'85 300DT has NO POWER but runs fine--why?

[v8volvo]

I'm helping a friend with an '85 300D turbo. It's a lovely car, clean, everything works, 166k miles, and it runs smooth and clean, but has NO POWER AT ALL. It starts well and has a fine idle, not slow or anything, and the throttle linkage goes all the way. But put it in neutral, floor it, and there is no smoke, it just leisurely revs up, like a gas engine at light throttle. I can hear the turbo spool so that is not the issue. In gear it cannot climb even the slightest hill. What could be the problem? Initially I thought it was slow pump timing but since it starts and runs so nicely and does not smoke I think not. Could it be badly adjusted valves or a clogged fuel or air filter? Those are my next suspects but I'd think it would stall with a clogged fuel filter and smoke black with clogged air.

I know I've noticed throttle butterflies on some older 240D's--do the 300DTs have them too? If so could it be out of calibration and staying closed?

Thanks in advance...

George