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The heater valve opens when vacuum is applied and closes with no vacuum. Per other posts the opposite should be true.
My car is a low miles 1992 XJS. The valve appears to be original. I have no heat until I manually apply vacuum to the valve.
Is it possible that something slipped in the valve or is it different for 1992?
Here are images that I have borrowed from another post that illustrate how it is supposed to be.
This is how it is said to work, but my car is the other way.
I've removed the valve from the car and discovered that it indeed self-reversed its function. It slipped internally. With the help of a screwdriver I was able to move the drum inside to the intended position.
A new one is on order.
Come to think of it, this slippage could cause the valve to be partially closed.
So if your car does not heat very well it might be worth looking into.
Partially open with the AC on would also affect cooling performance.
Last edited by Dave Neuhaus; Mar 23, 2026 at 10:39 AM.
So I replaced the valve with one from eBay, but the new part was leaking fluid, a lot of fluid.
Then I bought an expensive one from another supplier that looked slightly different. It leaked so much vacuum that it would not function.
2 bad valves right out of the box. It appears that only made in China valves are available.
Vee (member) says that this plastic valve is superior. Third times the charm, hopefully.
So I replaced the valve with one from eBay, but the new part was leaking fluid, a lot of fluid.
Then I bought an expensive one from another supplier that looked slightly different. It leaked so much vacuum that it would not function.
2 bad valves right out of the box. It appears that only made in China valves are available.
Vee (member) says that this plastic valve is superior. Third times the charm, hopefully.
"Four Season 74803"
This is an increasingly serious, problem. Aftermarket parts for our cars arefrequently poor quality. Any genuine NoS OEM parts you can find, either new or in good second hand condition, are worth buying.
Aftermarket, made in China usually, parts are a problem for ANY older car.
As far as most Chinese manufacturers are concerned, if it looks like an original part, it is as good as an original part, the actual function or longevity doesn't matter.
I can confirm, plastic part works great…. Pot metal aftermarket parts leak right out of the box more often than not. Stick with the Four Seasons if replacing
Heater valve is working opposite what it should
