350 mag engine miss.

[Buttercup]

I posted this in the sport boat forum, but thought this may be the more correct forum!

2008 210 Select Fission w/ 350 mag mpi motor package w/350 hours.

This problem occurred suddenly, the first indication was a one time noticeable stumble (both my wife and I felt it) as we were cruising at about 2800rpm/25mph. We had been cruising for the better part of 45 minutes.
The next time I used the boat about a week later this happened:

From a cold start, the engine misses just off idle and will not accept much throttle without missing severely. If I nurse the throttle along it will plane up in about 75 yards or so. It progressively gets better as the engine warms up and after a few miles she runs perfectly. After the engine is warm, the throttle response is smooth throughout the rpm range, I can nail the throttle from a dead stop and she planes right up. If I let the engine cool off, she starts sputtering again in direct relation to how warm the engine is.
I’ve replaced the water separating filter cartridge and the rotor and distributer cap. I ran almost all the existing gas through it and filled the tank 1/2 full and added a can of SeaFoam... 1 ounce per gallon.
I haven’t run much of the 1/2 tank of fuel through yet, I’m hopeful the problem is just bad fuel.

Anything else I should be looking at?

 

[Jeff Delbuono]

Not sure but it could be the engine temperature control module. Its on the thermostat housing. My 5.0 efi ran real rich with the smell of gas and white smoke at start up. After it warmed up in ran fine. That module tells the computer to run rich at cold and thin at warm. Might be giving a bad signal to computer. Changed mine and solved the problem. Just a thought cause i dont now the 350 engine set up you have

 

[Buttercup]

Thank you.
I think I’ll take it to a mechanic with the ability to check for error codes. I’m guessing if the problem is this or another sensor, it should show a code.