What year is it? Did you get any warning horns? I think you have tryd some of the main things already, So you really are at the point where your going to need to be hooked up to diagnostic testing equipment to see what the engine is doing in real time as it is doing it. Also there is a chance you have a bad intake manifold gasket that is failing and could be leaking air and or water into the cylinders and could burn up the intake valves, And the way to find that is to do a leak down test And you could try sprying some carb cleaner on the manifold to see if the rpms changes some, Some times that works some times it does not. Here is a Thread of another guy with a 6.2 that idles rough also, His first Post went like this, #1 his complaint was this [6.2 rough idle, not running right
Have a 2002 mercruiser 6.2 mpi. Ran all day fine, on the way back to the boat ramp experienced a lack of power. Pulled out spark plugs and cylinder 1 and 3 the electrodes were melted. Put new plugs in it, checked and cleaned distributor cap, rotor, iac, and took it out for a test run. Seemed to run better, had plenty of power for hole shot. After it warmed up it would barley idle. Seemed to be misfiring and on the same side that the plugs were melted on. Also at wot the engine would make a little popping noise but still had power.. So I went home and pulled plugs out and they all looked like they were burning fine.. Really stumped.. Help anyone?] Post #2 was this [Well yea sorry for the other threads.. I though the problem was resolved because the problem seemed intermittent.. well Ive about narrowed it down. All of the cylinders on the right bank seem to be running to hot. The plugs are a little glazed. All of the cylinders on the left bank are burning ok. The miss firing is occuring on the right bank where I know they are running to hot (glazed new plugs and melted 2 old ones) So maybe not enough fuel? Also the fuel rail on that side is the furthest away from the fuel supply. But i think im narrowing it down. For some reason the cylinders on the right bank are running too hot.. Thanks for any help guys...] His last post#3, He found and fixed this,[The problem was the intake manifold gasket on that side failed and was leaking air and water into two of the cylinders and burned up the intake valves.] Now I am not saying this is the problem just something to keep an eye out for is all, Good luck with it I hope this helps you out some. Boat teck.