Mercury 40hp efi 2015

by Eddie
(Gibraltar)

When I engage gear the water pressure coming from the tell tale is reduced nearly to a minimum, sometimes even zero output. The engine obviously overheats and the warning signal comes on. As soon as the speed is increased the water comes out at the normal pressure, quite strong and the engine runs normal. Again when I slow down, minimum speed, the pressure drops to almost zero. When in berth and in neutral it is also very poor and again overheats. I have recently noticed that if I bring up the trim the water pressure comes back to normal. I'm completely lost so any advice would be most welcome. I've changed the impeller, twice, and the thermostat. I've also cleared all the passages where the water circulates.