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Ben - yes, your best bet to check the HCV is to get the car up to operating temperature and then get a good vacuum source on the valve. You are right, the valve is always closed and applying vacuum opens it and allows coolant into the core. If you perform this test and get heat, then the problem is in the thermostat mounted on the heater case inside the car (and I'll be that is where the problem is because they tend to go bad with age). If you don't get heat with this test, then the HCV is bad. If you want to test the thermostat, hook a vacuum gage on the hose that connects onto the HCV. See if it's getting good vacuum or not.
Paul
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