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  1. #1
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    Question 250 EFI Overheating

    I have twin Mercury 250 EFI's Offshore, 1996. Runs in salt water, I am having overheating issues with the starboard engine. I have replaced the water pumps, thermostats and popoff's. Engine runs fine at 2500 RPM but at 3000 RPM will overheat in 10-20 minutes. I have 5-6 lbs water pressure in both engines at 3000 RPM and about 2-3 at idle. Solid pee stream and 5lbs water pressure, even when overheat alarm goes off. If I idle down the engine cools off after a few minutes and alarm goes off. When overheated there is steam coming off of lower unit above waterline. Engine has always been flushed with fresh water, Inside passages seem clean at thermostats and at popoff. I saw in the parts manual that there is a filter screen in the engine but was told that to get to it you have to take off the powerhead.

    Any help would be greatly appreciated, don't know what to do next.

  2. #2
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    water pressure seems low, is this your normal set up or have you changed something,is bottom clean?

    21 SuperBoat VF200 Yamaha SHO
    Bender Clan memberEroshibend Yamamoto
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  3. #3
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    Yes bottom is clean, I have had this issue since I bought the boat a year ago. Both engines have the same water pressure, they max out at about 6lbs on my guages at anything above 2500 RPM. Only stbd. engine has overheating issue.

    Does anyone know of a place to get onto the dump side of the cooling passages that I can try to flush, without pulling the heads?

  4. #4
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    maybe the wter tube is leaking at the base of the adapterplate. water pressure is low on that year motor to bring up pressure stretch the poppitvalve spring and re install, also make sure the adapter plate isint plugged up. and make sure t-stats are the correct range 143 degrees. also make sure the lowerunit isint plugged up with crap on the inlets.
    1988 reaction19-sold
    1978 action20 w/ 250 efi-sold
    2003 quartershot T4-sold
    1978 16 action with 15'' 2.4 -sold
    1979 mako21 w/ evinrude 150 faststrike-sold
    1995 18 ACTION COPY w/ CCMS HO2FO!-sold
    1981 24' manta w/ a v8

  5. #5
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    Since both engines have the same water pressure I amassuming that it is a flow issue. I checked the thermostats and they open at approx. 140.

    I also just replaced all of the parts in the poppet.

    How do I get at the adapter plate?

  6. #6
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    pull the powerhead.
    1988 reaction19-sold
    1978 action20 w/ 250 efi-sold
    2003 quartershot T4-sold
    1978 16 action with 15'' 2.4 -sold
    1979 mako21 w/ evinrude 150 faststrike-sold
    1995 18 ACTION COPY w/ CCMS HO2FO!-sold
    1981 24' manta w/ a v8

  7. #7
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    I have 2003 225 EFI and the thermostats in mine are 130's. I just replaced my thermostats because they were taking too long to open.

    Side note: I called a merc dealer which I won't name, they said my motor needed 143 degree thermostats. However the factory put 130's in. Sometime they cross refence these motors incorrectly and wind up with the wrong parts.

    It happened to PowerPlay25 also,,,He's needs 120's for his pro-max's and they sold him,,,get this,,,143's. He didn't question it untill his alarms were going off at idle.

    Be careful. HV
    Boats love hundred dollar bills.

  8. #8
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    Did you roll the impellers in the housing correctly with the direction of shaft rotation so they are pushing the water as opposed to acting as scoops?

  9. #9
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    I'm sure the impellers are in the right direction. (wouldn't they flop over if in backwards anyway?)
    I think I must have a partial blockage somewhere. Is there some way to access the waterjackets without pulling the powerhead. The boat is in the water and I don't have a trailer. How hard is it to pull the powerheads in the water or does the lower unit have to get removed also?

    Also regarding thermostats I don't have any problems at low speeds, just at RPMs over about 2800rpm.

    (I had a mechanic tell me a year or so to run the engines with out thermostats and they overheated right away)
    Last edited by vdm; 08-20-2008 at 12:11 PM.

  10. #10
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    A blockage is more than possible. Depending on how many hours you have on the motors I would consider freshing them up. At this point your motors are going on 13 years old, as you know. Maybe Simon can give a good deal on a refresh job. The power heads aren't that hard pull. Send them off and I'm sure he will dyno test them before he ships them back so no surprises...just an idea, I know everyone's budget is different.


    Also, I wouldn't run a motor without thermostats period. I would just replace them.
    Boats love hundred dollar bills.

  11. #11
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    i feel like i have the opposite problem strangely...

    i have a 99 250 EFI offshore

    my motor never gets hot! .... i have 5psi at idle and it gets to 10psi no problem anywhere around 4000-5800ish ...

    my temp gauge seems to always be pegged at 100 degrees like it's not working... but i know it works bc when i run the motor hard for 10-15 minutes straight in some nasty water it creeps up to maybe 115-120 is the highest i've ever seen so far

    is that ok? is there a such thing as too cold, or is that where i'm supposed to be at? ... at an idle... the temp gauge needle will never move from the bottom of the gauge which starts at 100F

  12. #12
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    Quote Originally Posted by shibby1485 View Post
    i feel like i have the opposite problem strangely...

    i have a 99 250 EFI offshore

    my motor never gets hot! .... i have 5psi at idle and it gets to 10psi no problem anywhere around 4000-5800ish ...

    my temp gauge seems to always be pegged at 100 degrees like it's not working... but i know it works bc when i run the motor hard for 10-15 minutes straight in some nasty water it creeps up to maybe 115-120 is the highest i've ever seen so far

    is that ok? is there a such thing as too cold, or is that where i'm supposed to be at? ... at an idle... the temp gauge needle will never move from the bottom of the gauge which starts at 100F
    Shibby,

    It seems that the temps these 3.0L run at varies through out the years. Mine is a 2003, fortunatly I have smartcraft. I read 129 degrees at idle and 123 degrees running on plane. My pressure can get as high as 20-23 pounds at WOT. 12-14 at cruising speed.

    I do have analog gauges and I can tell you they are WAY, WAY OFF... My temp gauge reades 180-190 degrees on plane...!!!! Water pressure gauge is pretty close though, within 2 pounds.

    Your right though, these motors do run nice and cool on average. VDM must have a blockage of some sort.

    HV
    Boats love hundred dollar bills.

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