W124 Coolant Pressure Build Up
 

I have a 1992 300D. I recently replaced my twist cap for the coolant reservoir because the gasket was coming off. Soon after I replaced it I had the top radiator hose leak. Then I had and overflow of the coolant reservoir both on different hot days. The temperature gauge in the car never showed any abnormal levels. I put the old cap back on and haven’t had any problems. The temperature is always right around 80C. Could the new cap be defective? Could I have hot spots building pressure?

Thanks,
Jay

Have you tested them? (both)
 

If you have a cap tester or some type of scale you can compare the spring weights of both. I did get a bad one some years ago, I kept dumping coolant until I got another.
If I'm guessing right you've got a 603 or other alloy head motor??
BTW I took out the highest sending unit in the thermostat cap and was :eek: 'd to see I could get in over a liter of coolant mix to the head when the radiator and reservoir were 'full.' I'm sure that prevented a hot spot or two.

You are correct it's alloy.

Are you saying that you found are some critical areas without coolant? How did you know to do that procedure? Was there some blockage preventing it from venting and allowing collant in that area?

What does a sending unit look like?

Thanks,
Jay