These days, the engine temperature hovers around 80 degrees when the engine warms up. I think this theperature is warm enough to produce hot air.

I have been thinking about this for a couple of days. So far, the members have come up with 2 two solutions: thermostat or monovalve.

Thermostat is a very easy replacement. I am wondering if the engine is cooling off as the heater air temperature decreases. I know that in the manual, if the thermostat is removed, the engine will overheat. That should produce plenty of hot air. If the thermostat is stuck shut, the warm water will be diverted to the radiator and the engine will have a hard time warming up. I do not feel that the engine has a hard time warming up.

In this problem, there is a correlation between engine RPMs and heater output. The higher the RPMs, the less heat the heater outputs. I am thinking that it has something to do with water pump pressure because as the RPMs increase, the water pump increases the water pressure. On the other hand, as the RPMs' increase, the water pump suction must increase as well.

So, that leads me to the monovalve again. Is it possible that a weak spring in the monovalve causes the monovalve to be sucked shut, thus cutting the flow of coolant? But, I wonder if the pressure from the water can force the monovalve open.

Looking at the layout of the monovalve and recirculating pump, the recirculating pump is on the output of the monovalve. I know that when the recirculating pump has been disconnected, I have more heat. I guess what is happening here is that the recirculating pump is sucking the monovalve shut. I think this symptom supports the theory of the weak monovalve.

I will go check out the monovalve again.