2018 250 pro xs stutter at wide open

[slim110]

Ive got a 2018 250pro Xs. S/N 2B529435.

I've had an issue for a while now and my dealers can't seem to pinpoint it. While running at full throttle, and anything above 5500 rpm I get a random stutter almost like its hitting a false rev limiter. I've got smart craft gauges and the tach has always been rock solid at any rpm. You can watch the tach dump 2-300 rpm and it immediately jumps backup. You can feel and hear the engine when it does it every time. It does this every time ion the water regardless of air temp etc.. and at random rpm above 54-5500 rpm. I've gone through coils, plugs and highspeed pump and still the same issue.
Now on the last one or two trips, I let my fuel level dropped below 25%. Typically I never let it drop below 1/3. But below a 1/4 tank both times it ran up to 5900 rpm with no issue and no stutter.

Ideas? Any help would be greatly appreciated.

[lpugh]

For test purposes try it with the gas cap off. I think you probably have fuel demand valve on that That should be checked, I am basing this on you statement of when the tank is low on fuel it seems to be better. possibly because of a larger vapor space allowing it to run longer before tank vacuum is high enough to cause fuel starvation
Don will probably respond to this soon

[slim110]

I did actually try that once early on thinking that may be an issue. Still did it.

[EuropeanAM]

Take a really critical look around the back of the engine to see if there are any notable WATER SPOTS, or a line that might indicate water spraying. Especially around the spark plugs themselves.

Have see a few hairline fractures or pin-holes over the years, the only way you can catch them is visually observing the "clues".

[slim110]

I’ll look at that today. Thank you!

[slim110]

Just checked. No water spots or spray spots on the back anywhere are of the plugs,coils etc. there are water spots on either side of the case that look like back wash coming in from the top of the cowling. But nothing at all on the back side.

[EuropeanAM]

I'll throw a few more out there for you- just make sure that ALL of the basics have been covered first (Compression, Spark, Correct Plugs with .043-.044" gap, Air/Fuel Pressures correct). Your DEALER should be addressing this for you.

Couple other areas to look at (not very common, but easily over-looked):

-Check the Tensioner spring tension with an INCH pound Torque Wrench. Set to 70 lbs/in, wrench should click BEFORE you reach the end of the tensioner's travel.

-Check for a yellow plastic cap bouncing around in the intake, or lodged in a reed block. Originally in the flywheel, they sometimes end up loose and get sucked into the intake.

-Check the bottom of the tuner to make sure the divider is intact (and not broken off)

-Have direct injectors and fuel injectors tested/validated (or if you have access to another complete setup, try that to see if the problem persists). An intermittent DI, or a high-flowing injector can cause symptoms like this.

[slim110]

Compression test, plugs, basics were all done by multiple dealers. So those should be good. I found that plastic cap some time ago floating around in the bottom.
Will start checking the rest of that stuff this week.
I really appreciate the help. It’s getting frustrating to the point I’m considering selling the boat. But if I can get it resolved I’ll be super happy.

[CoastieWess]

have you figured out this problem yet?

Compression test, plugs, basics were all done by multiple dealers. So those should be good. I found that plastic cap some time ago floating around in the bottom.
Will start checking the rest of that stuff this week.
I really appreciate the help. It’s getting frustrating to the point I’m considering selling the boat. But if I can get it resolved I’ll be super happy.

[slim110]

Still at it. I can’t figure it out and neither can anybody else. Sadly I’ve just accepted it as normal operation. In its defense it’s never left me stranded and runs like it should everywhere else as long as I don’t run it above 5500-5600 rpm. This is my first Mercury and will probably be my last.

[EuropeanAM]

Curious- are those RPM's coming from SMARTCRAFT or from an Analog Gauge (if analog... you may be turning more than you think)....

[slim110]

Smart craft gauges. Not only can you see it in the gauge you can feel and hear it as well.

[slim110]

It’s hit 5900 before so I know it’ll run that high, but this issue started at about 160 hrs or so. I’ve got 260 hrs on it now so I’ve been chasing it a while.

[slim110]

After fighting this issue fro close to a year, engine finally let go about two weeks ago. Looks like the #5 seized up and broke the rod, putting it through the crank case. Luckily Mercury is covering everything. So I'm hoping whatever was causing my initial issue is going to be resolved with a full rebuild. I know they're putting new injectors on it as well so we'll see.

[EuropeanAM]

Be sure to report back.

[slim110]

Will do.