# Plug not firing - Mercury 115hp 4 stroke



## Quack Wacker (Dec 20, 2006)

Hey, 

My father has a 2004 115 Mercury EFI 4 Stroke that one cylinder is not firing. Besides the plug wire, which is what I am assuming is bad what other things could cause a cylinder to not fire? 

By the way there are new plugs in it today and it is still doing it, that is what leads me to the plug wire/boot. 


Thanks, 

Eric


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## WALLEYE MIKE (Jan 7, 2001)

Bad spark plug.


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## Lucky Dog (Jul 4, 2004)

Does the motor have a coil pack for each cylinder? a bad one would be my guess.


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## kroppe (May 7, 2000)

Wires could be the issue. To verify, take the suspect plug out, and put the wire on it. Have your dad crank the motor while you hold the plug (by the boot) against the cylinder head. As the motor cranks you should see a strong spark. If it is weak or non existent you have an ignition problem. Take a known good plug (repeat the above test on another plug+wire to verify) and do the spark test again with the good plug and suspect wire. If there is spark, the plug is bad - replace the plug. If there is no spark, the wire is bad, or something upstream in the ignition system - a suggestion is to replace the wire and repeat the test with a known good plug.

Caution: if you touch any metal part of the plug with your hand while the engine is cranking, you will likely be shocked. Doesn't feel good, speaking from experience.


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## Quack Wacker (Dec 20, 2006)

wasn't the plugs they are all firring, but when we get on the throttle a little to full throttle one cylinder isn't running right. 

I hear that there are possible fuel injector problems with these motors and I could see how a clogged or an under performing fuel injector could cause this problems. 

So, it is off to looking at the fuel injectors.


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## kroppe (May 7, 2000)

Seems like you have ruled out the ignition system. 

One quick/easy/relatively cheap thing to check, before replacing injectors, is the fuel line. My Honda 90hp (4 carbs) would start and idle fine. But give it some gas and it would sputter and conk out sometimes. The primer bulb was easy to pump up, but the fuel line from the gunwhale to the engine was more than 10 years old, so I replaced it. Fixed the problem.


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## Quack Wacker (Dec 20, 2006)

_OutdoorHub Mobile, the information engine of the outdoors_

Boat is about 8 years old, so I will have to check the lines to see how the are.


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## Quack Wacker (Dec 20, 2006)

Turns out all of the fuel lines inside layers were completely gone and had plug one injector causing the problem with the one under performing cylinder.

_OutdoorHub Mobile, the information engine of the outdoors_


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## gunrod (Jan 16, 2001)

Hope you fixed the problem.

This is one my favorite forums on these boards, especially since I'm not that mechanically inclined. Even if you don't can't find the problem you can always find someone else who has the same experience to help you out.


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## anon12192013aazz (Dec 10, 2010)

I had a similar problem with my 115HP Merc. Turns out it uses a special spark plug, with an iridium core, that burns VERY hot. It also burned a hole in my !#)*(&!#! wallet, when I found out how much they cost! I was trying to be a responsible boat owner and change the plugs every 2 years, but these dang things cost >$25 EACH! Suffice to say, I now keep 1 new plug on-hand, but I'm not changing them out until/if the motor starts to run poorly. :yikes:


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