I've just replaced the heater valve on my 1975 MGB Roadster which was previously jammed open and very hot! However I now find that the heater works but with no hot air. I presume there is an airlock somewhere in the system. Very simply, whats the best way to get rid of this?
First, turn the heater valve to its closed position. If you then disconnect the two heater hoses at the ends furthest from the heater, and connect the one that doesn't go to the heater valve to a garden hose, trying to get the fit fairly watertight. When you turn the hose on, you will find that all sorts of discoloured water and crud comes out the other hose. When the exit water is running clear, get someone else to turn the hose off (but leave it connected to the tap) while you put your thumb gently over the other end - you are not trying to pressurise the heater, just stopping air entering the pipe when the flow stops. Connect this free end to the heater valve as quickly as you can, (remembering to put the jubilee clip on) so that the minimum amount of water is lost. You are only trying to join the hose to the heater valve at this stage, so don't waste time doing the clip up. Then disconnect the hose, putting your thumb over the other pipe to minimise water loss and put this pipe back where it came from. Then do up the hose clips on both pipes. Start the engine and let it warm up until the thermostat opens and allows any air in the cooling system to go into the radiator. Then with the engine still running, open the heater valve. Once the cold water in the heater has been replaced by hot, you should get hot air.
Jim
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