350 Mag MPI rough idle when cold

[viscose]

I have an 02 Sanger dlx with a 315 hp Mag MPI. The motor does not want to run at all when cold. To get it to start I have to give it throttle and then warm up the engine before it will stay running at idle. My first thought is to pull the plugs, check and replace (any suggestion for plugs -- also I use the boat at 3600' to 6250'). If that doesn't work I am stumped. I was told by the previous owner that the boat was cold blooded which is fine, but I'd like to try to fix it.
Also, when the boat is warm it doesn't start right up; the starter must be cranked for about 5-8 seconds to fire the motor. And, if you don't get it started on the first or maybe second try, you have to again throttle it to start it.
Any help is appreciated!

 

[newport dave]

Re: 350 Mag MPI rough idle when cold

2002 was a transition year for that engine. Is your engine a MEFI 3 or a ECM555 engine? Have you done the basic tune-up type stuff (plugs, cap wires, ect.)? Checked for trouble codes?

 

[viscose]

Re: 350 Mag MPI rough idle when cold

I'm not sure of the type of engine but there is a Motorola logo where the plugs connect to the engine. I have not yet done the basics but am off to the parts store tonight to get new plugs in hopes that clears the problem. I have done a bit more research and it appears that there may be a burnt piece of foam restricting air into the throttle body I need to check as well. It should run well enough for the weekend with new plugs and if the problem is still there I'll be back.

 

[newport dave]

Re: 350 Mag MPI rough idle when cold

A Mototrola logo would indicate you have a ECM555 engine. The "burnt piece of foam" you mentioned is called the IAC muffler and it could very well be the problem. I would check/replace that before replacing the plugs. The part number is 35-863829. You can just pull it out to see if the idle improves. You will get more intake noise without it, but you will know if thats the problem.

 

[viscose]

Re: 350 Mag MPI rough idle when cold

I replaced the spark plugs which had excessive gap, pulled out the IAC muffler and it ran well, at 700 RPM but ran very well. With this out my top speed was only about 41 mph (but I think I am mis-propped for my altitude). at 1000' feet higher with the IAC muffler of unknown status it ran 42 mph. I don't know if the IAC muffler would affect top end power but I expected that my mph would be 43-44. After I had run it for maybe 25 minutes with the IAC muffler out it wouldnt start. I rigged a piece of fabric in it for the rest of the weekend and it ran fine. the rough idle is gone and it starts right up with the IAC muffler valve completely open. thanks for your help!