I just swapped the engine in a 92 Legacy and used a new water pump and thermostat. I replaced the coolant and tried to be careful about bleeding the air i.e. lifted the front of the car, topped up the coolant, opened the drain screw on the passenger side, warmed it up, let it cool, and topped off the coolant again.

 

Now when the car heats up the level of the coolant in the overflow tank slowly rises. I have not driven for extended periods yet. The longest was 20 minutes and the overflow tank just started to overflow.

 

Any suggestions about what could be causing this? Would an air pocket in the engine do it? A small head gasket leak? (there doesn't appear to be bubbles in the coolant). Perhaps just a bad radiator cap? Any idea what is normal for the coolant level to rise as the engine heats up, if at all?

 

Any comments much appreciated!

A little rise is probably normal. As the coolant expands it gets forced into the coolant expansion tank. When the endine cools, it should get sucked back into the engine. Because air expands more than coolant, it would make the change more visable until all the air gets eliminated.

I've had the same thing happen at coolant changes, but it never overflowed that I was aware of. The coolant level in the tank would actually drop when cooling and I topped it off for a few days untill all the air was out. There is a full or normal line on the tank don't overfill when the engine is hot or it will have no where to go except out the overflow tube. If it keeps on occuring, it could mean something more severe.

Had this happening to me last year, turned out that both my hg's were blown. I'd go get the coolant sniffed for hydrocarbons.

It looks like the radiator cap was to blame.

 

I changed the radiator cap this afternoon, bled the air and went for a long drive. The coolant level in the overflow tank only went up about 1 cm. So I think that was it, hopefully.

 

thanks for the comments.