But Wait. I like your fix, but have question about diagnosis.

If the pump is disabled, it does not move hot h2o to the heater core, so no heat. If this pump is failed this would indicate not enough heat -- not too much heat like we have been getting.
If the monovalve is sticking OPEN, one would get too much heat, or heat at erratic times. If the monovalve is stuck CLOSED, (even with an aux pump that worked all the time), one would get no heat. Disabling the pump effectively bypasses the hot h2o flow to heater core plumbing and the faulty monovalve at the same time. I am thinking this is still monovalve.

Can anyone confirm?

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David
1986 300E Anthracite + ECodes + MB Mileage Award