I had chased coolant leaks and finally fixed them all but I now have issues again. '84 300D, fine around town and short 1/2hr to 45 minutes everything appears normal, no coolant leaks anywhere. I drove to Washington DC last weekend and I noticed the overflow tube puked some coolant into the fenderwell. It did the same thing on the way home. Not enough coolant to see a visible drop in the plastic tank but puking some nonetheless. As an experiment I loosened the radiator cap one click so the cooling system would not pressurize and drove the car out to western MA on business this week. Ran fine, no puking coolant, no overheating just the coolant level rising about twice as high as it normally does in the plastic tank, due I suppose to expansion? I tightened the cap again and drove home. Now I have coolant leaking from somewhere under the waterpump. Not a large amount, maybe 6oz max. So here is what I am thinking:

1. I have a small headgasket leak. My coolant hoses are firm even when the engine is dead cold. Not as firm as when hot running but definitely there is pressure in the system at all times. Could the combustion gasses be "burping" out of the system during extended high-speed cruising causing the puking of some coolant out the overflow tube? I know that headgasket issues are almost unheard of in 617 engines but its possible.

2. My "new" leak under the waterpump is either coincidence, the waterpump just decided to start to die after the two long drives or maybe the seals need time to "seat" again after pressurizing/depressurizing the system a few times.

3. Where is the coolant level supposed to be on the plastic tank? I fill it to the little line/arrow mark when cold. The coolant level rises about a 1/2" above that when fully warm, due I guess to expansion? Sound normal? Should the coolant level be at that line when hot not cold? Is there a "minimum" mark on the tank? Should I drain it to that point instead?

4. I am sick and tired of dealing with this issue and I will be contacting Evans Cooling this week to consider the possibility of using their NPG+ coolant and going the a non-pressurized cooling system. I realize this may just mask another problem but if it runs well, doesn't leak anything and doesn't overheat I will be happy.

5. Is there any harm in running standard coolant with the radiator cap loose? When I did it there was no sign of overheating, no leaks, no issues? Maybe hotter weather would give me trouble but then again maybe not... As long as I keep a close eye on the temp gauge I don't see how it would hurt anything.

Oh yeah, at no time has the temp ever exceeded 95*C so its not overheating. Any ideas on this problem would be great. Sorry for the long post, just trying to put it all down. RT

__________________
When all else fails, vote from the rooftops!
84' Mercedes Benz 300D Anthracite/black, 171K
03' Volkswagen Jetta TDI blue/black, 93K
93' Chevrolet C2500HD ExCab 6.5TD, Two-tone blue, 252K