The heat won't turn off on my 1988 XJ40

 

Hi Richard,

I believe that in your '88 there is a separate heater water control valve mounted in the heater hose near the firewall in the engine compartment, part 1 in the diagram below. Its default position is open, allowing hot coolant to circulate through the heater core. It is shut off by vacuum, so if the vacuum hose is cracked or detached from the small elbow pipe visible on top of the valve, or the vacuum source is clogged, the valve will always be open and you'll have heat at all times:



Cheers,

Don

 

Than you. I'll look for this. Does the valve ever go bad? I see they are available at a reasonable price.

 

Yes, the valves do fail, but check the vacuum hose and if you have a vacuum gauge test it for proper vacuum to the valve.

Cheers,

Don