Yamaha 8 HP Outboard Thermostat

Yamaha 8 HP Outboard Thermostat

May 17th, 2010  |  by Marilyn  |  Published in Maintenance and Repairs  |  2 Comments

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). We did that because the telltale wasn’t flowing water, and it still wasn’t after we serviced the water pump. So we took off the head cover to look at the thermostat.

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’t do anything about this, except clean off the deposits.

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.

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 – hurrah!

Some things to note about the thermostat – we tested it to ensure it behaves to spec. It’s 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.

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. 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.

Access to the thermostat was easy – it’s 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, which includes the thermostat and surrounding parts.

Moral of the story – flush with fresh water after use.


  1. Mark DeJarnatt says:

    February 1st, 2011at 6:54 pm(#)

    I had to go on to this step for my outboard too, thanks for the excellent instructions. I also had a bolt shear off, and with hindsight 🙂 might soak those with something to break them lose next time.

    Thanks for sharing the knowledge!


  2. Jack says:

    May 6th, 2014at 2:14 pm(#)

    Hey Guys!

    Thank you for posting. I have the same engine. Bought it in Florida last summer and it has never been reliable for my C&C Mega 30 located at Folly Beach, South Carolina. Was about ready to dump it and spend another thou. on a different motor. I need another trailer for my skiff and that thousand is reserved for it.

    Picked up the impeller kit yesterday and found your instructions THANKS! Re-assembled everything and cranked her up about an hour ago. I know it takes a short bit of time for the thermostat to kick in but still no squirt. Found your other post about the thermostat and took the cover off, did a double take…. there is not even a thermostat on this motor! Empty hole.. Whoever worked on this motor last must have had some mighty good scotch or maybe even some moonshine as I noticed the old gasket at the impeller housing was on the wrong side of the plate as well.

    Thank you for posting this information, it’s straight on and very helpful…. Think I’ll have a beer now and pick up a thermostat tomorrow.


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