Hi all,

My 1987 300D developed a strange problem suddenly. The cooling system remains under high pressure even when stone cold (left overnight). Yes, I have replaced the radiator cap. The car has never done this in the previous ~4 years I've owned it. The only way I can think of abnormal pressure in the cooling system would be from exhaust gas entering. How do I test if this is happening? The coolant looks clean and smells fine (to me.) There is no oil in the coolant or vice-versa. I suspect either the head gasket or a crack in the head (shudder.) I'm at a loss here... Help!

Dave M.
Sacramento, CA
1987 300D - 228kmi (weird cooling system problem)
1987 300D - 234kmi
1984 300D - 205kmi

If I'm not mistaken, this model has an aluminum cylinder head. Have you flushed your cooling system and changed anti freeze often? If not, you may very well have suffered corrosion which is showing up as a small passage from combustion chamber to water jacket.

You need to go to a radiator shop and have them pressure test the system.

Best of luck,

To answer the question, you can get a small kit from NAPA to check if exhaust gas is entering your coolant. The test is very simple, you set it in the filler cap and suck air into a blue fluid. If it changes color, then you will know. I have used this test many times...it works. cost about $35.00. But will last for a bunch of tests. I agree...something is odd. as the coolant cools everything should contract. give us some more hints to go on

One last comment...if exhaust was entering your coolant.... you would see the Temp guage showing a hot condition in a big hurry