I'm new here and a new owner of a 99 mercury 2 stroke 200 hp efi. The motor has a cooling issue I am trying to sort out. I would appreciate any help I can get. The motor runs great. However, at idle it has water pressure around 12-15 lbs and pegs the gauge at anything above about 2000 rpm. The pee hole has a strong steady stream. The motor runs strong with no alarm until I try to run around 5000 rpm (about 60 mph) then I get the dreaded steady beep of the overheat alarm. The alarm then shuts off as soon as I get out of it. According to the former owner, this only occurs at higher water temps (upper 80s here right now). I suspect either a restriction of water flow or a bad alarm. I would genuinely appreciate any advice on how to trouble shoot this scarey problem to a solution. Thankks for any help.

Under normal operation your water pressure should increase to 20-25 psi then decrease to 15-18 psi. This is controlled through the relief valve or poppet valve and the purpose is so that at high RPM the flow is slowed down so that the water can actually absorb the heat. Your relief valve is most likely failed in the closed position. As water pressure increases, the relief valve opens and dumps excess water out the exhaust bypassing the heads. It is located on the starboard side of the block. You'll have to remove the lower cowling to access it.

clean poppet and replace t stat