After driving on a road course yesterday I noticed that my coolant reservoir was overflowing and spraying coolant around in the engine bay, even though the whole time I was out there I was way below half on the temperature gauge (car has always run cool), and the fluid wasn't boiling. It also sits pretty high after just regular driving on the road. What could cause this? The radiator was replaced before, as well as my water pump not too long ago.

-Matt

 

You could have an airlock in your coolant system, or your thermostat is effed, but that usually results in overheating

 

If it's blowing out into the bottle, it's because you've got a crack in the block/head or a blown head gasket. It takes at least 14psi to lift the spring on your radiator cap, and the only thing that can do that is extreme heat with the coolant expanding, or combustion chamber pressure. Been down this road with my wife's D17A2, it had a blown head gasket, eventually she ran it out of coolant and it over heated. While it was running though, it would just fill the overflow, and the temp was normal.