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2018 F115LB Spark Plug Issue

klossfam

klossfam

New member
Hopefully a fair question but at 159 hrs (I've owned for 51 and did the 100 Hr Service) I started to get a misfire at all throttle positions. I thought fuel but seemed good and has spent it's whole life on 90 Ethanol Free.

I checked the obvious and replaced the water sep filter and primary fuel filter. I then pulled each injector plug one at a time but still a rough idle one each individual injector. I then did the spark plug check and very obvious (see below) that cylinder 3 was the issue. I replaced the plugs and it immediately ran perfect at all RPM ranges.

My question is, what are the possibilities of a simple non-firing aka bad plug vs an underlying cause? Has always ran perfect up to Labor Day when the rough running started. I won't have much run time left in 2026 to test more as the season is short on the Great Lakes.

I know these NGK plugs were legit (a lot of counterfeit plugs out there now) and they were perfect from hrs 108 to 159. I did test the resistance of the #3 plug and it was the same as the others (in the normal range). Thoughts?
 

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It is not abnormal for a single plug to become defective at any given time. Many reasons why a plug can fail.

The resistance between the top of the plug and the center electrode can be just fine but if the connections between those two points is internally grounded then it won't be sparking.
 
boscoe said:
It is not abnormal for a single plug to become defective at any given time. Many reasons why a plug can fail.

The resistance between the top of the plug and the center electrode can be just fine but if the connections between those two points is internally grounded then it won't be sparking.
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Thanks for the follow up. That's a good point that passing a resistance test doesn't guarantee a good plug.

Since I had the boat out, I did pull the injectors as we have a great injector service company nearby with great pricing.

My thought was a ton on putting around as the Yamaha is on my Bennington pontoon, so clean injectors to start it's 9th season of use. Almost the same as heavy trolling use profile of operation.
 
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