Firstable, there is a simple way to check if the vacuum capsule (not sure of the exact name of this piece) on the top of the injecdtion pump is OK.
You remove the hose which is connected to the vacuum capsule and when you put a finger at the end the hose (with the engine runing) you should feel a suction. If not, that means you got a leak somewhere. If yes, you have to replace the vacuum capsule.

Good luck !

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in France : 240D 1981 380 000 km
240D 1984 252 000 km
300D 1978 325 000 km
in the US : Ford Tempo 1993 70K