Possible leaks are heater core (unless blocked on BOTH sides of the heater core), head gasket, heater hose clamps (this was the leak on the TE that drove me nuts), heater core (if connected), radiator.

Sadly, "mysterious" coolant loss is often a head gasket.

Do not run plain water, the engine will corrode terribly, and you will also end up with a ruilned radiator and heater core. You need the boilover protection in the tropics as much as we need the freeze protection in this part of the world, plus the protection from corrosion.

A leaking head gasket will usually pressurise the cooling system cold -- is the upper radiator hose hard with a cold engine? If so, head gasket time (or head if you've let it go with plain water some years).

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!