MG Car Repair: 68 mg midget cooling system fault, blown head gasket, radiator cap

Question
Hi Barrie, I have an issue with coolant passing through the overflow from overheating.Recently replaced water pump, removed thermostat, new radiator cap and still it throws coolant through the overflow and overheats.Removed thermo housing and i have flow, started vehicle and yes pump has flow too, made a hell of a mess :).Maybe radiator cap is wrong, too shallow, it does not use much force to compress into lock position, although it appears to be long reach. Any advice?

Answer
Hi Lei-Tsi
This is a common problem with the MG Midget.  I have three suggestions:  

1) When you drained the engine to fit a new water pump you also drained the heater matrix.  Before you fill the radiator you must turn the heater control to HOT, otherwise you will get an air lock in the heater matrix.  You may need to undo the upper hose to the heater in order to bleed air from the system.

2) It is also possible that you have a bigger problem.  I would do a compression test on the engine to make sure that you do not have a blown head gasket.  This would cause similar symptoms.

3) You say that you have removed the thermostat.  Did you fit a new one?  Running the engine without a thermostat will cause localised overheating, because most of the coolant will pump around the bypass hose and not through the engine.  If you remove the thermostat then you must fit a blanking sleeve instead.