1990 90 hp Johnson V4 cylinder burn down

[jaylo600]

I just picked up a winter project yesterday and tonight I dove into it. Like the title says I have a melted top of one of the pistons but the other 3 look great. I knew the engine had problems but I bought the boat on a gamble. I was just wondering if anyone could tell me why only one piston went bad and not the one underneith it also ? I apologize as I don't know wich # cylinder it is. I have read the good and bad about the VRO systems but there is no way that it was to blame in this incident. Is it possible there is a dirty carb and it caused a lean condition in one cylinder only ? I will become more educated on this project as I go but I'm trying to pick some brains out there so I know what common things to look for on this motor. Thanks for reading !

[jayyyy]

A melted piston. Are you sure it melted? Long before a piston got hot enough to melt it would have scuffed the cylinder and broke the piston, wrist pin, or rod. I could be wrong though.

[Dhadley]

Where the piston is damaged and what the actual damage looks like will tell the tale. Could be as simple as someone had the wrong spark plugs in the motor. Tell us more about the damage, a pic will really help........

[jaylo600]

Johnson 90.jpg Here's the photo, as you can see it's definately started to melt. Very little damage to the cylinder wall, so a bore or even a hone will fix it. Just wondering how this could happen. I've had this on fan cooled snowmobile engines before and it was a drty carb causing a lean condition that burned a hole clean through the top of the piston, but this outboard is totally new to me.

[Sprky]

That is detonation. My guess is heat. If you look at the diverter between the cylinders is appears as though it has completely blocked the water path around the #2cyl.

[retiredfornow]

How does the head look?

[jaylo600]

How does the head look?

The head is fine, it has no signs of burn or melting at all. When I pulled the head there was large flakes of metal peeling from the top of the piston but the head is in good shape. There was no spark plugs installed either so I don't know what plugs were being used.

[jaylo600]

That is detonation. My guess is heat. If you look at the diverter between the cylinders is appears as though it has completely blocked the water path around the #2cyl.

Exactly what is the purpose of the diverters, I was wondering about those things.

[Sprky]

Exactly what is the purpose of the diverters, I was wondering about those things.

To divert water around the cylinders in a manner that promotes the best cooling possible.

The diverter in the middle has occluded the water path around the top cyl. Pull it out and install a new diverter. Use a piece of fuel line or hose of some sort.

[Dhadley]

Don't worry too much about boring or not, it will need to be bored. First, with that much combustion heat it will be out of round beyond spec. Second, even if it is still in spec the highest cost of rebuilding (per cylinder) is the cost of the piston. The cost to bore vs hone only is minimal. And obviously we're not going to reuse that piston. No sense buying a new piston and putting it in a worn cylinder.

[jaylo600]

I never even concidered the heat damage to the bore, but your probably right, it will need boring. I'm just glad I have all winter to take my time and rebuild it properly. When I get it torn down completely I will post some more pics of the block and other three pistons.

[emdsapmgr]

The diverter being out of place affects the cooling to that one cylinder. It is possible that the prior owner never got an overheat warning horn.

[jaylo600]

Okay, I'm dragging this thread up again now that I'm all torn down and ready to order my parts. I could of sworn I read somewhere that guys have used pieces of fuel line to replace the diverters. Just wondering if this is possible or should I just order the oem ones from brp ? Also , I've had my brother help me with this motor and he believes that the water travels through the centre of the diverter as well and the reason for the meltdown is due to the fact that one of the diverters was plugged solid. Any thoughts on this guys ??

[daselbee]

The problem is indeed the swollen water diverter. Your question regarding the water flowing thru the diverter....not the case. The fact that the diverter is hollow but plugged is irrelevant to the problem. Who knows why OMC chose hollow tubing to do this job....but it could have been engineered with solid o-ring stock just as well.

[jaylo600]

The problem is indeed the swollen water diverter. Your question regarding the water flowing thru the diverter....not the case. The fact that the diverter is hollow but plugged is irrelevant to the problem. Who knows why OMC chose hollow tubing to do this job....but it could have been engineered with solid o-ring stock just as well.

I have o-ring stock at work , I wonder if I could make it work ?? Thanks for the reply.

[daselbee]

well, just be sure that whatever material you use, that it does not block the flow up and around the cylinder outside walls. The diverter is just supposed to block the water flow thru/between the two cylinders.
O-ring stock may have a permanent curve in it from manufacturing, and might shift position...so, I suggest that you use the originally designed deflector material.

I was just rattling on about how they could have/ might have used something else....

The newer V6s have what appears to be 1/2 in. o-ring stock in at least 6 places for diverting.

[jaylo600]

ya I decided to order the oem ones (prt # 0322411 for anyone interested), I don't want to open this engine up again.

[Faztbullet]

Who knows why OMC chose hollow tubing to do this job

They used it as it was a stock item, it is the non reinforced fuel line from the old double line engines split.

[toddschubert]

Good call on ordering new deflectors. Other material may come apart or deteriorate in some other fashion. When round they allow water to flow around passage closest to head bolt hole. I just had to replace mine as they were the culprit in an overheat prob. after I replaced entire cooling system. Don't ya just love taking that thermostat on and off?!! lol

[daselbee]

Thermos on these are absolutely NUTS! Absolutely my lease favorite job. It is almost easier to pull the powerhead to get to the thermos than fight with that cramped, ridiculous engineering design.

[jaylo600]

Thermos on these are absolutely NUTS! Absolutely my lease favorite job. It is almost easier to pull the powerhead to get to the thermos than fight with that cramped, ridiculous engineering design.

I can see what you mean, fortunately I have my powerhead off and the bottom shroud off also, nice and easy now ! lol

[toddschubert]

I can see what you mean, fortunately I have my powerhead off and the bottom shroud off also, nice and easy now ! lol

I found that loosening the lower cover helps a little but that's no fun either. Small fingers and tiny incriments

[BrianCrosley]

I just picked up a winter project yesterday and tonight I dove into it. Like the title says I have a melted top of one of the pistons but the other 3 look great. I knew the engine had problems but I bought the boat on a gamble. I was just wondering if anyone could tell me why only one piston went bad and not the one underneith it also ? I apologize as I don't know wich # cylinder it is. I have read the good and bad about the VRO systems but there is no way that it was to blame in this incident. Is it possible there is a dirty carb and it caused a lean condition in one cylinder only ? I will become more educated on this project as I go but I'm trying to pick some brains out there so I know what common things to look for on this motor. Thanks for reading !

ANSWER=Your #1 top Piston melted because your timing is advanced.

[emdsapmgr]

I really doubt the timing is the issue. If it were advanced, other 3 cyls should also show distress and they look fine. Likely the bad cyl ran very hot, which basically means it ran lean-due to the misplaced water diverter. That mis-shaped diverter has completely closed off water circulation around that cylinder.

[BrianCrosley]

ANSWER=Your #1 top Piston melted because your timing is advanced.

I have been working on these motors and rebuilding them for 25 years, and this is what I have found, it may be beneficial to you to get your timing checked.