Heater valve vacuum problem


Oct 2, 2002
My heater valve is opening during boost and even light driving affecting my AC air. I'm sure it's the valve as I bypassed the heater core and I get cold AC all the time.
I assume a bad or missing check valve or something else?
There is 1 check valve on the drivers side going to the metal vac line. That one is ok.
Are there others?
Any tips on how to troubleshoot?

Also, when I turn the car off the heater valve maintains a vacuum source for a long time.
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Check the heater valve first on the bench with and without vacuum make sure the spring is strong also so the valve can cycle ok.
And make sure the valve guts themselves aren't rotted and leaking passing water at all times regardless what the vacuum and spring are doing. Run some water through it from a hose barb while testing.
If all that checks out make sure the white vacuum line on the A/C head is doing it's job.
The C60 vacuum diagram is in the shop manual linked below.

The only position of the A/C switch that "shouldn't" allow any water flow is the MAX. position which most everyone uses for best operation of the A/C make sure you are in that postion.
Other positions of the A/C mode control switch will allow the water to flow IF the "cold" "hot" slider is moved off the far left COLD position. So if you aren't in MAX and your slider is defective or just off the cold position water will flow through the valve.
That's all I can remember for now off to repair my blown brake line with a stainless one after I finish my.... :coffee:
I rechecked the check valve but with a compressor and it does leak.
Also thanks for the suggestion regarding putting water through it. With a garden hose it does leak even though it appears to be closed.