I do not know which manual you are looking in, but the Yanmar Service
Manual for the 2QM20
specifies that in the raw-water cooled version (2QM20(H)), with a 2.2:1 reduction drive, the thermostat will begin to open at a temperature of 42 C (108 F) and be fully open at a temperature of 52 C (126 F). In 1975, the Yanmar part number for that thermostat was 124770-49200, but it may have been revised since.
In the fresh-water cooled version (2QM20Y(F)), with a tank, heat exchanger
and recirculating coolant
, the manual specifies that the thermostat will begin to open at a temperature of 71 C (160 F) and be fully open at a temperature of 85 C (185 F). I do not have a part number for that type of thermostat, but I do know that it is supposed to be located on the fresh water tank
(heat exchanger), at the point where the water flows into the tank from the cylinder head
. Whereas on a raw-water cooled engine, the thermostat is housed on top of the exhaust
manifold, linking into the cooling water bypass circuit.
So you appear to have one model of engine (raw-water cooled) while quoting the thermostat specs for a different engine (fresh-water cooled). It would appear that Compass790 has put his finger square on the root of the problem, that you are stuck with a conversion. Which then leads to other questions: Do you in fact have a fresh water conversion and is there a thermostat on the fresh water tank
? If so, then the manual seems to indicate that you should not have any thermostat on top of the exhaust
manifold, but rather the that the raw sea-water should be pumped directly into the heat exchanger
and then flow directly from the exchanger into the exhaust manifold, without passing through a thermostat. Only the fresh water loop should be passing through a thermostat, the one on the inflow into the tank.
Let us know how it all works out.
Originally Posted by bigdogandy
Maybe a bit off topic, but what’s the harm in just taking the thermostat out entirely?
Andy, the thermostat is necessary to ensure a proper operating temperature for the engine. A diesel
engine needs to be warmed up to a specific temperature range in order to run and properly burn fuel
, which is why the engines in trucks and construction gear
have glow plugs to preheat the cylinders. If you simply do a full-flow of cooling water from the get-go, then the engine will have difficulty warming up enough to facilitate full combustion, especially in a cooler climate.