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Thread: Engine Overheats at Idle RPM

  1. #1
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    Default Engine Overheats at Idle RPM

    I just got a new Marine Power 6.0 L motor installed in my 2860.

    The boat runs fine when I first take it out and cruise around, but after an hour if I back the boat down to idle RPM's 700-800 the temp. gauge will instantly heat up to above 200 degrees (normally runs about 150) then as I try to power up the boat appears to be missing.

    Once I am able to get back up to even 2000 RPM's the engine runs fine and the temp comes back down to 150 degrees.

    I only have about 13 hours on the new re-power and it just started doing this.
    Any ideas???

    Thanks in advance.

    Rod
    Bayliner 2860
    Valdez Alaska

  2. #2
    Member Dupont Spinner's Avatar
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    Default

    Freshwater cooled????

    Look to ensure they didn't get a hose kninked. Then look at your impeller, freshwater cooler exchanger(if you have one), and oil cooler for any blockages.

    Also check to ensure the belt is self adjusting to tighten itself. Also check to see if the freshwater pump is turning correctly (the right direction).

    Mine will stumble sometimes after a long run when getting back on step from idle. They richen these motors up because of the freshwater cooling. Something required to run in a 75* plus lake waters not our Alaskan waters.

    I have the 2859 and mine runs 180* always. It may spike up after a long hard run but just for a couple of minutes.

  3. #3
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    Default More info - overheating

    My boat is fresh water cooled.

    I just got back from the harbor were I tried a couple of different things.

    My boat at low idle (in the boat slip not in gear) is about 600-650 rpm's.
    That is when the temp. climbs up above 200 degrees.

    As soon as I bump the rpm's up to even 800 it will immediately cool back down to 150 degrees.

    It acts like either the sea water pump or the thermastat is not functioning correctly. Both are brand new and there is less than 15 hours on the boat.
    I hope the additional information will help.

    Thanks again.

  4. #4
    Member Dupont Spinner's Avatar
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    Default

    I am assume you are still running the Merc Bravo II setup???

    How long does it take for the temps to climb? If they climb quickly it is a problem, most likely on the freshwater cooling side and it could possibly be a stuck/partial opened Tsat.

    Do you have an infared thermometer you could use? Check water hose temps from the outdrive, oil cooler, exchanger all the way to the manifolds and compare incoming versus outgoing.

    Also check your freshwater temps at the exchanger, Tsat housing(below and above tsat), water pump, block and head temps.

    Did you remove the 350 for the 6.0? What was salvaged from the 350 to the 6.0, pulleys, alternator, etc?

    If left to idle does it climb past 200*? I know that 200* sounds bad but you still have plenty of room before boil over. What does concern me is the temps climb high at idle and move down when idled up.

    Merc use to require a second water inlet, a thru hull, for freshwater cooled boats because of cooling problems. I still believe this is a lower 48 lake boat problem not something I would consider as a requirement for Alaska's waters.

    I hope I am helping....it's hard doing remote surgery. I will in Valdez the end of the month if you are still in a quandry over this. I actually think we have met once before already a few years back. We spent 2 or 3 weeks in the harbor. We had the three Rotties.

  5. #5

    Default Food for thought.

    Check your outdrive water intake. I had a little metal bolt-on hose that has a rubber hose attached to it with a hose clamp corrode away to crumbs, it was all hanging in there but not really connected anymore. But it made the boat act like the thermostat was stuck, it actually melted a riser hose, then we fixed the riser, then the thermostat, THEN discovered the real problem.

  6. #6
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    Default Anti-freeze won't suck back into FWC

    Just got back from working on my boat.

    As detailed earlier, my anti-freeze was nearly empty. After a further investigation my overflow tank was way to full and the overflow line from the motor only seems to flow only one way, it does not go back into the motor FWC. Thought it might be a faulty cap (I replaced).

    I had someone mention today that my overflow coolant tank needs to be higher than my fresh water cooling unit and it is not it is mounted about eight inches lower than my FWC. He mentioned the anti-freeze will not run up hill to get back to my cooler, is this correct ?

    If I need to mount my overflow tank high I can, however it will be difficult to do so with my current setup.

    Please advise??

  7. #7

    Default Mounting height theory...???

    First off, mounting height may or may not be relevent, I think the real issue is the over heating to make it boil over in the first place. Secondly, I have four trucks and one leaks anti-freeze until its basicly empty, but it never over heats from it... I think you could run a motor with less than recommmended amounts of coolant and not have the issues your having -yours is boiling over. Did you replace the thermostat with the correct temperature rated thermostate for your motor??? They all have a temp range. Do you have a clogged or rusted riser, elbow, or manifold???

  8. #8

    Default Eliminate the thermostat issue

    Remove your thermostat completely from the engine and reattach the hose, fill with anti-freeze, and turn it on. Then watch the gauge to see if it still over heats, if it stays cool than your problem was a bad thermostat.

  9. #9
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    Default Thanks - I will give it a shot

    Thanks for the advice. I will give it a shot. The engine is brand new with only 25 hours on it. But, who is to say the thermostat may not be working correctly. I also agree that something is not working correctly. I will pull the thermostat all the way out and try that first.

    I definitely want to fix it before something else goes wrong.

  10. #10
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    Default

    I called the engine distributor today and he suggested checking my FWC filler opening and neck to make sure it had not been damaged.

    Sure enough the filler neck had a dent in it probably preventing the cap to properly seal while it was under pressure. I think I have it straightened out now and it appears to be functioning correctly.

    Thanks for all the help in trouble shooting my issue. It was very frustrating to have a brand new motor with less then twenty hours on it have these issues.

    Thanks again :arr !

    Rod
    Bayliner 2860
    Valdez, Alaska

  11. #11

    Default

    It sounds like the pulley on your water pump is to big which makes the pump turn slower.

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