I recently had to replace the water temperature sending unit. I had broken the connector screw while installing a new water pump pulley while doing a serpentine belt and high output alternator upgrade.. Yesterday after completing the alternator upgrade project, I started the engine for the first time since replacing the sending unit.. After about five minutes the temperature buzzer and warning light activated. I had good raw water flow coming out of the tailpipe. In addition the heat exchanger felt cool. In fact the coolant in the heat exchanger was not even hot. The housing for the thermostat felt hot. The hose leaving the thermostat housing and going into the heat exchanger was also quite warm. The belt was not sliping and the water pump pulley seemed fine, and there were no coolant leaks anywhere. The expansion tank and heat exchanger had plenty of coolant. I know the impression at this point is a faulty sensor however there was a slight difference in the exhaust as if the engine were indeed overheating. In addition the thermostat housing felt quite hot. You could keep your hand on it but it was uncomfortable. And that was with me shutting the engine off almost immediately when the buzzer water temperature sensor warning lamp went off.
How hot should the thermostat housing be as normal operating temperature?
I added additional coolant to the heat exchanger and checked the expansion tank hose. I also started the engine with the cap off and watched to see if it took any coolant or any was circulating. It did not take any coolant from the heat exchanger.
So I think the possibilities include a stuck thermostat, a faulty sending unit, or something clogging the flow of water into the heat exchanger. May be bad bearings in the waterpump.
However, I wonder if another possibility exist. and that is the reason for this post. Can there be trapped air in the coolant system that does not allow the coolant to circulate? (I did not drain the coolant when changing the heat sensor. I simply took the heat sensor out and little coolant drained around it. I replaced the heat sensors sending unit with a new one, using proper threading preparation and carefully torqued.) I don't think so and have not been able to find any reference in my yanmar manual or diesel troubleshooting guide that would indicate that but thought I should ask.
How hot should the thermostat housing be as normal operating temperature?
I added additional coolant to the heat exchanger and checked the expansion tank hose. I also started the engine with the cap off and watched to see if it took any coolant or any was circulating. It did not take any coolant from the heat exchanger.
So I think the possibilities include a stuck thermostat, a faulty sending unit, or something clogging the flow of water into the heat exchanger. May be bad bearings in the waterpump.
However, I wonder if another possibility exist. and that is the reason for this post. Can there be trapped air in the coolant system that does not allow the coolant to circulate? (I did not drain the coolant when changing the heat sensor. I simply took the heat sensor out and little coolant drained around it. I replaced the heat sensors sending unit with a new one, using proper threading preparation and carefully torqued.) I don't think so and have not been able to find any reference in my yanmar manual or diesel troubleshooting guide that would indicate that but thought I should ask.
