Ford focus 2014 1.6 engine coolant goes below minimum level after driving it for a while, but when the cap is removed (Engine off and cold) the coolant comes back to max level. I bought the car recently and it had a leak in the expansion tank thus replaced the cap and tank (both brand new).

Tom's answer is probably the best for your problem:

On 11/11/2020 at 9:29 AM, TomsFocus said:

The expansion works the other way round.  Hot coolant expands and would overflow the reservoir when the engine's hot if there wasn't a built in air gap for the coolant to expand into.  This is why you mustn't fill higher than the max mark.  As it cools, it contracts again and shrinks back down but should remain higher than the min mark.

You've got an air lock in the system.  Fluids can't be compressed so if the coolant disappears then it has to be into a pocket of air.  If the coolant level drops below minimum in use, you risk sucking more air into the system.

Ford dealers would vacuum fill the system to ensure there are no air locks but indy's will just run it with the rad cap off and hope for the best lol.

 

How can I fix airlock in system?

if you got an airlock take the cap off the expansion tank leave engine running until the water starts to boil, then you`l get the air bubbles comeing out as it boils over.

6 hours ago, unofix said:

Tom's answer is probably the best for your problem:

 

Well explained and spot on