This will sound like an odd problem but the heat in my XJS is intermittent. I can drive the car and the heat will be nice and toasty then it will stop being hot and actually turn cold. The fans keep blowing cold air but not a drop of heat is coming out. Then I drive the car again and the heat is back. Drive it again and the heat is gone. It will disappear while driving. Since the heat is there sometimes and I am not changing any settings I don't think it can be a valve. But, maybe not. I don't know what to start looking at. Any advice would be greatly appreciated.

 

My first thought is that your cooling system needs proper bleeding. An air pocket is travelling thru the system and when it reaches the heater core, no heat. If you need info on bleed procedure, give a shout.

Next......

When the heat goes away, does the airflow change at all? That is, does it change from the lower outlets to the center outlet, or the outboard dash outlets? If so, this would suggest a control problem----the system 'thinks' heat is no longer being requested.

As for the heater valve, disconnect the vacuum hose to the valve. This will cause it to remain open. Does the problem go away?

Cheers
DD