I've only been driving for a few months and was doing my regular vehicle checks and noticed that my coolant level was below minimum. I topped it up with about 0.5 litres of water before my father caught me and told me it needed engine coolant (No one explained this too me!). Got some 50/50 mixture from halfords and filled the expansion tank to the max marker and ran the engine, everything fine.

Until i was doing a callout after work the following day i noticed the heaters were working (Thought the heating was just poor) and then after about 20 minutes I heard a bubbling noise. Checked under the hood and saw nothing obvious but by the time i got home there was steam coming out from under the hood and the top of the expansion tank was wet.

I've taken it too a mechanic and he's had a good luck replaced the header tank cap and thought it was sorted. But 10 minutes later bubbling noise and then 20 minutes later steam from under the hood and wet top of expansion tank.

Since then I've checked coolant levels daily but not driven the car. The level isn't dropping but there is a white scum on top of the engine coolant, from a little bit of research i think it's from putting tap water in there. I've checked my oil and it's black both from dipstick and when looking in filling point.

I haven't got a clue what the issue is and I'm hoping someone here may have an idea of what's going on?

Thank you in advance!