Engine trouble on 4.3 merc

[irishladdie]

Hi All,
Went out on the boat the other day and had problems with the engine. Mechanic came out and got it going again after changing 1 spark plug, but said to go back to the marina slow as there was still a problem on 4 cylinder.
They checked it out yesterday and came back and said the the piston was melted, saying that something had gotten into the engine water and blocked the supply to that cylinder. What i need to know from you guys is this.
1 Surely there is some sort of strainer on the intake of the water supply to stop this type of thing from happening.
2 If this was the case why only affect 1 cylinder
3 The heat required to melt a piston, ( if this is right as i live in Turkey and he is Turkish) would be way to high for an engine to still work. Maybe he means it has seized
4 Is there anything else you guys can think of that might have cause this.

This engine was completely rebuild at the beginning of May and had no more than 2.5 hrs running time on it, so is it possible that it was not build correctly.
I am now reluctant to let this mechanic work on my boat again, if the problem is from the rebuilding of the engine.
So hope some of you guys can give me some pointers.
Thanks

[cunuck]

I just got over the same problem melt 1 piston .I think it is called hydrolock it happens, riser fail or gasket or whatever lets water in .You must find the source or it can happen again . I just spent 3K for my fix and it was riser gasket failure $2.00 part.also check your shutters for there condition.I would be there when they take the intake manifold off to check for water .good luck

[ss3964spd]

Hydrolock and melting pistons are two different symptoms and are caused by two different problems.

Hydrolock occurs when water gets into a cylinder. As the piston is on it's way up in the cylinder during the compression stroke the intake and exhaust valves are closed. If water is in the cylinder during the compression stroke, and since water cannot be compressed, several things can happen; 1) the engine stops rotating immediately (locks up) 2) the piston breaks 3) the connecting rod bends or breaks 4) the crank shaft breaks. If the engine hydrolocks you will need to find the reason water was able to get into the engine.

A melted piston occurs when it gets too hot, which is typically caused by a lean fuel condition in that cylinder. In other words, the air to gas ratio has too much air/not enough gas. The incoming intake charge - which consists of a mixture of air and gas actually cools the intake valve, the cylinder, and the top of the piston just before the mixture ignites. A lean fuel mixture means there is more air and less gas which results in the mixture burning MUCH hotter than it should. After repeated cycles the temp in that cylinder rises dramatically and the piston, which is made of aluminum and has a lower melting point than the rest of the components, melts.

You will have to know if the piston broke or if it melted (those are symptoms) before you can determine what the actual problem is.

Dan