MG Car Repair: overheating rad, radiator overflow tank, mgb gt

Question
hi Barrie,

i have a 1978 mgb gt, with a fairly new rad. On a recent outting, the car over heated, this caused the seal around the thermo switch to disintgrate and spew collant and water everywhere.I though it would be just a simple case of replacing the seal, so i did. after running the car from cold for 15 mins, the rad was cold, then suddenly the top left of the rad (beside the thermo switch) started getting very hot, steam was rising from the rad and the gauge was almost in the red. i turned the car off, i could hear the water boiling in the rad, and from what i could see, no water or coolant had left the overflow tank, at no point did the fan kick in or the engine falter. is this a case of water pump failure, electric fan, thermo or clogged pipes?...maybe all of the above?

any help would be much appreciated!

thankyou

Answer
Hi Kenny.  Whilst all your suggestions are possible causes, it could be something much simpler.  When you replaced the radiator, did you turn the heater control to HOT?  If not, then the heater radiator wil have been full of air when you filled the tank up.  This air bubble will pressurize, and eventually something will blow.  I doubt that it boiled, it just bubbled noisily.  

First off, you must fit a new seal to the Otter switch in the radiator header tank.  Next, I would fit a replacement pressure cap to the radiator overflow tank.  I do not like the 15 psi caps, so I recommend a 10 psi cap from an earlier MGB.  At 15 psi it is more likely that the Otter switch will be blown out of the rad again.

As an electrical check, unplug the Otter switch wires and join the wires together with a paper clip.  With the ignition switched on the fan should kick in immediately.

I would then turn the heater control to HOT and remove the pressure cap from the overflow tank.  
Take the plastic bung out of the radiator and fill it to the brim with with a 25% anifreeze mixture.  Refit the plastic bung.  Half fill the overflow tank with neat antifreeze and replace the pressure cap.

Run the engine until it gets warm, then switch it off.  Feel the top hose, don't trust the temperature gauge.  

Check that the radiator is still full, topping it up if necessary.  The overflow tank should also be checked to ensure that it is at least half full, but not full to the brim.

If all this fails, get back to me.