Mysterious broken spark plugs + Bad compression test (With Pics)

[tahmad575]

Hello boat kings,

I recently swapped out both my ECU's and the boat has been running fine up until a couple days ago my starboard engine started to only rev to 6k rpms. Port engine is good runs smooth up to 9k rpms.

No smell of fuel on the starboard oil dipstick but I decided to purchase a cheap compression test kit off eBay for $20
Big mistake because it is completely useless

To my surprise when I pulled out the spark plugs 5 of them were missing the electrode tip and core! The 3 that are good and intact have proper gaps as well. all 8 spark plugs were changed at the beginning of the season with ngk cr9eix irridiums, Plugs were checked and all were good when I swapped out the ecu's last month.
I wonder what could have caused the tips to break? also a slight amount of oil was on the threads of some of the spark plugs as seen in the photos

Test results:
Starboard
#1 50psi broken plug
#2 45psi good gap
#3 30psi broken plug
#4 50psi good gap

Port:
#1 50psi broken plug
#2 50psi broken plug
#3 125psi good gap
#4 55psi broken plug

These are obviously not accurate because you could hear the compression tester leaking where the gauge hooks onto the hose with each crank. On port #3 ALOT of water was spraying out of the compression tester tube where it connects to the gauge..wonder where the water came into the #3 cylinder from ?
Engines were cold and throttles we're at full open, someone in a previous thread said his numbers didn't vary significantly from cold or hot readings but regardless I think this was a completely inaccurate test because the gauge and connection that came with it are trash

As for the plugs ...it's a mystery. The boat was running fine two days ago on the Hudson River except for the starboard engine which only revved to 6k rpm about 5 minutes into my voyage. I decided to leave it at 5k rpm and use the port at 8k rpm, that's how I cruised for about an hour, no changes in rpms or performance really. Just don't know how my port could have performed so well with 3 broken plugs inside the engine..must have happened after? I litterally just winched it onto to the trailer and drove to my warehouse and then did this test..unless they broke during the drive?

 

[Beachbummer]

Any chance it's a bad batch and the plugs failed this way by having its internals shoved in? the electrode tab(not sure what this piece is called, the metal piece that lands the arc) looks intact. I suspect your compression measurements are not correct. Did you open the throttle to WOT to take it? did you crank until the pressure stabilized?

I recommend new plugs and new test.

Best of Luck!

 

[tahmad575]

Yeah so for some of the broken plugs the tips look broken off and the core intact but for 2 of them the white core piece and tip are both broken off, I don't think the internals were shoved in because the ground electrode ( L curved metal peice) would be broken too
I think somehow they broke off and got spat out through the exhaust valve after probably causing some cylinder damage

Also yes I did the test at full open throttle and cranked until the reading was steady

 

[Liveto99]

I asked my dealer about different plugs and he said if you just keep the motor stock he sees rental agencies gettin a couple thousand hours out of them with people killing them on Jet skis. He said don’t upgrade the air cleaner, don’t upgrade the plugs, just leave it stock and do the service on it.

 

[rkluck]

I am no expert but I have some thoughts. First your compression check is probably meaningless since you could hear leaks and the compression is so low. The #3 that had higher compression is the one that you said had water. The liquid may have helped so seal it more resulting in a better reading. So the first question is are you sure that was water? You should not have water in the cylinder. That would be a a more serious problem in the engine. You said everything has been running and then you had the issue with the RPMs. No other issues right? I would first just swap out the plugs. Hopefully she will run fine and at least you know everything is working mechanically. Did you change the plugs? Are you sure they were set correctly? There have been instances of bad plugs as well. Usually she won't idle well with bad plugs but you didn't mention that. I would start with good plugs set properly first. That will tell you a lot.

 

[Cambo]

Do you know what cylinders had the plugs with broken electrodes? Those pieces could be in the cylinder try to feed a tube in and vacuum them out then turn the motor over with no plugs to try and eject the pieces re vacuum. Get the debri out

 

[Wishfull1]

Probably not what you want to hear, but I had a Seadoo that did something similar, cracked the electrode ceramic off.

Trashed the pistons, locked up the rings and scored the cylinders. Hopefully yours blew out the exhaust without damage.

 

[Neutron]

Doubt the pieces came out the exhaust unless they were microscopic because the valves open very little so anything going by would have to be smaller than the tip of a pencil.
If they are jamming your valves open, your compression readings would be very low.

 

[swatski]

Doubt the pieces came out the exhaust unless they were microscopic because the valves open very little so anything going by would have to be smaller than the tip of a pencil.
If they are jamming your valves open, your compression readings would be very low.

I was hoping you would chime in!
This is a head scratch-er, isn't it?

The OP is a bit confusing as far as which engine does what RPM at WOT, but I find it unfathomable for an engine to run its max RPM under load without all cylinders working...

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[tahmad575]

@swatski at wide open throttle the left port engine is smooth and good running at 9k rpms the right starboard driver side engine was at 6k rpms not reaching it's full potential
Idle on both engines was fine and yes no other issues just low rpm on starboard
@Cambo where can I buy that vacuum tube? Or could one of those long thin magnet pickup things work?
@Neutron do you think I should try starting the engine with new plugs or wait until I get an endoscope and look for damage and stuck valves?
@rkluck yeah I'm pretty positive it was water it was odorless and clear maybe it came in from the exhaust somehow?
@Liveto99 so far it's all stock, only thing was irridium spark plugs I figured they were stronger but I'll use the regular ngk cr9eb now

 

[Neutron]

@swatski at wide open throttle the left port engine is smooth and good running at 9k rpms the right starboard driver side engine was at 6k rpms not reaching it's full potential
Idle on both engines was fine and yes no other issues just low rpm on starboard
@Cambo where can I buy that vacuum tube? Or could one of those long thin magnet pickup things work?
@Neutron do you think I should try starting the engine with new plugs or wait until I get an endoscope and look for damage and stuck valves?
@rkluck yeah I'm pretty positive it was water it was odorless and clear maybe it came in from the exhaust somehow?
@Liveto99 so far it's all stock, only thing was irridium spark plugs I figured they were stronger but I'll use the regular ngk cr9eb now

I wouldn't run/or crank that engine at all till its inspected with a scope. They are cheap now with bluetooth to your phone. Check Amazon.
The less damage you cause the better.
Waiting is your cheapest route right now. I know it sucks but better to see nothing in there and the try new plugs. You put plugs and run it it could be bad news if chunks are still in there

 

[tahmad575]

@Neutron that's true i'm going to harbor freight to pick one up so I can find out within a couple hours from now

 

[Brad460]

I wouldn't run/or crank that engine at all till its inspected with a scope. They are cheap now with bluetooth to your phone. Check Amazon.
The less damage you cause the better.
Waiting is your cheapest route right now. I know it sucks but better to see nothing in there and the try new plugs. You put plugs and run it it could be bad news if chunks are still in there

Agreed- get a bore scope in there and take a look. Keep in mind the bore scope magnifies everything and so what might look like a large scratch ..etc may be very inconsequential...regardless, look for shrapnel...

 

[tahmad575]

Got a $80 boroscope from harbor freight, really bad camera quality but didn't find any shrapnel, around 2 - 3 scratches per engine on the cylinder wall. I'm guessing the plug tips disintegrated and exited though the exhaust valves

Then I hooked up a thin clear tube to an air compressor, put it through the spark plug hole and shot air through each cylinder to loosen up any debree, looked again with the scope and couldn't find anything.
After that I used the same thin tube and gorilla taped it inside a shop vac hose to suck out any debree that I couldn't see, pretty good suction on that thing so it would have gotten anything out.

Proceeded to replace all the spark plugs with ngk cr9eix irridium's. The manager at Advanced Auto said he had seen that happen to plugs before but on cheap small engine plugs, said it could have been a bad batch like what @Beachbummer said, also he said it could have been a fake ngk supplier out there. I purchased these from Amazon. If it happens again I'll buy direct from advanced Auto since he said they buy direct from ngk.

Went out on the river today, really calm and right off the bat both engines performed smoother than ever, at wide open throttle I got up to 9,700 rpm on the starboard engine (14 years old, as old as the boat) and 10,000 rpm on the port engine (was sbt rebuilt in 2017). I don't like doing that because I'm afraid it's not good for the engines so I try to keep them at or below 8500rpm.

I appreciate all the help thanks ?

 

[Jgorm]

That's bizarre. Are you sure the ecu is designed for your engine? Detonation can crack plugs. I'd look at the new plugs after a short wot blast and cut the throttle. Examine for afr and detonation.

 

[Beachbummer]

After trying iridium on my seadoo, I found they did not perform any better than OEM on that application at least. Get the oem one next time and you will see for yourself.

Glad the boat is working well!

 

[rkluck]

Glad to hear all is well. There have been several reports of bad spark plugs. Now that you know the engines are good you may want to verify you have the right ECU. Since she is running good and normal I am thinking probably they spark plugs were defective or not installed correctly. You can easily check your plugs for awhile and make sure they are looking good.

 

[swatski]

After trying iridium on my seadoo, I found they did not perform any better than OEM on that application at least. Get the oem one next time and you will see for yourself.

Glad the boat is working well!

I believe that's been pretty well established in both MR-1 and 1.8 Yamaha engines, there is no "upgrade" to OEM plugs. Iridium plugs are not considered an upgrade, based on this thread pictures I would probably want to stay away. I ran iridiums in a 1.8 engine for a part of a season, no difference from OEM.

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[Brad460]

That's bizarre. Are you sure the ecu is designed for your engine? Detonation can crack plugs. I'd look at the new plugs after a short wot blast and cut the throttle. Examine for afr and detonation.

This is a great point..

Usually the ground electrode will melt in the case of detonation, but nonetheless the plug will be damaged.

If the timing it too advanced the engine will start knocking. In addition if the ECU is not designed for your engine the ECU may not be able to properly control that knock.

OP- Did you see any signs of detonation on the pistons?

 

[Neutron]

Anything other than oem plugs are just a waste of extra money. Stick with stock.
If there is no benefit then why.
If its to go longer between plug changes than that is a bad reason. Changing them once a year will prevent damaging threads