Thread: Cooling System Pressurised

Hi and welcome!

Coolant flows through the heater core all the time. Heater 'off' just closes airflaps.

Damp in the footwells is more likely from leaks around windscreen or even door seals, etc. (unless it obviously smells / looks like coolant).

Sounds like something is wrong with the cap on the expansion tank. Are you sure it actually pressurised and not sucking air in after a few days standing?

Normally as they heat up coolant expands in the system and into the expansion tank. If the pressure is over a certain level the cap will let air out (or coolant if the tank has been overfilled when cold).

When turned off and the system cools there is a little spring loaded valve in the cap that allows air back into the tank, so if you check the level when cold there is very little pressure difference from ambient.

I'd suggest having a look at the cap, make sure it is the correct one for a pressurised expansion tank, the air return valve is working (with just your fingers, spring is quite weak) and don't overfill the tank. Level should just be a bit over the brass plate in the bottom.

cheers, DL

Great advice thank you will check cap first hard to say whether it was pulling air in when i opened or blowing it out, will definitely pay more attention this weekend and check before embarking on pulling heads off, then try to get to the bottom of the damp foot wells, had sort of hoped it was the heater box as that is almost preferable over the alternative of body water leak hunting.

Leaky heater or windscreen are certainly options.
Have you pulled up the carpets, because if the leak has been going for a while it may have rusted a hole in the floor, and need welding up.
While you have the carpets up, vacuum any carp out, slap on a coat of rust converter and then a couple of coats of paint, preferably with a bit of rust converter in it. That should help in the future.
Might pay to also uncover the doorsills and see what's happening underneath.