Page last updated July 17, 2008



Outboard Thermostat

If your particular outboard motor has a thermostat, you can easily replace or service the area but you will need replacement gasket for the exhaust covers, if you have an engine similar to a Mercury outboard 650 or 700. The thermostat is housed inbetween exhaust covers. There are also a few pieces or components that should be replaced within the thermostat housing. Some thermostats have their own housing and gaskets, so you may need a repair manual to see what it is exactly that you need.




As you can see from the picture to the left, there are a few pieces that should be replaced. Some are prone to deteriation, while some will last a lifetime. The deteriorated plastic piece on the left is a diaphram. It seals the water in the engine and allows the washer to expand, hence the need for the hole on the exhaust cover..


If you obtain a thermostat kit, it should have all the necessary replacements parts needed (minus the exhaust cover gaskets).



It is always a good idea to service the thermostat when you first get a used engine. How can you tell if you have a thermostat? On the 650 and 700 Mercury outboard, the housing will have a small hole for the poppet valve to expand (see above pic). That small hole also clogs up and should be cleaned. I did not even know the hole was there until I removed cover and noticed the area was completly covered in buildup.

My 650 had a thermostat inside but the 700 did not have one. Both engine blocks were almost identical and the 700 had the location for one, but it was not installed; thus it was missing the small pinhole.


Cleaning - When you have the covers off, get a small nail or pliers, anything with a sharp area and scrape the area inside the poppet valve hole and the thermostast housing area. If you look to the left, where the plastic resides in the powerhead, this area is where the sand accumulates. These areas tend to accumulate sand and lime build up; it will limit the flow of water. When I opened mine, it was completely or seemed to be clogged up with calcium like deposits / sand especially where the poppet valve sits into powerhead.

The rubber gasket that was attached to the poppet valve had dryrot and so it was replaced.

The thermostat was replaced but only because I received a new one. My old on was fully functional.

To clean your old one, use soap and water.

Scrape if needed. To really clean it and if it is made of copper, you can submerge the thermostat in KETCHUP or anything acidic for a 10 - 15 minutes and rinse; If you don't believe me, toss a penny in some ketshup and see.

To test thermostat, making sure it still works.

1. Boil water and remove from heat

2. With a pair of needle pliers, grab thermostat and submerge in hot water.

3. The thermostat should open up and you may hear a pop. Observe and make sure spring contracts and that there is a difference; the spring should grab the valve that blocks water and take it in when it ocntracts, observe. If you notice spring cracked / broken or not catching plug at end, then you should replace thermostat.

If you open the exhaust cover and you do not have thermstat in place, you can place one in, but if your motor did not come with one, then it probably does not need one. Again, my Merc 700 did not have one but all the openings were there. The only thing I needed to do to put one in was put the poppet valve and thermostat in place. I also would need to drill a small hole on the outside exhaust cover, but I don't think I will retrofit one in at this time.


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