as far as a merc 3.0, the 1990 and older motors are all 2-piece rear main seals and come with a smaller flywheel the 1991 and newer 3.0 have a 1-piece rear main seal and come with a larger flywheel. other than the short lived EFI 3.0, there have been a few changes in the ignition system, however the base motor is the same for 1990 and older and 1991 and newer
On this note, I understand that the minor changes that the 3.0 went through from 1990 to about 2005, include horsepower upgrades ranging from 115hp all the way to 140hp. Apparently some of that had to do with the way the HP was measured (at the crank vs. the prop) and use of bigger carburetor head? They also named them differently throughout the 90s between the 3.0L and the 3.0LX? The LX meant bigger carb head for higher power rating and/or meant it had power steering? Not sure - info found during research was very confusing. Furthermore, after 2000, Mercruiser made the 3.0 MPI and the 3.0 TKS???
Then I read where the best way to simplify it all is to look at ROM ranges at WOT. If it reads 4200-4600, you have something closer to the 115HP. If it reads 4400-4800, then you have something closer to the 135HP.
In the case of the one I'm close to getting, it is a year 2000, which reads only Mercruiser 3.0 Liter. But according to the dealer, it does have power steering. The label on the carb also reads 4400 to 4800 RPM at WOT, indicating that it's 135HP. Is that a correct assumption?
Can anyone experienced with the Mercruiser 3.0 comment, conform or shed some light on this confusion?