mercury 2004 225 hp misfiring

I have a 2004 mercury 225efi which idles rough and is smokey but has spark on all cylinders but when going along under load cylinder 2 is not firing. I have changed this coil and tried a borrowed computor pack but it is no better . Any help would be great

Same problem I had mate...try changing the MAP sensor or at least cleaning it out with MAP sensor spray.

No.2 cylinder will always foul first with a rich mixture (so I was told by the Merc techs)

Yes, it could be the MAP or you could replace that and still have issues - take to a dealer and have them plug it in to a DDT. Yes, that will cost a few bucks, but since the sensors on these are not exactly cheap, it could save you hundreds compared to the "swap and check" method...

You will not get any errors on the DDT. I had a frustrating 6 to 8 months chasing exactly the same problem. All sensors come up within factory parameters, the MAP is just slow to react and will lead to a rich condition fouling number 2 plug.
You have a few options, spend $300 getting a dealer to plug in a DDT and find nothing, spend $125 on a new MAP or spend $20 on a can of MAF /MAP sensor cleaner. The cleaner will work if the MAP is only plugged with oil...which 9 times out of 10 is the problem.
I have remotely mounted the MAP from a pipe and hose out the intake manifold to the MAP, this has eliminated the oil contamination problem.

good luck!