The MGF Register Forums

MGF VVC 1996 : Engine coolant boiling, bubbling and steaming from the expansion tank. The valve in the expansion tank cap let steam out. (cap recently replaced) However, the radiator was hardly warm.
My conclusion: thermostat not opening. QUESTION: Are there any other likely causes I should consider? Should I also replace both jiggle valves? Any other suggestions?
Frank-in-Sweden

First. Bleed the system.
2nd Proper MGPart cap. The MG parts fail, forget the aftermarket parts.
3rd, Check the compression pressure of each cylinder. Low pressure indicates a HGF,
Check the radiator for failure by loss of fins on older radiators.

Thermostat seems most obvious but also check water pump is circulating coolant properly,
you should be getting a steady stream back into the reserviour when idling

As these engines, (K-series), are notorious for head gasket failure, you could also get the coolant checked for combustion gases. This is done using an electronic device.
Of course with a cool radiator, thermostat failure does seem the most likely candidate