E300D slow coolant loss
 

I'm experiencing slow coolant loss with my '94 E300D wagon. It seems to go oddly though- I recently did a 600 mile trip with no loss, then on a 100 mile trip the level in the expansion tank went down around 3/4 of an inch. I can't see any obvious leaks, all hoses are in good condition, temp stays around 80-82 up to around 85-90 when working hard. No signs of head gasket trouble. With the history of the car I have I know the water pump/thermostat/radiator haven't been replaced in the last 4 years.

Have the cooling system pressure-tested for leaks.. the cooling system as many components that could leak, so the only reliable way to test ALL components is to pressure-test. You could have a leaking expansion tank.. they tend to grow hairline cracks over age, but with the symptoms you describe it doesnt sound like that could be the problem.

Replace the pressure cap, too -- they go bad and allow the coolant to boil in heavy traffic or after high speed runs, causing occasional coolant loss.

Pressure test is the only real diagnostic, but the caps are cheap.

Peter

Thanks for the replies. I saw another 'couple of posts about experiencing the same thing and one turned out to be the thermostat gasket and other was the plastic fitting on top of the radiator for the hose to the expansion tank. I'll have to check after a long run.

If that were happening, wouldn't water boil off before the coolant? IOW, wouldn't the concentration of the coolant increase because water is being boiled off?

When coolant boils you can smell the antifreeze so it boils off also.

I think that antifreeze is an alcohol based brew and is hydrophilic. It makes a pretty tight mix with water and I am guessing that they only separate under controlled conditions. I am not sure what it would take. I have seen antifreeze recycling machines but I don't know if they separate the water from the antifreeze.