124 cooling systems, as they age, become problematic. It could be any one of the components, if not a combination of two or more. When you first start your car cold, you should hear the fan roar as if pulling air, then it should subside as the engine warms up. Then, as the engine goes beyond the the prescribed temp, you should be able to hear it pulling air once again. Also, your aux. fan should engage at some point as well, and anytime the AC is used. I would also think about replacing the water pump if it hasn't been done at some point. Also bear in mind that a head gasket can cause overheating without any other noticeable symptoms, a hydrocarbon sniffer can rule this out.

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1987 300TD Turbo 5-spd
1991 350 SD Turbo (Frankenstein 603.970 crank, rods and head on a .50 bored 603.963 block )
1982 300D euro 617.951, 4-spd
2003 E500

2000 GMC C2500 6.5 TD (Double ott, ex DOT truck)
1980 Scout Traveller SD33T (Resting!)
1997 GMC K2500 Burban 6.5 TD (mid engine rebuild)