After racking my brain about why I'm throwing so much steam out of my exhaust after running hard (starboard more than port), I replaced the elbow to manifold gasket with one that has 2 open slots and 2 restrictor holes instead of slots. Didn't fix the problem...new impeller, new T-Stat, backflushed...nothing. 1999 502 MAG EFI. Manifolds and elbows are on their second season, aftermarket from GLM.

Then today I stumbled across a Merc bulletin here:
http://www.boatfix.com/merc/Bullet/99/99_10.pdf

Merc changed up the gaskets that should be used. Since I do not have bypass hoses feeding water to the elbow, I should be using the gasket with 4 open slots rather than 2 restrictor holes.

How is it that my marina and the aftermarket manifold/elbow manufacturer do not know about a bulletin put out in 2001?

I swapped out the gaskets today and will be taking her out shortly for a test run. I'm really hoping this allows more water flow through the manifold and elbow and cures this mystery.

What was the verdict with the new gaskets??

verdict is....still not fixed! Took an infared gun out today. The side that is steaming is running about 30 degrees hotter, around 137 compared to 105 on the other side. It reads hot from the manifold all the way through the elbow, with the top of the elbow obviously the hottest. This tells me that either their is a crack in the manifold or it's plugged up somehow but either way it's not getting enough water to keep it cool. Any thoughts?

Sense you changed the impeller what was the condition of the old one? broken in pieces, did you get all the pieces out? if not that could be the restriction

block could be full of sediment making one side run hot...shoot the block with IR gun see what you have down low...seen this a few times...