I purchased a '90 744 Turbo that had a leak in the induction system. The PO had doctored the engine settings to allow it to run pretty good. The leak eventually got to the point where the car would not start.

My mechanic told me that the car idled at 3500 RPM after the hose was fixed, the engine was running quite rich. I could not tell because Turbos have a little black in the tail pipe when they get older, and the car ran pretty good.

One problem that could cause symptoms like yours is a rich mixture. This might sort of act like a choke on a carburator.

PS the PO had also jammed the thermostat open so that the car could run a bit cooler because the electric fan was siezed.
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2 8s & 2 7s 600,000 miles total