Quote:
Originally Posted by tangofox007
With a properly functioning monovalve, that is true. But with a torn diaphragm or a poor quality aftermarket valve, the increased dynamic coolant pressure at higher engine RPM will mechanically overcome the spring pressure of the valve and push the valve to the closed position. Hence, good heat at low RPM and no heat at highway speeds above at 45 mph.
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Just to add - a neglected cooling system can cause problems with the Monovalve plunger too. I went through that with the SDL last year. Running water, or letting the coolant stay in too long promotes rusting of the components in the cartridge and will cause it to "stick". Mine would stick open or stick closed randomly, when I removed the cartridge, it had a very "gritty" feel to it when operated, just enough to make it bind up. New cartridge has it working perfect.