Here are common suggestions for lower Rpm of the D3 150 hp
When the boat was launched in May the engine turned 3000 +50 rpm ?
If yes
Bottom is dirty ( doesn’t take much slime to load the engine)
Prop is fouled ?
Trim tabs extending ?
Air filter element clean?
When you changed the primary filter was it dirty ? If yes did you replace secondary filter?
Do you know what your max boost pressure is at WOT when the engine is turning 3000rpm? Has boost pressure changed ?
Is the boat equipped for cruising? If the boat is light with no gear and 1/2 tank of fuel and 1/2 tank water 2 passengers you should see 3050 to 3100 with factory prop. When loaded full fuel, full water , equipped to cruise, 2 passengers 3000 rpm with a clean bottom. Add 2 more passengers and it would be normal to loose 100 rpm. The D3 Volvos are 2.4L engines good motors but it takes all they have to push a R25 and maintain recommended rpm. Any additional load from bottom slime, extra gear, additional passengers, head winds and waves. One or more of theses could cause a drop in max recommended rpm. My money is on a slimy bottom. 3 to 4 months in the water is plenty of time to have a freshly painted bottom to have some growth.
If the engine is running ok, normal temperature, check your fuel burn numbers. Compare the numbers to what you were getting when the engine turned full rpm. Compare your 2100 rpm or what ever rpm you normally run at fuel burn numbers. If you see a slight change in fuel burn this would also be an indication it is taking more power to push the boat.
