2018 4 stroke 115 hp Mercury - Misfire (?) under load after 1 hour

[Mauro Brisola]

As car mechanic I’m used to fuel injection. But I’m in the second Mercury outboard that I had bought new, Then I’m not used to outboard problems.

I had before a 2013 Mercury 4 stroke 60 hp. In the same boat setup, 16 foot rib. Used it 2 and half years and about 200 h before selling boat. It worked flawlessly.

Now my “almost new” 90 hours 2018 Mercury 115 hp 4 stroke is scaring me. Because a intermittent problem that I did not understand yet. Scaring becacuse I go far with wife and son, to places without support or tow.

First time problem occurred was 3 months ago. After about 1 hour cruising at 4000 rpm I felt like a small hit in the hull. In fact was some kind of engine misfire, ignition related or not. Because some minutes after it started to repeat in less time interval. It is a split second problem. But when the interval of the “misfires” started to decrease I had to slow down. Slowing down I was able to continue to marina.

This time I thought about fuel tank level too low. Probably causing fuel aeration. With less speed and boat trim fuel feed was ok. But I think I was wrong. Even because there is no gradual loss of power but a sudden “bump”.

Used boat some time times after that without any problem. Including long runs.

Yesterday I had same problem with almost full fuel tank. 20 gal. Again, after more than 1 hour of cruising about 3800 rpm I felt one “bump in the hull”. Continued in the same rpm/speed. Less than one minute after it start to repeat. Again returned to marina slow/out of plane to avoid problem repetition. Closer to marina I put it on plane again. It went well maybe 5 minutes and started to misfire again.

In the square SC1000 Mercury monitor there is no fault messages or alarms. Engine temperature always about 129 F (54 C).

My doubt is if it’s fuel related or not. Maybe a dual output ignition coil with problem? Hard to know because it occurs just under load and after a long run.

After in the marina yestearday the idle was perfect. Did the flush normally.

I will replace water separator filter and main fuel filter before next run. But both are original Mercury and less than 30 hours old.

I apreciate any hint from who has experience with it. Tks

 

[QBhoy]

Hi. This was happening exactly to me on my 2016 identical engine. It’s the CT model 115 I have. Spent a wee while thinking and looking around things like fuel filters, plugs, leads etc etc.
would take a wee miss fire (or so I thought). Was convinced it was a miss. Split second loss of power almost. Then…
One day at the beach…was washing the boat. Engine was tilted up. Looking at my lovely propeller…and noticed something not right behind the prop. The white plastic exhaust bore seal ring behind the prop had came loose. Turns out it’s a bloody very very common thing to happen on these gearbox about that age. On the 115 CT and 150’s etc. The locking tabs come loose and let the ring rattle about behind the prop. Then means that occasionaly you get exhaust gas bubbles sent over the prop blades, causing a split second of ventilation…acting almost identical to a miss fire. I used sealant to stick it back on. Never a bother since.
I hear since then, merc have changed the design in the last year or two. Was very common issue. Look it up online. Even YouTube guys have posts on it. Worth a look, in case it’s that.

 

[Tonycat]

As car mechanic I’m used to fuel injection. But I’m in the second Mercury outboard that I had bought new, Then I’m not used to outboard problems.

I had before a 2013 Mercury 4 stroke 60 hp. In the same boat setup, 16 foot rib. Used it 2 and half years and about 200 h before selling boat. It worked flawlessly.

Now my “almost new” 90 hours 2018 Mercury 115 hp 4 stroke is scaring me. Because a intermittent problem that I did not understand yet. Scaring becacuse I go far with wife and son, to places without support or tow.

First time problem occurred was 3 months ago. After about 1 hour cruising at 4000 rpm I felt like a small hit in the hull. In fact was some kind of engine misfire, ignition related or not. Because some minutes after it started to repeat in less time interval. It is a split second problem. But when the interval of the “misfires” started to decrease I had to slow down. Slowing down I was able to continue to marina.

This time I thought about fuel tank level too low. Probably causing fuel aeration. With less speed and boat trim fuel feed was ok. But I think I was wrong. Even because there is no gradual loss of power but a sudden “bump”.

Used boat some time times after that without any problem. Including long runs.

Yesterday I had same problem with almost full fuel tank. 20 gal. Again, after more than 1 hour of cruising about 3800 rpm I felt one “bump in the hull”. Continued in the same rpm/speed. Less than one minute after it start to repeat. Again returned to marina slow/out of plane to avoid problem repetition. Closer to marina I put it on plane again. It went well maybe 5 minutes and started to misfire again.

In the square SC1000 Mercury monitor there is no fault messages or alarms. Engine temperature always about 129 F (54 C).

My doubt is if it’s fuel related or not. Maybe a dual output ignition coil with problem? Hard to know because it occurs just under load and after a long run.

After in the marina yestearday the idle was perfect. Did the flush normally.

I will replace water separator filter and main fuel filter before next run. But both are original Mercury and less than 30 hours old.

I apreciate any hint from who has experience with it. Tks

I have the same problem with mine , just wondering if you ever fiqured it out?

 

[jbcurt00]

I have the same problem with mine , just wondering if you ever fiqured it out?

Start a topic about your motor and ask questions there

Seems Mauro started this topic and never back

Closed