Why are my spark plugs disintegrated?

[JetboatBrad]

I keep having the same problem with one of my cylinders destroying the plug. Anyone have any ideas? The one before that had the ground electrode fried off and the threaded section belled out from, I’m assuming, heat. I just put this one in there 8 hours ago. Just checked it 2 hours ago. Anyone have a thought?

 

[osteodoc08]

Wow! Do you have a bore scope you can look at the top of the piston? When my 2 stroke plug looked like that, it ate a hole through the piston. That’s scary looking.

 

[JetboatBrad]

Wow! Do you have a bore scope you can look at the top of the piston? When my 2 stroke plug looked like that, it ate a hole through the piston. That’s scary looking.

No I don’t have one. I’ve never seen anything like it and I’ve been around cars most of my life. I can somewhat see the top of the piston and it’s pretty black. Did you ever figure out the cause?

 

[tdonoughue]

Looks like deposits to me. See How do I "read" a spark plug? | NGK Spark Plugs

You know it is not duration on that one. So that leaves you with oil and fuel...

 

[osteodoc08]

Can’t directly compare a 2 stroke with the Yamaha 4 stroke. Mine was an overheat. But that’s the only time I’ve ever seen a plug like that.

 

[tdonoughue]

So I have been trying to research what would cause the blue-green color on the insulator there. I think that is a clue. No joy yet (some suggestion from automotive that it might be antifreeze, but a) we don't have that and 2) antifreeze burns white). Usually that color is associated with copper... Don't have lots of additives in your gas tank, do you?

 

[JetboatBrad]

Looks like deposits to me. See How do I "read" a spark plug? | NGK Spark Plugs

You know it is not duration on that one. So that leaves you with oil and fuel...

Thanks T, I agree it does look like deposits on the pic but what I’m seeing on my plug is the plug itself being eaten or burned away. It’s not a foreign material on the end of the plug. The threaded section of the plug is starting to burn away. I’ll take a pic of a good one side by side to give you a reference.

 

[osteodoc08]

Do you have a way to check compression and leak down? Do you know how to ohm out your coil and does anyone know what the factory values are?

 

[JetboatBrad]

So I have been trying to research what would cause the blue-green color on the insulator there. I think that is a clue. No joy yet (some suggestion from automotive that it might be antifreeze, but a) we don't have that and 2) antifreeze burns white). Usually that color is associated with copper... Don't have lots of additives in your gas tank, do you?

No no additives. Winterized with stabilizer but that’s it. Thanks for looking on that.

 

[JetboatBrad]

Do you have a way to check compression and leak down? Do you know how to ohm out your coil and does anyone know what the factory values are?

I need to take it to someone to get a compression test. I swapped coils with another on different engine last time with the same end result. I have another on order but I don’t think the coil is the culprit. This is the 3rd plug now in the last month

 

[tdonoughue]

Odd that the base of the plug is melting away and you don't have more evidence of overheating on the electrode. Though that goes back to the fuel/overheating/timing issue theory...

 

[Cambo]

How many hours on the boat ? Did it run odd that lead you to check ?
that is roughlooking

 

[JetboatBrad]

Odd that the base of the plug is melting away and you don't have more evidence of overheating on the electrode. Though that goes back to the fuel/overheating/timing issue theory...

Would a previous overheat cause this? Earlier this year, I launched in a marina with a previously unknown bunch of weeds. I parked the boat at the dock and let it idle while I parked the truck. I came back to an exhaust overheat alarm. I couldn’t have been gone more than 5 minutes parking the truck. I hopped in and put it in reverse and immediately got water flowing. I’ve seen pistons melt on a overheat or the lower end start knocking but generally it’s bad right away. Here it is side by side with a new one

 

[JetboatBrad]

How many hours on the boat ? Did it run odd that lead you to check ?
that is roughlooking

Yea it started missing. A little over 400 hours on it. I could feel it and shut it down. I thought a fouled or faulty plug and it wouldn’t start when I tried to make it back to the launch. Pulled a charred hunk of metal and porcelain out

 

[tdonoughue]

You could have an oil leak from an overheat letting oil into the combustion chamber. Or you may have munched an injector in the overheat, so it is now spewing too much gasoline. Just spitballing here...

 

[JetboatBrad]

Yea I’m at a loss too. I would think there would be oil fouling the plug if excess of oil. Never had that particular injector problem so I don’t know what to look for. Seems like that comb chamber is extremely hot for some reason. I’m able to chip away at the threaded end of that plug like it’s just flaking off. Idk what could cause it to get so hot

 

[tdonoughue]

Can you pull the injector? I've never done that, but I am deducing that one way you can get that chamber too hot is to have too much fuel... I wonder if the tip is melted off.

 

[JetboatBrad]

Can you pull the injector? I've never done that, but I am deducing that one way you can get that chamber too hot is to have too much fuel... I wonder if the tip is melted off.

Yea let me see if I can. I was going to look at that next. I’ll keep you posted. Thanks all for the input

 

[Cambo]

Would a previous overheat cause this? Earlier this year, I launched in a marina with a previously unknown bunch of weeds. I parked the boat at the dock and let it idle while I parked the truck. I came back to an exhaust overheat alarm. I couldn’t have been gone more than 5 minutes parking the truck. I hopped in and put it in reverse and immediately got water flowing. I’ve seen pistons melt on a overheat or the lower end start knocking but generally it’s bad right away. Here it is side by side with a new one

In post 13 you are showing the spark plug next to another plug it looks like there is some type of sleeve / threaded insert on the burned up plug ?

400 hours... wow in two years are you running in salt water ?

how does the oil look any possible water contamination or gas smell ?

still have the warranty if so I would bring it in immediately so it can be documented .

 

[Gym]

I was looking at the same thing @Cambo mentioned about your fouled plug. Looks like a threaded sleeve. That larger threaded area on the plug will absorb more heat due to its greater mass. That could cause your plug to overheat.

Another possibility, as somebody mentioned, is injectors. If an injector is squirting droplets of gas rather than atomizing it you may be getting detonation in that cylinder rather than a clean burn. You may want to put 5 - 6 cans of Sea Foam into a full tank of gas to clean the injectors.