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Author Topic: M25XP Overheating at wits end  (Read 5680 times)

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Kevin Henderson

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Re: M25XP Overheating at wits end
« Reply #15 on: November 10, 2014, 07:48:04 PM »

 :clap :thumb:
Let me start off by saying that I would be completely lost if it were not for the excellent sources of information I receive from this association and this website.  THANK YOU EVERYONE!   

After receiving some fantastic suggestions for isolating my problem, I went to the boat yesterday with my work list.  These are the steps that I followed and the results I found. 
1.    I removed the thermostat from the system to allow the coolant pump to flow.  I started the engine and lifted off the manifold pressure cap to see if I could tell if the pump was working.  I didn’t really look like any different than any other time so it was not definitive of a test to see if in fact the pump was “pumping”.
2.   I reinstalled the T stat and then sought out a way to isolate and remove the hot water heater coolant line from the problem.  As suggested, I brought with me a small length of 3/8 inch rubber hose bought for 4 dollars at a local auto shop. I removed one end of the coolant line running from the water heater lop that runs into the coolant pump.  I then removed the other end of the coolant line that runs into the base of the Tstat. Having removed the hot water loop from the problem I then used the short piece of hose to directly run a closed loop of the coolant without it having go to the water heater.  I started the engine and ran under load.  The temperature of the engine never rose above 175!  AT LAST… I had discovered where the problem really was… somewhere in my hot water heater coolant loop. 
3.   Now focused... I proceeded to investigate the HW heater coolant loop.  I was certain that I had properly burped the system earlier in the season and in fact had motored several times with no problems.  Never the less I decided to thoroughly flush out that line and properly burp it again.  I attached my par pump to one end and started to suck the coolant out to a bucket.  The coolant I removed was very thick and slimy… smelled hot (from my overheat) and appeared to be a bit discolored.
4.   After cleaning out that line (I didn’t have any more coolant on hand) I grabbed a couple gallons of distilled water I had aboard and attached my par pump to the coolant hose and one end in the distilled water.  I pumped the entre line with water till I was certain no air was still in the line.  I then reattached the coolant lines from the hot water heater loop to the pump and the Tstat and started the engine.  I ran the engine under load at 2500 RPM for nearly a half hour and the temperature did not reach over 175!!!!  FIXED!!!
Lots of lessons to be learned here and a couple questions.
1.    I may have, in fact, been dealing with an air lock all along in the water heater coolant line.  I’ve read previous horror stories about the dreaded air lock and now I have lived it…. I think that may have been part of my problem.
2.   Can old coolant get “gunked up” in the coolant line and possibly combined with an air bubble, create enough of a block that the coolant pump cannot pull fluid?
3.   Another little item also proved to be my downfall during this situation.  Somehow, earlier in the season, my little pressure relief valve located on top of the thermostat became a bit loose, this allowed a release of pressure and slow coolant loss… coupled with the air lock blockage to overheat. 
4.    Using a small PAR pump to  flush the coolant line and remove all air is very easy and in my opinion works better that the burping method from the thermostat.
THANK YOU TO EVERYONE WHO OFFERED SUGGESTIONS.!  And I hope that my experience will help others before they experience the dreaded hot water heater coolant line air lock!   :abd:
The sail, the play of its pulse so like our own lives: so thin and yet so full of life, so noiseless when it labors hardest, so noisy and impatient when least effective.
~Henry David Thoreau


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Re: M25XP Overheating at wits end
« Reply #16 on: November 10, 2014, 08:14:58 PM »


I hope it holds.  I haven't seen coolant go to gel, but I suppose if there was "stuff" in it maybe.

However - I don't doubt whatsoever what you found and the immediate results, but one thing (kinda major).
The water heater loop has nothing to do with the cooling of the engine (once the Tstat opens,) so the result (no overheat) doesn't make sense with the cause (gunked up WH hose.)

That loop is separate from the loop that runs from the block, thru the Tstat, thru the exhaust manifold, thru the heat exchanger, etc. and back to the block.   And once the water heater gets hot, it doesn't 'suck any more heat" out of the engine coolant.

Before the Tstat opens, though, it does have a function. 
It runs off the coolant pump -- basically the coolant pump circulates coolant from the block, thru the water heater, and back to the block (underneath the thermostat.) But, that's only to keep a little coolant circulating so no hot spots develop anywhere in the block.  "Theoretically" you could pinch off the WH loop and it would not make the engine overheat (it might have hot spots around cylinder walls, but generally not overheat)  because the Tstat would open and the Hx cool it down.
I believe more likely the actual cause was an airlock in the loop with the heat exchanger -- I had not concentrated on that because you said that air was all burped out.

This is all presuming that your WH hoses are plumbed "normal like" 
Do they originate at the top back of coolant pump and at the base of the thermostat housing???

Ken K
Twenty years from now you'll be more disappointed by the things you didn't do, than by the ones you did.
So throw off the bowlines.  Sail away from the safe harbor.  Catch the tradewinds in your sails.
Explore.  Dream.  Discover.   -Mark Twain
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