We had to replace the check valve on my Son's '91 940T. The car would crank over a bit more than "normal" before it would fire up. Once started, it ran fine. It is checked with a fuel pressure gauge attached to the fuel rail. when the car was shut off, the fuel pressure would quickly drop to zero. It should've maintained (random number off-the-top of my head...) like 40lbs of rest pressure.

Check for leaking vacuum lines
properly attached turbo ducting
Dirty throttle body

Check for stored codes, too.
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RussB in Ct '96 855T - '91 944T