Hi, The thermostat on the raw water cooled Yanmar 3QM30H has always been a bit of a mystery on the use of one thermostat or two thermostats. While there appears to be two thermostat holes in the housing, the service manual shows a diagram of only one thermostat and the discussion is in regards to always "a thermostat". The Yanmar 3QM30 parts manual also lists a quantity of 1 for the thermostat (105582-49200). My engine came with one thermostat, so when I rebuilt the engine, I put in one thermostat and it has been working fine. I wished I took better detailed photos of the thermostat housing, because I recall convincing myself that based on the housing, a second thermostat would would do absolutely nothing.
Note for the raw water cooled engine, there is only one thermostat from Yanmar that fits so you can’t screw up and mix thermostats of different temperature settings. Early 3QM30 were converted to fresh water using the Sen-Dure fresh water heat exchange kit. With the Sen-Dure kit, the engine used the same thermostat, but one replaced the temp sender. The engine ran hotter because the heat exchanger was not as optimal in cooling water as exchanging hot for cold raw water. But since the thermostat opened at lower temperature, the engine was not as efficient in coming up to the hotter operating temperature. Later Yanmar came out with the fresh water cooled 3QM30F using Yanmar’s own heat exchanger. The 3QM30F uses a different and physical bigger and hotter thermostat which is mounted on the Yanmar heat exchanger and get rid of the old one which was mounted on top of the exhaust manifold. - Paul E. 1981 C&C 38 Landfall S/V Johanna Rose Fort Walton Beach, FL http://svjohannarose.blogspot.com/ > On May 28, 2018, at 9:16 PM, cnc-list-requ...@cnc-list.com wrote: > > Date: Mon, 28 May 2018 20:27:42 -0400 > From: wwadjo...@aol.com <mailto:wwadjo...@aol.com> > To: cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > Subject: Re: Stus-List thermostat on 3QM30H > Message-ID: <163a94b1b92-c9a-17...@webjas-vab075.srv.aolmail.net > <mailto:163a94b1b92-c9a-17...@webjas-vab075.srv.aolmail.net>> > Content-Type: text/plain; charset="utf-8" > > Thanks for info. > > Sent from AOL Mobile Mail > > On Monday, May 28, 2018 Rick Brass via CnC-List <cnc-list@cnc-list.com > <mailto:cnc-list@cnc-list.com>> wrote: > > Bill; > > ? > > A ?raw? water cooled engine passes water from outside the boat ( in your case > lake water) through the engine for cooling. A ?fresh? water cooled engine has > antifreeze in the engine and a heat exchanger, and the lake water only passes > through the heat exchanger to cool the antifreeze circulating inside the > engine. > > ? > > Josh?s note about making sure you have the correct thermostats installed in > your engine is spot on. > > ? > > A raw water engine uses a low temperature thermostat and has a low operating > temperature. I would have guesses 120-125, but Josh found a spec of 108 for > the thermostat. Your operating temperature when the engine is under load > should be around 140-145. That helps to prevent deposits inside the engine > that will block the cooling passages. > > ? > > A fresh water engine runs at a higher temperature ? diesels are more > efficient at higher temperatures. You use a 165 degree thermostat and the > engine can get to 180-190 under load. > > ? > > One consequence of the lower temperature in a raw water engine is that there > is increased production of unburned hydrocarbons and soot. Back in the day, > when I was a Yanmar certified technician and teaching diesel mechanics, the > consensus among the mechanics was that the exhaust elbow on the Yanmar 2 and > 3 cylinder marine engines should have the carbon buildup cleaned out every 50 > to 100 hours ? at least yearly ? as preventative maintenance. > > ? > > On a raw water engine like yours, that would be a very good idea. > > ? > > Rick Brass > > Washington, NC > > ? > > ? > > ?
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