There are 3 things that might be a problem.
The manifold can look NEW and be BAD!!
You have to take it off and inspect the end for blockage. Do a flow test.
Went on an overheat job.
The same setup as this one.
The manifold, I could actually push in the side.
The paint was the only thing holding it together.
The heat exchanger insert, it gets clogged and stops the flow of cooled water.
Sometimes a flush works, sometimes a boil out at a radiator shop works,sometimes an acid dip works.
Take it to a shop, don't try the acid yourself.
This part is CRAZY expensive.$1000+
Maybe e-bay for a used part "855453 Volvo marine heat exchanger" $160
The water pump: the impeller, or the cam wear plate wears down and not enough water is pumped into the motor.
My first guess is the manifold.
Any scoring on the pump cover? Closeup pics??
How about back inside the pump at the rear?
Remove the pump.
Reassemble it.
Try blowing through the pump as you turn the impeller.
If you can blow through the pump as you turn it?
You probably need a new pump.
Another test, put it back together.
Start it on a hose or in the water(best) then disconnect the outlet and rev the motor.
If it doesn't push a LOT of water as it revs up then the pumps probably bad.
One more pump thing.
The cam inside the pump it's over the inlet and squeezes the impeller to make suction.
The cam can wear down and not work good at high speeds.
Or like I suggested in my other post: the exhaust manifold or the heat exchanger.
How old are they?
Do a visual inspection on the end of the manifold.
ANY clogged outlets?
