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  1. #1
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    question about operating temperature

    I have a 2001 Optimax 3L DFI 225, SN 0T305247. I was on the lake Sunday and the water temp is 39 degrees. If I got on plane and did under 4k rpm with the motor trimmed all the way down, the temperature gauge never got above 75. Once I trimmed the engine up and increased my rpm to 4500-5000 the temperature climbed to 118-122. Is that normal or a sign of a problem. I ran the engine about 5 miles with it lower speed and no trim and it stayed at 75 degrees. On a side note, is there a feature on here where I can type in a keyword and it will show me messages with that keyword? I couldn't find anything.

  2. Member
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    #2
    The FAQ thread in the stickies has answers to every question, how to safely run your motor in cold water, all the governing variables of motor temps and water pressure including the system design, how to service everything in the system, what to look for under normal operation. The short version is on plane temps don’t mean a thing as long as you aren’t overheating and you are coming over 120 F at idle before any planing. Your boat may need the upgraded thermostats but I will let the experts tell you that.

  3. Member
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    #3
    I fish a cold water reservoir in NE WA and when surface temps are 40° or colder I'll see similar on plane temps as you, BUT, as JT said the critical thing is whether or not your engine reaches at least 120° before accelerating AND read the recommended cold water operation topics in the FAQ. My personal practice in cold water is to watch thermostat cycle before accelerating as temp begins to climb towards thermostat temp the second time.

    Proper Warmup and Cold Water Operation

    Engine Shutdown Precautions after a HARD RUN (Especially in cold water)
    _______

    Phil
    '09 Hewescraft ProV
    '09 150 Optimax


  4. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #4
    ^^^^ Both above responses point you in the right direction.

    Your engine already got 130 degree thermostats/covers (as well as all the other recommended upgrades).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor

  5. Member
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    #5
    What does the "stickies" mean?

  6. Member 06 SB's Avatar
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    USN Retired
    2020 Basscat Caracal
    2020 Mercury 225 ProXS 4s



  7. Member
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    #7
    I read the stickies, and I understand now the dos and donts of running a cold engine. I also understand if my thermostat was not functioning, it can cause the engine to run cold. I have been on the lake 4 times so far in February, and each time I am having the same issue. When I launch the boat, I start the engine and let it idle while I park the truck. When I get back to the boat the engine has warmed up to 122. When I run to my first spot it stays between 114-118. I fish for an hour and start it to run again and its around 100 and it warms up to 118-122 and cools to about 114 by the time I get to the next spot. It does this until the 4th or 5th run. When I start it, its around 95 and as I sit there it drops. If I start to go it keeps dropping. No matter what I do it drops. It always goes below 80 and the other day it got down to 68. It has followed this pattern 4 different times. Im at the point where I run to my furthest spot first and work my way back because I know at the end of the day I will be idling back and I want to be close. Has anyone seen or heard of this before?

  8. Member
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    #8
    Replace the poppet valve for starters....its probably 18 years old



  9. Member
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    #9
    Need to remember also thermostats only control temperature at idle rpms. Once poppet begins to open, water flow is changed, and when poppet is fully open, the thermostats are out of circuit. If your engine will not reach at least 120° at idle, need to inspect thermostats and/or poppet. If you know for certain your thermostats are correct (should be 130°) and pass function test, then I'd go with Savage's advice above.

    Very important to remember your "on plane" temps will be what they will be. Weekend before last, and with surface temps of 37.4° - 37.8° my "on plane" temps were 68° - 70° BUT my engine reaches thermostat temp every time at idle.
    Last edited by SilverFox579; 02-23-2020 at 07:53 AM. Reason: correction
    _______

    Phil
    '09 Hewescraft ProV
    '09 150 Optimax


  10. Member
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    #10
    The problem was a broken thermostat stem. The spring would push the thermostat open but when the spring cooled down the stem wasn't attached to the face that closes off the water flow.

  11. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #11
    Good catch.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor