No idea how you could tell that 4 of the 5 are "firing good". The main way is a "power balance" as long done w/ gas engines w/ spark wires. Disable each cylinder one at a time by cracking the top injector nut until it seeps fuel. See if each cylinder has the same effect on idle.

Could be some of your injectors are even for non M-B. Seems Bosch diesel injectors of similar vintage are almost the same for VW, Volvo, etc cars & trucks, but check Google Images. Main thing is that the pop pressures are about the same. The injection pump determines the volume flow to each cylinder.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans