First, if it only gets too hot in traffic, you need a new visco clutch, it's no longer working.

If it gets too hot on the highway, something else is wrong.

Your last problem is lack of coolant, these engines are a bear to get full of coolant. The cheap and cheerful way is to squeeze the upper radiator hose flat with one hand, then pinch the small hose from the rad to the reservoir tank shut, then release the radiator hose. This will pull coolant out of the tank into the engine and radiator. Repeat, keeping the tank full, until it won't take any more coolant, then start the engine with the cap off and the heater on full heat and add coolant as it sucks down out of the tank.

The correct method is to take the bleed plug out of the cylinder head (in the "sensor strip) next to the temperature sensors at the left front, on the M103, don't know where it is on the M104) and fill until it overflows, the re-install plug. Chancy, it IS an aluminum head and a steel screw....

You wil still need to add several quarts as the air works out of the head and heater core.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!