I have a 2005 Crownline with a MERC. 350 MAG MPI GEN III Engine. The boat has low hours @ 160. I noticed first time out 2 months ago it was running rich and using more fuel. Second time out 4 weeks a It was running very rich at cold start and idle and brownish tint smoke. Last week.. 3rd time out 500 rpm,very choppy idle, missing and backfired a few times. Pulled plugs and very black. Cap and rotor new. Flame arresotr good and clean. I'm getting NO alarms. Boat seemed too run fine at W.O.T. but burning more fuel. Any help would be great.
Thanks.

Temp sensor shot or injectors leaking could be the problem.

Do you mean the temp sensor located in the thermostat housing. My impeller was destroyed on sea pump. Also thermostat opened slow @ 190.
Thanx alot for help!

temp sensors usually wont read unless there in water, if your sea pump failed u may not have been getting water to sensor
i would repair pump.backflush system if any impeller pieces are missing, make sure your temp is ok if still running rich check fuel pressure,

My impeller was destroyed. Pulled pieces out of pump with needle nose pliers!. I don't think the sensor got and water although the temp gauge was reading 220 and alarm sounded....this was with the ear muffs on...quickly shut down.
Question.....
Why would this make my boat run so rich....plugs black as coal!
Thanks guys for and suggestions

Best way to back flush system?

You should also check your oil cooler for pieces of the impeller.

OK...not sure where located

i dont believe there is an oil cooler on 350 mpi ,just p/s cooler
i would dissconnect outlet from sea pump and dissconnect water feed to therm housing put garden hose in hose flush back towards pump see what comes out if your fwc this procedure will be different let us know

Yes....I only have power steering cooler. I already have inlet and outlet hide disconnected. There appears to be check valves on either side of motor below exhaust manifolds. I would assume to flush before them down to sea pump.
what is ....fwc?
Boat is fresh water use only
thanx