Calling all Merc Mechanics

[THEFERMANATOR]

My 150 MERC didn't spray a steady stream either.

This may be a stupid question, but are you sure it is the overheat turning the horn on? Try unhooking the overheat sensors one at a time to see if it is actually the overheat sensors doing it. There is also a chance that parts of your old t-stats are obstructing flow somewhere in there fropm the looks of what you removed. Try running it with the t-stat covers off fopr just a few seconds and make sure water is flowing up to that point with a good stream.