Here’s a nice experiment to carry out on a freezing cold morning. Before driving to work / school / shopping centre / Auntie Betty’s, look under the bonnet of your car and make a note of the level of coolant in the expansion tank. Chances are its fairly low. After you get to work or wherever, open the bonnet again and check out the level now. See any difference? (NB I mean check the expansion tank – DON’T open the radiator cap with a hot engine)
The coolant expands quite considerably when it gets hot – and what no longer fits in the radiator / cooling system gets pushed into the tank, only to be sucked back up when the engine cools down.
I’ve been tracking my coolant level closely because I reckon I’ve got a small leak in the system somewhere, but its hard to be sure with that level constantly going up and down as temperature changes.