Either cracked or overflowing, but why overflowing. I've had 3 caps on this focus so far.  Does expansion cap always release air.

I've just tipped some coolant out to make it halfway between marks

as the coolant and engine heats up they expand and often pressurises - water under pressure boils at a higher temp which is why we have a pressure cap that is meant to release excess pressure....

Note - the expansion bottle has a magic overflow - it leaks it out the bottom - as the lid doesn't evacuate excess pressure or coolant out the top - however - they are also rubbish quality and fail often with splits

BUT all through this thread things sound in a bad way - and you might have over pressure from a damaged head gasket...

i have 2006 ford focus water coolant has zero circulation into reservoir yet car never overheats ? can someone explain for me what is going on

4 hours ago, colt said:

i have 2006 ford focus water coolant has zero circulation into reservoir yet car never overheats ? can someone explain for me what is going on

The reservoir isn't part of the circuit. Coolant doesn't circulate through it.

Coolant circulates through the engine, the heater matrix and (when the thermostat opens) the radiator.

Coolant expands as it heats up, and the excess coolant needs to be pushed somewhere. That gets pushed up into the reservoir. Then when it cools down again, the coolant contracts and gets sucked back down again.

To avoid air being sucked into the system when coolant contracts, there always needs to be a thick head of liquid coolant above the bottom pipe of the reservoir.