I have a volvo penta AQ 131D with a 275 drive. Does fine at idle speed but gets hot at higher speeds. I have installed a new impeller and thermostat. I would think that since it cools down when at idle speed it is pumping water, but not enough. I do not know where to look from here. I ordered an engine workshop manual, however it has next to nothing about the cooling system. I would appreciate any suggestions.