I have a 1990 500SL with 66k miles. I replaced the radiator (the plastic neck broke), overflow and expansion tanks (including cap), thermostat, thermostat housing and hoses. The car never runs over 90 degrees, but will sometimes blow out antifreeze via the overflow hose. Also, the antifreeze will drain from the over flow tank into the block while driving and will return once I shut off the engine. I tested for head gasket leaks using a gas-test kit. there was no presence of gases in the coolant. i was told I could have air in the system, how would I get it out? i do appreciate any help you could give me.

Before doing anything else I would make sure that the proper cap is installed & working correctly! The cap should be marked 1.4, & that means 1.4bar(22psi) of pressure. Also the overflow hose should connect into another OVERFLOW tank under the rt front fender.

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I will take a look at the cap when I get home, this evening. The over-flow hose located below the cap is connected to the top of the lower tank. Which means the hose conncted to the bottom is connected to the top of the lower tank.

I had posted recently that after replacing everything on my cooling system, I had forgotten to SCREW THE CAP DOWN ALL THE WAY!!! Otherwise, even if temps are normal (which they were for my situation as well), the lack of pressure lowered the boiling point, and forced the coolant through the cap (which wasn't tightened down anyway).

This may be obvious to most, but I struggled with this for about two weeks before I figured it out...DUH!

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I will take a look at the cap when I get home, this evening. The over-flow hose located below the cap is connected to the top of the lower tank. Which means the hose conncted to the bottom is connected to the top of the lower tank.

I did check the cap. I called the dealership and did verify the p/n. i do have the correct cap and tank. As I mentioned previously, I did replace the thermostat housing. The replacement does'nt have a bleeder screw. How would I get the trapped air out of the engine?