Discussion in 'Outboard Maintenance' started by Flyline, May 28, 2009.

  1. Flyline

    Flyline Won "Do More With Less" Award!

    When it's time for a tune up for my 97' mariner 25hp 2-stroke and I replaced with brand new impeller, selenoid, stator plate, new NGK sparkplugs, rebuild/cleaned the carburator, clean the waterjacket gasket and reseal it. Everything run great like it is before and something is missing in my mind and it's a thermostat check-up.
    So I took the thermostat off and it's looks like it need a new one. I ordered a new thermostat for my motor.

    But.....I was told by a guy and marine mechanic that I don't need to run with thermostat in it. He has a 30hp johnson 2-stroke without a thermostat in it and ran so many hours on it.

    So I put it back without a thermostat in it and the motor pump the water a lot cooler when it's idling in the tank but never ran it wide open.

    Is a good idea to run without a thermostat or not?
  2. Un-shore

    Un-shore Well-Known Member

    if it came from the factory with a t-stat I would run it with one.

  3. Brett

    Brett > PRO STAFF <
  4. rkmurphy

    rkmurphy Well-Known Member

    To be honest, I think the guy who serviced my motor took the thermostat out. I don't know why he would do this, especially since I paid for it, but my tell tail is constantly flowing instead of intermittently as with a thermostat.

    My motor runs fine (for now!) but, if what I said is the case and he did remove it, the next service I get I will have it put back in. Like Un-Shore said, if it came with one it's probably best to leave it the way it was made.

    I won't be going back to where I had it serviced, even though they were fair and their prices were good, because I just don't feel confident. I'd rather pay more money for service that I know will be correctly done.
  5. Tom_C

    Tom_C Well-Known Member

    Up north with water temp lower in the winter I think thermostat is more important than here. I had a problem with mine Easy removed it 3 year ago. Other than the water pressure seem lower the motor runs great for an 18 year old motor.
  6. snooknreds2

    snooknreds2 Well-Known Member


    Water should always run out the tell tail pee hole weather you have a thermostat or not. The thermostat controls the cooling water that runs through the block. They are two different paths for the water to follow. That is why it is possible to have water coming out the pee hole but not getting to the block to cool it if there is an obstruction in the latter mentioned path.
  7. snooknreds2

    snooknreds2 Well-Known Member

    To answer the original question directly...
    IN FLORIDA, you should be ok with out running a thermostat, but I would advise against it!!

    Basically, with out the thermostat the water continuously runs though the block and head. This continuous flow does not allow enough time for the water to be in contact with the metal of the block. Since the water is just flowing by, it can not effectively absorb the heat from the block. so in hot areas of the world you do stand the chance of not cooling your motor enough and over heating it.

    Think about it this way, if you have a hot sheet of steel and run water over it, the sheet of steel will cool down but not as much as if you could have the water sit on top of the plate of steel and absorb as much heat as possible and then be replaced by "fresh cool water" when the water started to get too hot.

    Obviously you want your motor be run cool but as brett posted you do not want the motor to run too cold either. So where this becomes a problem is in colder water locations like up north. The cold water constantly running through the block never has a chance to heat up and you cool the motor too much by always having a flow of "cold water" passing though the block.

    So effectively with out a thermostat you stand the chance to run too hot in warm regions of the world and run too cold in the colder regions of the world. Hence the thermostats job is to effectively regulate the motors temperature no matter what the ambient water temperature is. I dont know about you but I think that sounds like a pretty important task and I would make sure to have a operating thermostat in my motor.

    I hope I explained that well enough...
  8. Kemo

    Kemo The world is flat....and shallow.

    SR2 wrote: 
    My 15 Johnson 2-stk pees instantly on startup, even when cold.  The pee stream never gets very hot.  My old Evinrude peed so hot you couldn't keep your hand under it after running hard at WOT.  How can I easily  find out if my Johnny has 2 paths for the pee?
  9. snooknreds2

    snooknreds2 Well-Known Member

    As far as I know the best way would be just to follow the hoses and logically deduce an answer.  To be honest I never really thought about it, I thought they all had the same basic flow paths:


    On the side of most motors there is a small hole that emits a stream of water. This is only an indicator that the water pump is working. Most of the water that comes from the cylinders is exhausted through the lower unit. During cold weather, the indicator might freeze even though the pump is working well.

    After a quick google search I found the information above, so I stick with my initial thought that all out boards have the pee hole indicator only as an indicator that the water pump is working, not that the water is actually going through the head and block of the motor.

    Also I just found these images that... well I guess might make sense if you look at it long enough...LOL
  10. gotts1963

    gotts1963 Well-Known Member

    When running with a thermostat should the pee stream trickle until the motor warms up enough. And when the thermostat kicks it should it be streaming constant? How hot is to hot of a stream. I was always used to a luke warm stream but know have a motor that's stream is more on the hot side but not scalding. Thnaks jeff
  11. islander1225

    islander1225 Well-Known Member


    Looks like the water pump i put on that bad boy is still kickin sweet.