Most gauges if disconnected or the power wire is cut will go to max. Look at that connector mentioned on the prior post. The one at the driver side of the engine between the 2nd and 3rd glow plugs. If loose or disconnected would cause the temp gauge to max out. But my first thought is the power plug at the back of the gauge cluster is loose, partially unplugged etc. that would affect both the temp gauge and the fuel gauge. I agree about the fuel filter lack of power issue but with the engine off and someone pushing on the throttle, can you see any slack, wasted motion before the throttle pivot goes to full throttle? on the engine linkage. Follow the linkage from the engine side of the firewall to the injec pump and verify it goes to full throttle mechanically.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.