Power fall-off above 1.5 bar

[George]

My '91 200 Avant is having respiratory or fuel problems above about 1.5 bar.
To cut to the chase, I had the turbo rebuilt about six months ago, and the
car took off like new, pulled1.8 bar and all was glorious, recently it has
started stumbling and loosing power above 1.5 and we are having a hard time
diagnosing the problem. New parts include the WGFV, coil, fuel pump, fuel
filter, cats, cap, rotor, wires and plugs, and all the vacuum hoses.
Yesterday we detected a bad engine speed sensor, and turbo over-run sensor,
which have now been replaced. Same problem.  

 

I have a very good wrench on the job and he is frustrated. He has confirmed
full vacuum, no leaking injectors or fuel regulator, good spark, fuel
pressure, and that the upper and lower intercooler hoses are good.
Alternator output is on spec. Diaphragm in the waste gate is good. Car
starts right up and runs perfectly until it gets up to 1.5, and will
continue to try to build boost, but starts stumbling. So, we can rule out
limp-home mode from the ECU.  The check engine light is on. 

 

Any sage advice, ideas, or whacky thoughts most appreciated....  Something
is getting by us here.

 

George Sidman