Raw water system not cooling the manifolds

Hi, I'm new to boating and have bought a 1994 campion victoria sundeck 290. It has twin 5.0 volvo penta engines. The previous owner installed new manifolds and risers to both engines and replaced both raw water impellers, immediately before i bought the boat. The port engine runs fine at 160F. The starboard engine runs at 180F (i changed out the thermostat, tested the old one which worked fine) Previous owner said that the starboard engine always ran at this temp.
Whilst running at 1400rpm maneuvering out of the marina, the starboard engine temp stays at 180F. When i increase RPM over 2000 the engine temp rises to 220F, at which time i shut it down.

Both of the new manifold/riser combo's are very hot to the touch. I have cleaned out the heat exchanger, some gunk, but is now fully serviced. I have performed a flow test on the raw pump and found that the flow splutters as i increase rpm. I have pulled off both hoses behind the raw pump and both are in excellent condition.

I am now left with no idea what is going on. Is the problem inside the stern drive? Is there any way to check the flow through the stern drive?

thanks for any advice.

When was the last time the raw water pumps received a major overhaul??? if there are worn out hard parts in the pump, a new impeller won't deliver what it should...could also be an air leak on the suction side of the pump...

Thanks for the reply. Not sure when a major was done on the full pump. doesn't seem to be much more than a pulley wheel with a shaft that spins the impeller. I plan to switch the two raw pumps and see if i get the same result. I have stripped back to the transom inspecting each hose, only 2 hoses. both these and the oil cooler appear to be n good condition. Is it possible that there's a leak in the stern drive leg? Not sure how air would get in as the leg is under water.

I'd look for a 'vacuum' leak between the outdrive and raw water pump...could be sucking air?

Further jeffnick's post.... The "neck", that fitting on the drive that the raw water hose attaches to is a wear item. It is prone to pinhole corrosion leading to vacuum leaks on the raw water inlet to the pump...and the "S" hose degrades with time.