Hi ford focus (2005) mk2, ( pre-facelift) 1.6 petrol.
Had a leaking heater matrix, which is now done, not a nice job!
Now when you run it up, get warm air in car fine, car runs up & fans kick in. But keep getting boiling inside the expansion tank, temp gauge stays normal.I have since replaced the thermostat & now on idle it is ok, a few bubbles just as the fan kicks in but nothing serious, however if you run it with the cap off then it boils over when the fans kick in, stops when the fans stop. Just taken it for a 20 mile drive, performed as you would expect but again boiling inside the expansion tank and the pressure cap ( which is brand new, 2nd 1 in case 1st was faulty) releases & then coolant leaks on the ground.
Any ideas people?

 

It will boil with the cap off. Sounds like head gasket, needs a sniff test to see if it's exhaust gases.

 

Have done sniff test & cylinder leakage test, both passed fine