The monovalve is just a heater valve that controls the flow of coolant to the climate control systems. I would probably fix it soon because it isn't too expensive (around $30), it's really easy to do, and I think that proper coolant flow is important to keeping the temp. of the engine constant. Oh, and it will really suck to have uncontrollable heat (or lack thereof).

Alex

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1983 300D (parked for four years)
2012 VW Sportwagen TDI Manual
2001 Miata SE
1962 Chevrolet Corvair Rampside