Hello: I have a 1976 U1300 (425.121) and the fanbelt driven air compressor is overheating. I have checked the air filter and inlet hose, the oil supply line and the return; all good. The shop manual states that this is a 18.5 bar system. I have removed the head and cleaned the reed valves. I honed the cylinder and reassembled. The compressor then made 14.5 bar, but it ran very hot. After shut down and restart max pressure obtained is 8 bar. The cylinder is once again lightly scored.

How is this compressor supposed to get it's cooling, oil only? Is the pump supposed to produce excess air volume and have this dumped overboard so air is constantly moving through the system? Any help is greatly appreciated, thanks.

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T.W.
1987 300D 170,000 Mi.
1990 350SDL 105,000 Mi.
1979 450SL 147,000 Mi.
1976 U 1300 Unimog 4400 H
1963 Piper Cherokee 160 1800 H