i have twin 6.2 mpi mercs. I had not used it for about a year, there was a sensor issue and life got in the way. I just serviced the boat to get going again... oil, drive oil, raw water pumps, etc. one engine runs fine, the other overheats within a few minutes of the dock. I assumed thermostat and changed it.... same behavior. It doesn't overheat without load.

blockage? Something else? I need some ideas to check. Anyone have a "how to" on backflushing the engine?