The PVS valve is a temperature sensing valve found on the distributor vacuum advance line, and is installed in the coolant outlet elbow. During prolonged periods of idle, or any other situation which causes engine operating temperatures to be higher than normal, the valve, which under normal conditions simply connects the vacuum advance diaphragm to its vacuum source within the carburetor, closes the normal source vacuum port and engages an alternate source vacuum port. This alternate source is from the intake manifold which, under idle conditions, maintains a high vacuum. This increase in vacuum supply to the distributor diaphragm advances the timing, increasing the idle speed. The increase in idle speed causes a directly proportional increase in the operation of the cooling system. When the engine has cooled sufficiently, the vacuum supply is returned to its normal source, the carburetor.
- Check the routing and connection of all the vacuum hoses.
- Attach a tachometer to the engine.
- Bring the engine up to the normal operating temperature. The engine must not be overheated.
- Note the engine rpm, with the transmission in Neutral, and the throttle at curb idle.
- Disconnect the vacuum hose from the intake manifold at the temperature sensing valve. Plug or clamp the hose.
- Note the idle rpm with the hose disconnected. If there is no change in rpm, the valve is good. If there is a drop of 100 or more rpm, the valve should be replaced. Replace the vacuum line.
- Check to make sure that the all season coolant mixture meets specifications and that the correct radiator cap is in place and functioning.
- Block the radiator airflow to induce a higher-than-normal temperature condition.
- Continue to operate the engine until the temperature or heat indicator shows above normal.
If the engine speed, by this time, has increased 100 or more rpm, the temperature sensing valve is satisfactory. If not, it should be replaced.