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  1. #1

    Join Date
    Dec 2012
    Hudson, MA

    1997 SLT780 Overheating - Bad/Stuck Thermostat

    I have done some googling on the subject but if I have blatantly missed a thread already detailing this out please let me know. This is my first jet ski that I bought not running. The thermostat was stuck shut and caused one piston to overheat. I have read that some recommend just removing the thermostat completely and let cooling water pass through all the time. This ski will only be run in new england fresh water so I am a little hesitant to do that as often our water is cold and may make for rough idling. However, I am thinking this is my best option for numerous reasons. Thoughts?

    Is there a DIY or guide out there that I have missed for removing the thermostat? Is there anything else I need to remove or dead end before firing it up in the summer without a thermostat installed? I just don't want to cripple myself by missing a part and putting the ski in limp mode.

    Any opinions or advice is much appreciated.


  2. #2
    john zigler's Avatar
    Join Date
    Aug 2006
    we have been removing them in customers machines for years. just take it out. make sure to let your motor warm up, before blasting across the lake, or up river at wide open throttle.

  3. #3
    Banned User
    Join Date
    Jun 2010
    Milwaukee WI
    If you run in cold water I would use the thermostat. It can be tested in a pot of hot water on a stove. Should open around 140 degrees.

    Was it the popoff valve that was stuck? Make sure that moves freely. Clean out any corrosion where the thermostat and popoff valve are. A light coat of grease on the popoff valve isn't a bad idea either.

    I recently installed a temp gauge on my ski. Running in cold water, it gets up to around 120 degrees at idle. When you start going and the popoff valve opens, it cools to about 70 degrees.

    If you run in warmer waters, just remove it, and I'm sure you will be fine. If you choose to remove it completely, make sure you really let it warm up before hammering on it like John said.
    Last edited by K447; 09-25-2013 at 09:49 AM.

  4. #4
    ripcuda's Avatar
    Join Date
    May 2010
    Newman Lake, WA
    Welcome to Green Hulk!

    I had an overheated cylinder on one of my skis I bought. It had gotten so hot that it turned from Fuji blue to black and compression was way down too.

    Have you tested compression? What #s did you get?

    The thermostat lives in the rear of the water rail... which is the rail across the top of the 3 heads that says "Polaris" on it. At the very end of it in the rear is the t-stat and popoff housing sealed by an "8" shaped o-ring. Disconnect the hose (water out) and unscrew the housing. Check the popoff and spring are not all rusty and free to move and test the t-stat in a pot of water as suggested above... or remove all together. I too run in cold water so I would keep a t-stat in my machine.

    Since this is a new ski to you... have you gone through all the HIGHLY recommended maintenance and upkeep items for it? Have you inspected and replaced (where needed) the entire fuel system, hoses, in-tank pickup, selector valve, fuel pump? Have you cleaned and rebuilt the carbs and checked their settings?

    Tons of valuable info at this link. Read and heed.


  5. #5

    Join Date
    Dec 2012
    Hudson, MA
    Wow. Thank you everyone for all the tips and advice; I really appreciate the prompt and informative responses. I did do a fairly large amount of research prior to buying the ski so I have checked most of the recommended maintenance items on my PWC that you guys list out. I think I will try to run without the thermostat (in fact I already have it out, just wanted to make sure I didn't have to do anything else). I had a hard time finding a standard bore piston for the 780... is there anywhere we can get drawings for these parts in case we had to go machine one? I know it would be worst case but all the available parts are oversized which means boring and full rebuild. Not sure what would be cheaper, full top end or machine your own pistons. Websites that claim they have standard pistons in stock are all backordered... finally found one on eBay.

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