Spark plug disintegrating in one cylinder

[JetboatBrad]

After I cried for about an hour once I discovered this spark plug blown out of the head. I proceeded to have to helicoil the threads and reinstall a new plug. About 10 engine hours later, I notice the port engine missing again. Looked any the plug L that sparks was blown off again and was just a charred knob. I thought maybe the first time I boneheadedly forgot to torque that one properly. But now that it did it again, something else is going on. Anyone have this problem ever? I can’t find anything on here like that. Thought maybe the piston was hitting it but it’s at the same depth on the head as the other plugs. Also I wasn’t able to find that missing piece of iridium with a magnet or vacuum either. Really hoping it blew out through the exhaust port.

 

[14SX190]

The threads look long.
what plug are you using?

 

[JetboatBrad]

The threads look long.
what plug are you using?

NGK- LFR6A
The first time it happened, which is the plug shown it was one that cross referenced. Second time it was the NGK’s. It’s odd in the fact that it doesn’t do it immediately. You’d think if the piston were to hit it, it would do it right away. It didn’t. It was a couple of outings later before it started missing again. Has me stumped

 

[JetboatBrad]

NGK- LFR6A
The first time it happened, which is the plug shown it was one that cross referenced. Second time it was the NGK’s. It’s odd in the fact that it doesn’t do it immediately. You’d think if the piston were to hit it, it would do it right away. It didn’t. It was a couple of outings later before it started missing again. Has me stumped. Also the same plugs in all 4 every time.

 

[JetboatBrad]

Just pulled the plugs again. The one on the right is the problem one and you can see where it’s depth in the combustion chamber looks so different from the others. I’m totally lost. Thanks In advance for any help you guys can give me.

 

[Sean R]

You still have YES? I was hoping your boat was a 2008 not 18. Thats sucks man.

 

[JetboatBrad]

You still have YES? I was hoping your boat was a 2008 not 18. Thats sucks man.

It does and the frustrating part is I can’t figure out why

 

[Ronnie]

Check the ignition coil for that plug. I had two plugs fail on me (Same cylinder different engines) last season and this season, the coil was shot on one plug and badly deteriorated on the other. I also had oil on the upper part of each plug, apparently the seal between the valve cover failed at that point and allows hot oil to exit onto the plug. Hopefully a new coil is all you need. They cost around $220 at my local yamaha dealer, $150ish online but after dealing with one faulty replacement coil I’ve decided to replace all of my coils with some from my dealer.

 

[JetboatBrad]

Yea thanks Ronnie. I got one on order because the first time it blew out, it cut the rubber about half way up. I just swapped that coil pack with one from the other engine so took that out of the equation but I can still see A little more vibration in the bad (port) engine. It’s not enough that it’s like a miss fire but still a little more vibration it seems like.

 

[tdonoughue]

Ok, I hate to even suggest this, but...

The other thing that could cause that (I think) would be if your timing chain or cam sensor were messed up. If the spark is too early on that cylinder, you may be blowing out the plug because the ignition happens while the piston is traveling up (further compressing into the explosion, rather than letting the explosion expand and push the piston down). That said, a 2018 is very early for one to have that kind of problem.

 

[jmargo]

Who did the helicoil job ? It is possible the helicoil is only the length of the clean threads and combustion gas is moving up the damaged threads to where the repair is sealed.

 

[Ronnie]

@JetboatBrad , if you can check the compression on the cylinder the the plug went bead on.

I lost the core of one plug in one of my bad cylinders and the electrode in the other. Now the compression on both of those cylinders is between 10% and 20% lower than the rest of the cylinders in the same engine. I plan to have the top ends rebuilt and all coils replaced this next off season. My engines have just under 500 hours on them. I think the coils will cost me close to $1k, the gaskets another $50, no clue about the labor or parts for the low compression cylinders but if I get out for under $2k I won’t cry hard or long. Definitely a lot cheaper than buying a another new used yamaha, especially if the next one isn’t already upgraded to my liking.

 

[JetboatBrad]

Who did the helicoil job ? It is possible the helicoil is only the length of the clean threads and combustion gas is moving up the damaged threads to where the repair is sealed.

Ok, I hate to even suggest this, but...

The other thing that could cause that (I think) would be if your timing chain or cam sensor were messed up. If the spark is too early on that cylinder, you may be blowing out the plug because the ignition happens while the piston is traveling up (further compressing into the explosion, rather than letting the explosion expand and push the piston down). That said, a 2018 is very early for one to have that kind of problem.

It only blew out once and I’m thinking that it may have vibrated loose. The second time, the plug was still tight in the threads but the combustion chamber end was totally scorched and L shaped piece was gone. I just took it out again today for a few hours. I pulled the plug and it looks ok. I’m thinking the helicoil threads is what’s Giving that extra length of discoloration.

 

[JetboatBrad]

Who did the helicoil job ? It is possible the helicoil is only the length of the clean threads and combustion gas is moving up the damaged threads to where the repair is sealed.

I Think you’re exactly right. I didn’t think of that before but would totally explain it.

 

[JetboatBrad]

@JetboatBrad , if you can check the compression on the cylinder the the plug went bead on.

I lost the core of one plug in one of my bad cylinders and the electrode in the other. Now the compression on both of those cylinders is between 10% and 20% lower than the rest of the cylinders in the same engine. I plan to have the top ends rebuilt and all coils replaced this next off season. My engines have just under 500 hours on them. I think the coils will cost me close to $1k, the gaskets another $50, no clue about the labor or parts for the low compression cylinders but if I get out for under $2k I won’t cry hard or long. Definitely a lot cheaper than buying a another new used yamaha, especially if the next one isn’t already upgraded to my liking.

I don’t know any other way of getting the compression back without rebuilding the lower end 30 over and new pistons and rings. Do you notice any performance difference? Or maybe a vibration because of it?

 

[JetboatBrad]

It only blew out once and I’m thinking that it may have vibrated loose. The second time, the plug was still tight in the threads but the combustion chamber end was totally scorched and L shaped piece was gone. I just took it out again today for a few hours. I pulled the plug and it looks ok. I’m thinking the helicoil threads is what’s Giving that extra length of discoloration.

Is there some way I can check for that? Or any other signs of that I can check for?

 

[Ronnie]

I don’t know any other way of getting the compression back without rebuilding the lower end 30 over and new pistons and rings. Do you notice any performance difference? Or maybe a vibration because of it?

The engines don’t spin all the way up to 7,600 / 7,700 rpm anymore, now they max out at 7,400 to 7,500 rpm. I’m assume I lost some top speed but not sure how much since the time out i got it to 47/48 on the speedo with 7 adults, fully geared up and a half tank of fuel. I hit 52 mph in the past but with a lot less weight in the boat.

If the acceleration is not as good as it used to be o can’t tell.

I was hoping to just replace the pistons and rings after honing the walls of the low compression cylinders but I have not taken it in yet. The shop I use repairs and maintains all kinds of pwcs and some jet boats. They also build and race custom freestyle skis (old school single person stand ups) so I will have the shop owner (who I trust) do whatever he thinks needs to be done).

 

[JetboatBrad]

Ok good to know. That’s not too bad really. I personally wouldn’t miss a few mph. Do you have a 21’? I can’t get mine over 42 but I’m at 7k feet

 

[Ronnie]

2010 242 LS. For context at wide open throttle (WOT) I’m burning 26 gallons of fuel per hour, at least it’s not super unleaded (me trying hard to see the silver lining).

 

[JetboatBrad]

2010 242 LS. For context at wide open throttle (WOT) I’m burning 26 gallons of fuel per hour, at least it’s not super unleaded (me trying hard to see the silver lining).

Yea definitely. I guess it could be a tad bit worse. Did you ever get any of the old blown plug out when you blew those? I couldn’t find anything and I tried pretty much everything. Vacuum, magnet, compressed air.