Mercury 125 Outboard overheated, stopped and won't turn over. heat seized?

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tanggly24

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I was Flushing my 2005 Mercury 125 HP 2 stroke outboard in November 2016 with fresh water as it is used in salt water. The muffs were hooked up but water wasn't peeing (it always takes a while with the muffs - scary long sometimes) anyway this time I was waiting, and went inside to answer the phone while my dad monitored the boat. He did not hear the overheating alarm and when I ran to shut it off it made a pop noise and stopped. Well... I put her away til the summer and feared the worst. So this summer I got her out and tried turning the fly wheel. it will turn almost 360 degrees and go all the way back but something is stopping it from rotating all the way. took out pistons (<--- EDIT--- thats a typo... i took out spark plugs )and I can see that the pistons all move in and out smoothly. I assume the crankshaft and connectors are intact. Took off the lower unit and does not seem to be a problem there as the rod rotates completely and turns the prop. Any ideas?
 
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UKMike

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What you have said makes no sense! Do you mean you took out the plugs and you could see all pistons working OK? If your flywheel won't keep on going until your pistons go through a full cycle you likely have a broken piston at the bottom or other broken obstruction in the crank.

Make a note of the piston cycles and see what number piston stops on the down stroke as this is probably where you have an issue!

Get a boroscope and take a look at each cylinder carefully....
 

tanggly24

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UKMike, I can turn the fly wheel almost a complete full turn , about 320 degrees, and then turn it back the opposite way. I can see all 4 pistons move in and out. I guess that tells me that the crank shaft is in tact and the connecting rods are intact. There must be some sort of obstruction. I can't think of what it could be. I can hold my thumb over each spark plug port and air will squeeze out as I turn the fly wheel. Any idea what could be obstructing it?
 

tanggly24

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Oh yes, your right. I meant i removed the spark plugs and could see the pistons moving. sorry. I did not remove the pistons ;)
 

flyingscott

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First thing you do is take the lower unit off and see if the flywheel frees up.
 

tanggly24

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Thanks. Removed the lower unit. Seems to be in good condition including the pump and impellor. fly wheel still does not rotate completely. It rotates by hand but brings up solid.
 

ondarvr

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Time to start ripping it apart, you'll be lucky if it's just a piston and rings.
 

UKMike

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It's likely to be one of the piston bottoms has dropped off, I've seen this before where one side of the piston drops off when they get too hot too often, you normally find it is just below the bearing.
 

tanggly24

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Thanks UKMike. thats fairly accurate. I got the engine stripped. The top piston was broken. actually the eye that the pin goes through the connecting rod was broke on one side. I ordered a new piston. I also noticed that the cylinder liner seems to be pulled back slightly and a piece broke of the edge closest to the crankshaft. My assessment is that the piston overheated so much that it grabbed the liner on the down stroke and pulled it back and then the eye broke on the piston. I'd like to change the liner but these do not seem to be readily available spare parts... Im an Mechanical Engineer so I have an understanding of how engines work but by no means am I a professional mechanic. Much respect to thus that are... If I could have found a shop to do this for me I would have... Both Marine engine repair places here were "too busy".

Anybody know if this liner needs to be replaced or can be pushed back in place. Ideally I am looking for a replacement, since its already apart.... but if I cant find one...
 

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Texasmark

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Thanks UKMike. thats fairly accurate. I got the engine stripped. The top piston was broken. actually the eye that the pin goes through the connecting rod was broke on one side. I ordered a new piston. I also noticed that the cylinder liner seems to be pulled back slightly and a piece broke of the edge closest to the crankshaft. My assessment is that the piston overheated so much that it grabbed the liner on the down stroke and pulled it back and then the eye broke on the piston. I'd like to change the liner but these do not seem to be readily available spare parts... Im an Mechanical Engineer so I have an understanding of how engines work but by no means am I a professional mechanic. Much respect to thus that are... If I could have found a shop to do this for me I would have... Both Marine engine repair places here were "too busy".

Anybody know if this liner needs to be replaced or can be pushed back in place. Ideally I am looking for a replacement, since its already apart.... but if I cant find one...

I never went to the "forge", but I am going to bet that there is enough friction in the casting of the aluminum block that you aren't going to successfully change it. Yes liners come out in farm tractors and OTR trucks, other heavy machinery that costs big bucks, but they are built that way and the block and liner built as separate parts. I don't know how sleeved, aluminum blocks are cast, but if something like a lawn mower or outboard motor, I'd say the liner is placed in the mold when pouring the block and no dice. But, that's a SWAG based on hunch.....logic? Common sense?????????? Not sure on that one. Grin
 

flyingscott

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https://www.lasleeve.com/marine/cylinder-sleeves they probably have one. You need a machine shop to do the work because it is a blind bore motor. Or you can send it right to La sleeve. Most new outboards do not have the sleeves cast into the block only pressed in that's why you here about them turning in the hole. Is this a 3 or 4 cylinder motor?
 
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