When we had our opti we checked & cleaned the poppet valve
hat's how its supposed to run....
...on the stats when under 1500 rpm, poppet opens by 2000 or so. When on the stats after about 20 minutes at idle speeds in gear your temp should reach a constant temp of around 145-155 on most boats. If higher, or if it doesn't reach a steady temp then the stats are clogged or sticking, or the pump impeller is worn. Once the poppet opens, the temp drops radically, down to just about what the water outside the engine is at, plus 5-10 degrees. This is the way Merc wants it to run, so I do not recomend playing around with the poppet. I've got an '02 150 Opti, purchased new, and have seen no issues at all with the stock cooling system- the motor seems to run just fine at all times.