the WGFV.

Steve B
San Jose, CA (USA)

-----Original Message-----

I have a late 1990 Audi 100 TQ with the MC2-engine (also known as the MC
dual knock-engine).
I have replaced all sensors including both knock sensor, the idle/full
load-switch and the air filter.
If I try to read fault codes, I get 4444 = no fault codes stored. I'm quite
sure I don't
have any turboleaks or any major vacuumleaks.

The car is in very good condition. The mileage is 131 000 miles. The car is
original, never had any
modifications done to it in any way.

The problem is that sometimes the turbo doesn't seem to boost at all, or at
least according to the
gauge in the board computer. This occurs - if it occurs - after driving for
some time (30-60 minutes).

Normally the gauge shows 1,4 at full boost, but when this occurs it shows
1,0, and I can feel
that the car feels more sluggish. Not that it is a monster when it boosts
1,4 (0,4 bars), but when it
supposedly doesn't boost at all? I'm sure it boosts something, but it's not
a lot..

What could cause this?

/Dag
Norway