Help. I have a '99 200hp Yamaha OX66 EFI Saltwater Series II, model SX200TXRX, ID# 6G6X-001721. Recently after I go up on plane and the RPM's are anywhere over 3000, it will run for about 90 seconds and the alarm goes off, the water temp light flashes in the tach gauge and the RPMs drop off. At no time have the oil lights gone off and the oil tank on the engine keeps the same level while running it always has. First thing I did was to clean the battery cables. Did not help. Next I replaced both thermostats and temp sensors. There was no scale or corrosion buildup in the water jacket as seen thru the t'stat holes. Same results. I used an infrared temp gun and ran it, constantly checking the temps on both heads at several places. All temps were consistent at 124-130 deg.'s. I swapped sensors from side to side. Same results. Unplugged pink wires and narrowed it down to starbord side head. Even pulled sensor out soon as alarm went off and shot the hole it came from- temp same as rest of head. Usually takes about 2 minutes of idling or engine shutdown for alarm to go off. The only help the local Yamaha dealer could offer was to suggest pulling the head and clean in case it had hot spots. Can a hot spot be so tiny and localized to have a 50 deg. temp change and not show up when checked with the heat gun? I can leave the sensor unplugged on that head and run it, but that's not what I want to do. Any ideas?
