F255 Brother in law revenge project is driving me nuts!

single 350 w/alpha drive heats up slowly from cold iron to 240+ degrees in 10 to 15 minutes. RPM has little effect on how fast it heats up. The only effect rpm makes is when idling and the temp gets to approx. 170 an increase to 1500 plus rpm will lower the temp to 160 then the climb starts again.
So far I have checked temps with an IR heat gun with the following results, all water hoses (large and small), exhaust risers front and rear, intake manifold front and rear, on opposite sides of engine are within 10 degrees of each other, even the engine block temps on opposite sides and the exhaust pipes themselves are the same temps. makes no difference if on a hose or in the water. currently in spring fed water so inlet temp is constant 72 degrees.
Engine starts easy and there is no water in the oil.

1. impeller replaced by mercruiser dealer during alignment and repair of engine mounts.
2. water hose from transom to power steering fluid cooler replaced
3. stuck thermostat replaced with 142 degree thermostat
4. engine mounted water pump replaced.
5. Suction side hose to pump replaced.
6. Exhaust risers clean and not plugged (water pours out freely, no spiting or sputtering, when front and rear caps are removed)

Is it possible flow from inlet side is too low? I have removed the hose to the thermostat and there is a lot of flow but how to measure volume? water volume out of engine?
what is range for water temp out of engine?
Look at these questions! I really am NUTS!!!!!

This may sound silly, but What temps are you getting on the temp gun? Do they match the gauge?

Also how much water is flowing out of the exhaust and what is the temp of the discharge water

Yes 220 IR 240 gage close enough

good amount equal to flow at t-stat

Hmmmmmm. Have you tried it with the stat removed? Or verified the stat opens at the correct temp? I know it's new, but.... If there were a blockage somewhere I'd think you would have uneven temps or reduced flow. Seems like it's just bypassing the engine and letting everything heat up.

I had something similar it also was with a new impeller. However with an alpha drive I believe that the impeller is housed in a plastic housing. When the marina replace the impeller they didn't replace the housing. Water would get to the engine but not anywhere what Was needed

3pointstar

That is my current line of thinking. how much flow and how to measure the output of the impeller?

it is opening able to see that with hoses disconnected.