Yanmar 3QM30 Thermostats

[Alasdair]

2 years ago I replaced the thermostats in my raw water cooled engine. As the engine had two thermostats I replaced both of them. In the Engine Manual it shows only one thermostat in the cooling water system though there are positions for 2 on the housing.
The engine runs fine with 2 thermostats fitted. I would like to know if both or only one is required?

[rbyham]

While a slightly dated thread... I have the same engine and also have two thermostats. The 3qm30 Yanmar is the engine in question. I plan to replace both thermostats but would love to better understand the cooling system in terms of why two? I can understand that one is to shut off the bypass line at warm up so that the engine runs at efficient temp but why and how a second thermostat comes into play is a puzzlement...

[Cheechako]

Not sure, but interested to know. My 3Qm was a FW cooled version. The thermostat was in the header tank which was remote mounted ...not on the engine..... on mine. I could not remove it out of the aluminum header tank/heat exchanger unit... kinda weird.

[rbyham]

Cheech... thanks for at least keeping the thread alive :-) Yes mine is RWC. Your FWC is quite different especially as relates to thermostats. Due to your being a pressurized system and not being exposed to sea water, it can run at a higher temp. This changes everything when it comes to the thermostat(s). You have just one but the 3qm30 RWC is a bit puzzling as regards the cooling system in that some apparently have 2 thermostats in series. To add to the strangeness I can find several 3qm30 owners on line who verify the 2 thermostats but can find no official Yanmar source. All online manuals from Yanmar show just the one thermostat in the housing. Strange...

[Alasdair]

It is a bit of a mystery?? Interesting thing is that if you examine the thermostat cover and housing you will notice that the second thermostat does absolutely nothing wether opened or closed.
The manual diagram only shows one to fit in the front position with the second position vacant. Perhaps the second position was for some kind of modification if the engine was to be converted FWC though this seems unlikely