Port engine stumble

[Neutron]

Working on an 2011 242 and have a weird engine stumble/ miss at random times. Seems worse when at full temp.
It does it at any rpm range. It is like a a cylinder drops out for a second then recovers. Only on port engine

What i tried:
Swapped all coils,plugs, injectors and rail, fuel pump with starboard engine.
No fault codes

Turns out this engine is has been replaced. Not sure the history on it. Owner just bought this boat from a dealer in PA. He is going to try and get some info on it.

Didn't do a compression test, or leak down test.

 

[DRock3d]

Can you take a video?

 

[Neutron]

Not currently at the boat but i will when i am there next time.

It will run just fine then for a second or two the rpms drop then recover. A few times it stalls out but will start back up.
Engine temp stays around 144 degrees. So its not running hot.
All the data on the YDS pro looks normal.
Voltage stays around 13.9

 

[DRock3d]

What does the wear ring look like?

 

[Neutron]

Not sure boat is on a lift. It sounds normal not noisy at all

 

[DRock3d]

I wouldn't go any further without pulling the pump

 

[Neutron]

I have never seen a pump cause a random misfire. Seen pumps cause a locked up condition, vibration, etc
If you take boat out and cruise at 6000rpms, it will do the same thing, stutter for a few seconds and recover back to normal. A swollen impeller housing isnt going to cause that

 

[Cambo]

Swap the engine computer they have been known to have problems from moisture maybe this is some type of new problem.

 

[buckbuck]

I know you will find it as you have the troubleshooting disciplines needed. May I suggest the Throttle Position Sensor?

 

[FSH 210 Sport]

Intermittent electrical problems are the worst! I think @Cambo is on the right track with tge ECU swap. The only other thing I can think of would be crank / cam position sensors or the pick up trigger on the flywheel.

 

[Neutron]

Not sure when i am heading back to his boat. But i will certainly try all of the above suggestions. I will take my scope to look in the impeller housing through the clean-out port as well.
Whats got me concerned is the very new motor. Looks like a reman, its painted black with a grey painted valve cover. The starboard motor is just bare aluminum. Both have new oil coolers so whoever owned this boat took care of it you can tell. The new owner just got it. And this problem was noticed as soon as he put it in water.

 

[Neutron]

Just an update in case anyone is interested or has a similar issue.
After doing a compression test, swapping, ecus, tps, throttle bodys, map sensor, checking grounds, cleanout switches and still having issue.
Started tracing every wire and connector. Got it to stumble and stall out every time i disturbed the wires going to the stator. Narrowed it to the crank sensor pickup connector which was very loose when connected compared to the same connector on starboard engine.
I depinned The connector on the female side which is the engine side crimped them down tighter and reinstalled. Didn't get it to stumble and more. Customer is going to take it out this weekend so hopefully that is it and all good.

I am going to return to the boat some time in the next week or so to replace the crappy connector with a Deutsch connector from my replacement kit. Then zip tie the engine harness, since the previous owners mechanic never put any back and its loose and flopping around.
All in i have 7 hours so far chasing this gremlin

 

[jdkaeser]

That was going to be my recommendation. I had a motorcycle that did the same thing. Took me weeks to trace down a broken wire going to one of the coils. Fun part was, it would drop a cylinder, I’d hammer it to keep it going, then it would start firing again, then I was doing a wheelie!

Bad connectors and broken wires are the worst! Glad you figured it out.