300TE: After driving, water builds up in expansion tank and dumps out
 

My 300TE stays just above 80C now that I changed the thermostat, but when I stop after a drive, the water builds up in the expansion tank, dumps about a tanks worth through the overflow pipe, and then settles back down so that the tank is empty.

Just replaced the t-stat, cap, etc. I cant figure out why it would be doing this. I bled the system as best I could. What would cause the system to swell like that and then drop back down?

Defective Thermostat, bad expansion tank cap, a leak somewhere in the system (worst case is a blown head gasket).

Most likely a bad expansion tank cap, its leaking. Inspect and perhaps just replace the expansion tank cap. You could also have a leaking expansion tank, they are plastic, they crack and they can leak.

Pressure keeps the boiling point of the coolant higher. If you have less pressure in the system, the boiling point of the coolant is lower and the coolant then boils and overflows the tank.

After the engine has stopped and sat awhile inspect the oil for foam or traces of coolant. This would not be a good sign.

You can rent a cooling system pressure tester from Auto Zone or one of the other mass marketers.

Test is done on a cool system. Pump it up to about 15-17 lbs. and look for external leaks.

The expansion tank tends to crack through the cap sealing area, venting the pressure into the overflow tank and then onto the ground. The fact that you don't get coolant pulled back from the overflow tank makes me think either your new cap is bad or the seal seat is bad on the tank.

Expansion tank on my TE was replaced at about 150,000 miles.

Peter

My 300e was doing these things last summer (the good news) this summer it just kept getting hotter and hotter. I would replace Tstats etc but kept getting hotter.

I finally checked the radiator > right after driving it < and sur enough one section was cool while other sections were extremely hot. I read in through the fan blades.

I replaced the radiator and the car runs great in these high 90 degree days.

BTW, I was told that since it has 238k miles on it that it needed a new head gasket, a new head, and also a complete rebuild.

The price of the new radiator and the easy DIY installation were nothing in comparision to the expensive suggestions above.

Haasman

Isn't it amazing what these guys will make up in order to seperate you from your money? The truly evil part is, the car would have had the same problem after all that work was done. Then, they might have found the radiator, ... after they replaced the water pump and the expansion tank and ... and ... and ... .

Good luck belagirl,

turns out it was the water pump, I began to get some weeping from the weep hole and now the problem is gone. I also threw in a new radiator since it was off.

What a pain to do that water pump! 3 bolts no problem, 4th bolt almost impossible. Seems typical for some of this engine work to spend about an hour just staring at it and wondering how they expect anyone to get to that bolt.