yeah its the girlfriends grandads he uses it on a day to day basis mechanic has told him cause its losing water and the cap keeps coming off its the head gasket but i beg to differ heater still blows red hot not mayo round the cap and no mayo down the stick oil is pure black as it should be think the mechanic is just trying to exploit him

The car needs a pressurised cooling system to not overheat and loose water! If the cap is faulty or loose, the car will not be able to run at its operating temperature and higher as water will simply expand so much it comes out of the cap, or boil off.

Try the cheap stuff first, cap and bottle before you rip the engine apart.

The water will not boil with the cap off unless the engine is worked hard, the presuration is to allow it to run above boiling point should the necessity occur.