4.3 Merc

[catcatcher]

I have a 87 4.3 that has a hole in the #6 piston. The guy I got the boat from had the same happen but I'm not sure what piston it was. The spark plug was broke and the valves looked to have been beaten by what I am guessing was part of the spark plug. I was thinking of just rebuilding the #6 cylinder since the motor is new with only 15 hours on it but I want to find out what caused the hole in the first place so that it won't happen again. I heard there is a valve in the exhaust and if it goes bad it will allow water to dump in to a cylinder. Does anyone know if this is possible or have any idea what could have done it?

 

[Bondo]

Re: 4.3 Merc

Ayuh,........

My 1st Guess is,....... It suffered Extreme Detonation,+ Consumed itself....

I heard there is a valve in the exhaust and if it goes bad it will allow water to dump in to a cylinder.

Ayuh,.......

Reversion,+ Backwash can be an Issue,........
But,......
Your Symptons aren't at all right........

Btw,............

Welcome Aboard to the forums here at iboats.com......................

 

[catcatcher]

Re: 4.3 Merc

Thanks for the welcome!

What do the symptons suggest?

 

[Bondo]

Re: 4.3 Merc

Extreme Detonation,+ Consumed itself....

Detonation can be indued by quite a few things,........

A few are,........
Over Propping,.......
Timing Too Advanced,.......
Poor Fuel Quality,.......
Or,....
A Lean running Carb.........

 

[flabum]

Re: 4.3 Merc

Detonation can be indued by quite a few things,........

A few are,........
Over Propping,.......
Timing Too Advanced,.......
Poor Fuel Quality,.......
Or,....
A Lean running Carb.........

Also, the water reversion is probably a result of your problem, not the cause.

Were the spark plugs tight? loose spark plugs can cause the engine to run lean and detonate a piston.

 

[catcatcher]

Re: 4.3 Merc

The spark plug for #6 was loose.

The original prop was replaced by the last owner with a SS so I'm not sure what pitch it should be. If it is the wrong prop that might explain only getting 3500 rpms.

The #6 cylinder had a lot of water in it when I pulled the head off. I drained the motor before I took the head off and the head gasket was in perfect condition.

The fuel was fresh. I actuallydrained the 20 gallons out of the tank that had been in it for over a year when I got it.

As for the carb and timing I didn't check before it happened and it's a lil too late now I guess!

Thanks for the help I am trying to figure out the problem so it doesn't happen again when I rebuild.

 

[Bondo]

Re: 4.3 Merc

The original prop was replaced by the last owner with a SS so I'm not sure what pitch it should be. If it is the wrong prop that might explain only getting 3500 rpms.

Ayuh,.........

That's a Serious Problem right there............
That motor was Lugging,....... Badly...........

At this point, I'd say it's the Cause of the Issue............
'course,...... There maybe contributing factors,.....
But you won't know til it's together,+ Running to tune it properly.......

 

[MikDee]

Re: 4.3 Merc

I have a 87 4.3 that has a hole in the #6 piston. The guy I got the boat from had the same happen but I'm not sure what piston it was. The spark plug was broke and the valves looked to have been beaten by what I am guessing was part of the spark plug. I was thinking of just rebuilding the #6 cylinder since the motor is new with only 15 hours on it but I want to find out what caused the hole in the first place so that it won't happen again. I heard there is a valve in the exhaust and if it goes bad it will allow water to dump in to a cylinder. Does anyone know if this is possible or have any idea what could have done it?

I would guess you mean the flapper in the exhaust y-pipe, to stop the back wash from coming in.

 

[oops!]

Re: 4.3 Merc

did any of the valves hit the pistons?

it would have happened to all if it was the t-chain

 

[flabum]

Re: 4.3 Merc

The spark plug for #6 was loose.

.

I would bet this was the main cause of the detonation. I have seen MANY engine tulip valves and detonate pistons due to loose spark plugs. A loose spark plug will allow air to come in past it as the piston draws down to pull in the fuel/air mixture. With the extra air, that cylinder ran lean thus detonating the piston.

My bet is that # 6 is tough to get to for some reason causing whoever changed the plugs to not tighten it properly or he just plain forgot. If he had the same problem, perhaps it's the previous owner doing his own work and not tightening things properly.

You probably got the boat with the engine already going so you don't know what prop is right. A bad running engine won't turn up (reason for only 3500). After you get it rebuilt, start with a smaller prop and work your way up until you get within the rpm range it requires.

 

[catcatcher]

Re: 4.3 Merc

You are right #6 is a lil hard to get to but I think it was more the installers lack of knoledge or jus plain lazyness! I found a few nuts and bolts loose so I guess it is my fault for not checking further. Then again at least it went after a few test runs before I ran it in the bay striper fishing!

Thanks again for all the help! I fell more confident rebuilding and not having it happen again.

 

[flabum]

Re: 4.3 Merc

You are right #6 is a lil hard to get to but I think it was more the installers lack of knoledge or jus plain lazyness! I found a few nuts and bolts loose so I guess it is my fault for not checking further. Then again at least it went after a few test runs before I ran it in the bay striper fishing!

Thanks again for all the help! I fell more confident rebuilding and not having it happen again.

Don't blame yourself! There is just so many things you can check while buying a boat. If you had gone through everything with a fine tooth comb, you'd spend more than the actual cost of the boat. All you can really do is see how it runs, look for anything obvious and go from there. The only thing that would have prevented you from buying this boat would have been a full marine survey. Most boats under 30" do not get a full survey due to the expense involved. Most people looking at a $5,000 - $10,000 boat will forgo a survey since a good survey will cost around $1500. At least with a survey, if the surveyor missed something crucial, his insurance would have to cover it.

After you get this fixed, look into an extended warrantee contract for used boats....it may be well worth it.

 

[catcatcher]

Re: 4.3 Merc

Actually I didn't check the boat out barely at all when I got it. It is a 87 19' Sea Ray Seville CC. I got it from the original owner that kept it in a gerage since he bought it new so the interior is in exelent condition. I figured with the blue book value on it in great condition being $5,400 I couldn't go wrong even if I had to put a new motor in it. Oh yeah I got it for half a days work that I split with another guy and gave him a Galaxy bowrider with the infamous 470 in it. I was just happy to get rid of that alone!

 

[flabum]

Re: 4.3 Merc

Actually I didn't check the boat out barely at all when I got it. It is a 87 19' Sea Ray Seville CC. I got it from the original owner that kept it in a gerage since he bought it new so the interior is in exelent condition. I figured with the blue book value on it in great condition being $5,400 I couldn't go wrong even if I had to put a new motor in it. Oh yeah I got it for half a days work that I split with another guy and gave him a Galaxy bowrider with the infamous 470 in it. I was just happy to get rid of that alone!

Good deal. Now go fix that motor! :0)