Coolant/AirCon problem E46 3 series

[parc77]

Hoping someone can help...have looked through some old threads but not sure.

My 318ti 2003 (2litre) (approx 56,000miles FBMWSH)

Warning light came on for low coolant level, checked it...it was low, bought the correct stuff to top it up and assumed all would be okay. However did a shortish journey of around 10 miles arrived at my destination and got an engine full of steam, it was dark by this stage so couldnt really see much.
Checked it the next morning and found the level had dropped, assumed there must be a leak somewhere, so decided to go for a cautious run around the block in dayllight so I could see where the possible leak was...noticed my aircon was not getting warm grrr! (Is this to do with the fact that the coolant level is low?).
Anyway it appears as though coolant is coming out of the filler cap as it was boiling up and coming out(obviously with the cap in place).
Not sure if I just need a new filler cap (seal broken?) and need to top up again or is there a blockage /thermostat is kaput?
Temp gauge is stable and halfway as it normally is.

Any ideas/advise would be much appriciated

[john_p]

Expansion tank faulty perhaps? These are known to fail. Maybe try replacing the cap first.
If it still overpressures I'd be testing for head gasket problems, but that seems unlikely

Make sure you use a mix of coolant and water to top up.

[paprika]

Yup, look at expansion tank.