Thermostat on Force 9.9

Discussion in 'Outboards' started by Joe Morrison, Jan 3, 2009.

  1. Joe Morrison
    Joined: Jan 2009
    Posts: 1
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Port Lincoln, South Australia

    Joe Morrison New Member


    I have a Force (Mercury) 9.9 Outboard and I am not all that knowledgeable when it comes to outboards.

    On the back of the motor there is a rubber housing covering a thermostat. The rubber housing has an outlet for water and there are two other outlets in the metal casing lower down.

    What I am wondering is: what is the function of this thermostat?

    If it is not working, what could be the consequences?

    What would be the consequences of simply removing it?

    I have been using the motor for very short trips to a moored boat. I am in the process of swapping it over to become the auxiliary on a power boat where it may be used for longer trips and trolling so I'm keen to ensure it is working properly.

    Thanks for any tips shared.

  2. CDK
    Joined: Aug 2007
    Posts: 3,324
    Likes: 148, Points: 63, Legacy Rep: 1819
    Location: Adriatic sea

    CDK retired engineer

    There is some sort of plenum on which the thermostat is mounted.
    Water, pumped up by the impeller pump on top of the lower gear housing is returning without circulating through the engine as long as the thermostat is closed. 95% returns internally to cool the exhaust, the rest goes outside through the "pee hole" to show that the cooling pump works.
    Only when the thermostat opens, the water can enter the plenum and cool the engine.
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