is it a turbo? if so there is a plastic tube that runs from the back of the injection pump around the back of the valve cover to i think the intake area. this tube can crack and if it does it will not enrich the fuel when under boost..you will swear there is no boost. at least this happened on my 82 sd. i am not sure your car is the same but good chance it is.

good luck.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.