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  1. #1
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    Water Coming from Thermostat Housing when motor is cold. Overheat at WOT

    Hi,

    I've built up a little 14ft 1976 'Swift Craft Stiletto'. I found a nice 1986 2.4 200HP Mercury Black Max on ebay to power it. A bit of a lucky dip but it's turned out well.

    I'm not sure if this is an issue, but as soon as I start it, water comes out from the thermostat housing outlet ... and its warm ... within just a few seconds of starting. I would have thought that no water would come out until the stats open ?

    It's like the T-stats are jammed open. They where changed recently (along with pump / impeller)

    Also at idle it never heats up over 120f so I don't believe the stats even open at idle (eg during a flush).

    I note that the poppet has been disabled on this motor which could explain why it runs cool at idle.

    It also overheats if WOT is held for too long (a minute). The left bank is hotter than the right by about 15-20. I suspect some blockage which I am treating with vinegar flushes atm (it was a salt water motor before I got it).

    Here is a short video showing it overheating briefly (195f) .. then I back off and its back at normal temp within 30 seconds. I am measuring head temp under head bolts.

    https://www.youtube.com/watch?v=33VZvnq3DGU

    So my question is, why is there water coming from the thermostat housing when the stats should be closed ?

    Cheers
    Steve

  2. #2
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    Here is a link to a thread that "might" help some.

    http://www.screamandfly.com/showthre...telltale-issue

    As far as the disabled poppet....just what was done?

    A stock 86 2.4 needs t-stats ( or restrictors ) and a functioning poppet.

    Also it doesn't take much of an opening to bypass the t-stat. and get a pee stream. I remember from many years ago, that some t-stats had a small open passage to allow a constant telltale stream when closed.

    If you don't have a water pressure gauge, you really need to consider adding one. Temp gauge only gives 1/2 the needed information...jmo

    HTH

    James H.
    Last edited by W2F a V-King; 01-23-2017 at 12:20 PM. Reason: added link

  3. #3
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    If you examine a thermostat closely, you'll find small dimples in the flange. The dimples are the "hidden" stat bypasses that nobody can ever find. So when the stats are closed, they still flow water.

    A non functioning poppet (assuming it's closed) will have no effect at idle but will cause overheating at higher rpm. The additional water flow is needed to shed heat, so that should be the area of investigation.

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  5. #4
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    And take James advice and install a pressure gauge asap.

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  7. #5
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    Quote Originally Posted by W2F a V-King View Post
    Here is a link to a thread that "might" help some.



    Also it doesn't take much of an opening to bypass the t-stat. and get a pee stream. I remember from many years ago, that some t-stats had a small open passage to allow a constant telltale stream when closed.

    If you don't have a water pressure gauge, you really need to consider adding one. Temp gauge only gives 1/2 the needed information...jmo

    Thanks for the replies from everyone. Yeah I noticed a 'Thermostat Vent Tube' in the cooling system diagram - http://www.biggerhammer.net/mercury/...5_blizzard.jpg

    Thought it might be something like that, but I gotta tell you that its pouring water out of those housings ... if its some kinda t-stat bypass, then its a big one.

    I'll get a photo of the poppet shortly. I havn't opened it on account of it been disabled. I would have hoped it had been jammed open somehow, but if its jammed shut, then that'll be the source of the high RPM overheat.

    Agree on the water pressure gauge, I was thinking the same thing myself.
    Last edited by Steve200; 01-23-2017 at 05:52 PM.

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    Here is a picture of my poppet. Does that tell us if its jammed open or shut ? Click image for larger version. 

Name:	20170124_112143.jpg 
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ID:	365353

  9. #7
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    Should be a hose going from a 90 deg fitting on the poppet to a 90 deg fitting on the top of the block. Just rebuild the poppet and re-rig it like stock and you should be good....
    Last edited by XstreamVking; 01-23-2017 at 07:48 PM.

    83 V-King, Mercury 3.0 back on the transom
    Stop wearing the face bags people. Don't be a sheep!
    Rebuild thread:
    http://www.screamandfly.com/showthre...-it&highlight=
    http://www.screamandfly.com/showthre...cs.&highlight=
    Videos

  10. #8
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    yeah, but there isn't, so what does do to the cooling system ? Does it mean less water at high RPM or too much water at lower RPM ? It idles cold around 120 .. not even t-stat open temp.

  11. #9
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    120 at idle isn't bad, as soon as you plane off it goes to the poppet and the stats only flow some of the water. No poppet vent hose is causing a restriction in the water flow when it needs it the most. The poppet is really just a pressure regulator. I just put one in my opti today. Idled at good temps but rose after planing off. Poppet was jammed with grass and shells. Now back to normal. Idles at 125 deg, runs out at 140 deg. Different motor but same principle.

    83 V-King, Mercury 3.0 back on the transom
    Stop wearing the face bags people. Don't be a sheep!
    Rebuild thread:
    http://www.screamandfly.com/showthre...-it&highlight=
    http://www.screamandfly.com/showthre...cs.&highlight=
    Videos

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  13. #10
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    ok thx for the info. Ive been confused on how the poppet works. I've read 2 things.

    1 that it opens at higher RPM to reduce the water pressure (dump water on the exhaust via the bypass) and
    2 that it opens at higher RPM to increase the water flow requirements.

    Anyway I am not sure why it was blocked off, there is a hole in the casing, so maybe they couldn't be bothered to fix it and just blocked it off. It's a 30 year old motor so who knows what has been done to the poor thing !

    btw I can run at 5000rpm all day long and it won't overheat, its the last 1000RPM of the rev range that pushes the temp to overheat.
    Last edited by Steve200; 01-23-2017 at 10:57 PM.

  14. #11
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    Yes, and that last 1000 rpm is where all the fun is at!!

  15. #12
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    Quote Originally Posted by W2F a V-King View Post
    Yes, and that last 1000 rpm is where all the fun is at!!
    Absolutely !

  16. #13
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    Well I am still getting a very solid flow of water coming from my stats still when the engine starts cold.

    I took the stats out and they are 100% .. they are only a few months old.

    So could it be that the pressure from the poppet valve being jammed open is enough to push the thermostat open ? I can't see how else water can be flowing through the cold t-stat

  17. #14
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    Prob silly question, do you have the rubber seals on the t-stats?

    83 V-King, Mercury 3.0 back on the transom
    Stop wearing the face bags people. Don't be a sheep!
    Rebuild thread:
    http://www.screamandfly.com/showthre...-it&highlight=
    http://www.screamandfly.com/showthre...cs.&highlight=
    Videos

  18. #15
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    Yeah I do and the right way around too ... I even put silicon inbetween the rubber and the stat to ensure water wasn't getting through.

    Maybe the stats arn't seating right ... but both of them ? Weird. That's why I thought maybe there is enough pressure in the system to force the stats open. It could be that they arn't seating right, one of the t-stat housing has some epoxy work on it.

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