{"id":471,"date":"2010-05-17T21:24:19","date_gmt":"2010-05-18T05:24:19","guid":{"rendered":"http:\/\/svrainshadow.com\/?p=471"},"modified":"2024-06-14T09:21:28","modified_gmt":"2024-06-14T17:21:28","slug":"yamaha-8-hp-outboard-thermostat","status":"publish","type":"post","link":"https:\/\/svrainshadow.com\/?p=471","title":{"rendered":"Yamaha 8 HP Outboard Thermostat"},"content":{"rendered":"

We have a Yamaha 8 HP 2-stroke outboard for our dinghy (1999 model 8SMHX). I reported earlier about how to change its water pump impeller (here)<\/a>. We did that because the telltale wasn\u2019t flowing water, and it still wasn\u2019t after we serviced the water pump. So we took off the head cover to look at the thermostat.<\/p>\n

What we found was a lot of sand and gunk, and a corroded thermostat. We pulled out the thermostat for cleaning. The head cover plate was seriously pitted on the interior face due to saltwater corrosion. The corrosion protection installed by the manufacturer is gone over much of the interior. We didn\u2019t do anything about this, except clean off the deposits.<\/p>\n

Van used a turkey baster and squeezed it hard to flush water into the telltale, basically creating a ram effect to flush out gunk. Van then gave the water cavities in the power head a vinegar bath by using a turkey baster to squirt vinegar up the telltale hole, and also into the hole for the thermostat. He left it in there 5 minutes to dissolve the various calcium deposits, and then repeated the process with water to flush out the vinegar.<\/p>\n

Eventually, the telltale started dribbling water. So we started the motor, and used a wire to clean it out several more times, giving the motor a good fresh water flush using a 32 gallon garbage can rather than the flushing muffs. We now have a very healthy water flow from the telltale \u2013 hurrah!<\/p>\n

Some things to note about the thermostat \u2013 we tested it to ensure it behaves to spec. It\u2019s supposed to be closed until about 50 Celsius, and open about 3mm by 60 Celsius. At first it was stuck open after we pried it out. But with some cleaning of the central shaft that slides during the opening\/closing, we finally got it to work correctly.<\/p>\n

We seriously considered chucking the thermostat, but then I read it can cause serious damage to an outboard to run at the wrong temperature. Since we could get the old one working, we decided to keep it. If we were going to buy a new one, I found the original Yamaha parts available at www.boats.net<\/a>. Or, you can get a Sierra version of this thermostat (part # 18-3623) available at a variety of places. I could never figure out if the Sierra kit that includes a seal means it includes the head cover gasket, or if it means the plastic grommet that surrounds the thermostat.<\/p>\n

Access to the thermostat was easy \u2013 it\u2019s just behind the head cover plate that looks like an inverted L. Removal was harder because the thermostat was corroded in place, so we had to pry out the white plastic grommet with a screwdriver and knife. That did some damage, but we still could reuse it. Check the exploded view of the cylinder crankcase here<\/a>, which includes the thermostat and surrounding parts.<\/p>\n

Moral of the story \u2013 flush with fresh water after use.
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