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  1. #1
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    1986 Mercury Black Max 175 overheating

    1986 Mercury Black Max 175 overheating

    New to the forum and some what new to boating.


    First, I have read all the threads related to overheating issues for black max engines seems like others might have had a similar issue but no solution was ever posted - hate that...

    Second, here is what I know. ..Bought the boat from an auction in Norfolk Va. I know nothing about past history of the engine or boat.

    I checked Plugs, spark and compression when it first arrived. Plugs looked od, slightly on rich side but would expect from 2 stroke. Compression uniform, slightly low, around 80 -90 psi when I checked it. Not sure how long the boat sat but no longer than a year. I have not rechecked compression since taking it out on the lake twice. Will put on the list to recheck.

    First time out, boat ran fine but when at WOT the temp climbed fast.

    I could hear the alarm, temp gauge up to or close to 220 F

    Boat has a water pressure gauge- when the boat is around 3000 RPM the pressure pegs at 35 psi.

    Low and mid speeds the boat temp is ok.

    Took poppet valve out, look OK and that PO recently replaced it. Water passages looked very clean.

    Any thoughts, not sure if need to take heads off and check water passages. I am assuming the thermostats are fine because of the water pressure I am seeing at WOT. They also look like they might have been replaced recently but have not yet pulled the covers off them.

    Thanks for any help.

  2. #2
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    I think re-check on compression and thermostat inspection are good ideas. Your water pressure is way high. I would inspect that poppit closely and be sure the seat is still staked in place. I have seen them come loose and follow the poppit not allowing it to open and relieve pressure. You also may have an adapter plugged not allowing water out but would look there as a last resort. Good luck and welcome to the site.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  3. #3
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    I’m leaning towards the worst on this one. T-stats and poppit recently replaced, boat ended up at auction?

    Compression and leak down checks are where I think this one is headed...

  4. #4
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    When you refer to staked in seat, presume that is staked to the body of the head? It also looks like they used an updated poppet since the gasket was hand cut to have one hole as my engine is suppose to have and not the new version engine that had two holes. Not sure if that could be an issue.

    Will check compression again, but would that cause it to run hot, all cylinders have the same pressure so assume head gaskets are fine.

    Thanks and again any help is appreciated

  5. #5
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    I would pull those t stats. I had the same problem and it was some marsh grass clogging one. You can check the operation by suspending them with a peice of wire in pot of water on the stove they should open before the water boils

  6. #6
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    The seat is staked into the block. You will see several dimples around seat. Stick your finger in there and see if it moves. Testing thermostats is a good idea but I would use a thermometer to see what temp they open at. 122 and 142 are well before boiling and I would hate to see them open "before boiling" and be opening at 190. Odds are your compression is fine and your gauge reading low. Should be around 120 psi and if it is really that low you could have aluminum building up in cylinder causing excess friction therefore heat.

    Rock
    Team Junk

    No sparkling wiggles in here, only dump truck grinches.

    "Screamin Heathen"

  7. #7
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    first off you have no idea of tha "history" of tha motor so start from "square one".....

    pressure to high.. ya have a restriction some where.. can be anywhere in tha water path... stats make no difference in pressure.. new ta you motor with heat problem, start at tha bottom and go up.. ya have no idea what tha problem is.. need to replace tha complete water pump and parts with new.. need to replace poppet assy. with new.. stats n be "checked" for function.. replaced if they don't work.. after ya "fix" those items and still have a problem that's when ya start lookin' for tha cause.. don't jack with tryin' ta fix "used parts" ona "unknown motor".. no tellin' what/where its gonna be but ya have a good start with new junk where it maters.... jmo.
    Last edited by tlwjkw; 05-28-2018 at 10:43 AM.

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  9. #8
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    Compression tested today- right bank (1-3-5) all 115 psi cold with throttle closed, so warm with throttle open another 5% to add- so we are good here....
    left bank (2-4-6) all 120 psi cold with throttle closed, so warm with throttle open another 5% to add- so we are good here....

    removed thermo covers and..................... surprised no thermos only what appears to be the end piece of poppet valve assembly-- like a cupped washer with a hole in it.


    What the hell not sure why, but that would seem to restrict my flow like crazy and cause over heating. Someone mentioned replacing the water pump impeller IClick image for larger version. 

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ID:	408714 would say yes but pressure is high on the pressure meter. Would replace that later as a more preventative work.

  10. #9
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    those are "restricter" washer used with a hot rod coolin' type system.. go back and re-read carefully what I posted earlier.. that's a starter for fixin' what ails tha motor..... get yourself an oem manual or your gonna STAY completely lost with jus guessin' what ya doin'...……………….. i'll tell ya again, stats and/or in your case, washers will not effect PRESSURE...………… by some t-stats.
    Last edited by tlwjkw; 05-28-2018 at 05:25 PM.

  11. #10
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    Rubber impeller pieces can get stuck in tube under power head. Might Remove lower and back flush from t stats down see what the flow is my guess get some new 143 t stats

  12. #11
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    will pick up t stats , seems like the 143 are correct for mine. Will also replace poppet. Water pump leave for now as pressure makes sense with rpms and the restriction that the restricter washers put on the system.

  13. #12
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    washers and/or stats have NOOOOOOOOOO bearing on pressure!!!!… have ya even looked at tha pump?... it could be makin' 5 lbs at high rpm and with a restriction somewhere in tha flow will show off tha chart!!!

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  15. #13
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    yes I know that, but in my post I mentioned I have 35 psi at WTO, 2000-3000 RPM about 20 psi, that to me ( and for sure could be wrong) that the water pump is putting out, the more rpms the turns, force and water, if the flow is restricted from what ever poppet valve, restricter washers the pressure would be up, flow down so cooling capacity is lost.

    Hence the good pressure means good pump but restricted flow?

  16. #14
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    you started this thread for an over temp problem with a motor ya no absolutely nothin' about its history.. I gave ya my answer... good luck with it!

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  18. #15
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    Have you verified that water pressure gauge is accurate...if it is almost sound like a blockage on the return flow side, like maybe a plugged mid plate..

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