300 Verado sputtering

[theredman]

I have A 2016 300 Verado with 475 hours on it. Yesterday when running over 4000 or higher it would start to spit and sputter. Just not wanting to run higher RPMs. Runs perfect till I reach the higher RPM level. I’ve never had any problems with it. This is the first time it’s done this. It was setting for 5 months with some gas in it, but added about 15 gallons fresh on top. Serial # is 2B172074.
model # is 1302V23LD. Thanks for replying

[EuropeanAM]

When was the last time that you replaced the fuel filters, and the spark plugs?

[theredman]

I haven’t replaced either. I purchased the boat with just over 400 hours on it. Planning on replacing all filters as soon as my mechanic can get to it.
Also wanted to ad I put marine stable in it over the winter and I noticed the water separator is about 75 percent full. Can that be A issue?

[EuropeanAM]

Water separator should be 100% full (of FUEL, no water).

I would recommend having your technician/dealer do a check of the basics, and then go through all of the maintenance on the engine (filters, plugs, thermostat, belt, water pump, gear lube, etc).

[theredman]

That wasn’t the water separator that had water in it. It was A pump. So I talked to my mechanic today he’s going to do A full service on it and see if we can figure out what’s going on. Probably start by replacing all filters. And check fuel delivery, and spark.

[EuropeanAM]

[theredman]

Update. Replaced fuel filter, water separator filter, and impeller. Changed all fluids, no codes on computer. Checked spark plugs, mechanic Said they looked fine. But still sputtering and cutting out at 4,500 RPM. So now what do we do?

[satbeachbill]

I have two sets of used spark plugs laying on my work bench that "look fine". Both times, a new set of plugs cured my issue. .....Sure left me scratching my head as to why.

[theredman]

Why would bad plugs only misfire at high rpm’s. It runs perfect until 4,500. Please explain

[theredman]

Going to take back to my mechanic. Is there anything else he should replace or check besides the plugs?

[EuropeanAM]

Everything. However... I personally would not go to that much trouble until the plugs have been REPLACED and you confirm that the condition is still present (or gone).

Plugs can be very deceiving, especially in the 4-stroke outboards.

[theredman]

What’s the best or recommended plugs we should replace with?

[EuropeanAM]

Mercury Part# 889246Q39

NGK: ILFR6GE

[Billydonwon]

Sounds like a coil. Had one do that under a load. Would not throw a code. Found a faulty coil. Replaced and fixed the problem.

[theredman]

Is there Anyway to check the coils to see if they’re bad? Or just replace them and see what happens.

[theredman]

My mechanic checked for codes, we did have any. So maybe it’s plugs and or coils.

[EuropeanAM]

Can also do a spark test with the G3 Diagnostic System, set the gap on the spark tester to 7/16" gap.

Also remember to check the boot portion of the pencil coils for burn marks or holes caused by "escaping spark".

[theredman]

Replaced the plugs, taking her out this weekend. I’ll update after. Thanks for your reply’s

[theredman]

Update, it was just the plugs. Brent Lintner down at portside marine did the work for me. Look him up. He’s in Avila beach Ca. Totally honest guy would not do any work unless he thought necessary. Boats back on the water running good. Thanks Bloody Decks for all your input.

[theredman]

Doing it again!!. I’m up here at Clearlake and it’s sputtering again. Was fine right after I changed the plugs but now doing it again. So frustrating.