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Sorry to be late, and maybe irrelevant to your root cause.
Have you replaced the preheater control valve thermostat inside the air filter box? With a failed thermostat, the preheater flappy valve inside the air filter box gets stuck to all hot all the time, drawing heated air from the exhaust manifold heat shield. Apparently can cook the AMM / MAF.
I'll guess your 1987 240 is not in CA or NY state-sold 240 as these require EGR as well as Germany. EGR reduces NOx by ingesting amount of exhaust to reduce combustion chamber temps. A failing EGR valve does not remain sealed when the engine is cold or the 240 is in overrun, like coasting downhill or at cold idle, with foot off the gas pedal for max manifold vacuum. With max manifold vacuum, the FPR allows more fuel to pass through reducing duel pressure at the fuel rail.
I guess you have resolved the ECT engine coolant sensor circuit?
You've tested the oxygen sensor with a digital volt meter? You can tell whether the fuel to air ratio is rich or lean after warm up or if the oxygen sensor is bad?
You replaced the FPR with a known good used FPR? You've resolved no vacuum leaks?
Some idea-rs.
Hope that helps.
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