So... my heater control valve isn't working. It isn't closing when it is supposed to, e.g. on start-up.

I tested it further by wiring the valve to the closed position... in that position, water is closed to the heater (I can tell because suddenly there's not the persistent warm air coming out the vents in the cabin), so it's not the seal inside the valve that's failed. It appears the problem is rather that the valve isn't closing.

Two possibilities 1) valve failed, or 2) valve OK, but just not getting vacuum.

Now, I could plunk down some dough to buy a new valve eliminate 1), but I'd rather test for vacuum before spending the money, if possible. So is there a simple (or even complicated) way to test if the valve is getting vacuum to begin with?

The car runs very well otherwise... I mention this in case it eliminates various types of vacuum leak.

Anyway, ideas and advice appreciated, as always.

 

it does not take much vacuum. put a hose on it and suck. if it closes then the valve is good. if it doesn't then buy a new heater valve. they are cheap and known to fail.