Yamaha sterndrive 4.3 not cooling, pulling my hair out

The motor will overheat on throttle, low throttle or idle it will keep perfect temp.
New impeller, the sea water pump spin's the way it should (removed the square nut on top for inspection) New thermostat.
And no leaking found in cooling system using smoke machine.
I have the lower unit off at the moment and run a water hose directly into water intake.
Here is what I have noticed.

On initial start up the water flows fine out the lower bellow, after it starts to warm up the water starts to come out of the lower prtion of the transom.
Do you guys think there might be blockage somewhere backing up the flow and setting off the seawater pump alarm?
Does anyone know how to backflush this type of motor?
This is my first boat but I think I'm fairly mechanically inclined.

*note when I changed the thermostat i did find some small gravel debris I removed the 4 hoses on the thermostat housing they seemed clean.
*Also where does the impeller debris end up when they start to come apart?

Thanks!
Max Wilson

Ayuh,.... Impeller bits usually end up at the next choke point, the T-stat,.....

If this motor ever saw saltwater, another choke point to inspect is where the exhaust manifolds meet the exhaust risers,.....

bondo said:

Ayuh,.... Impeller bits usually end up at the next choke point, the T-stat,.....

If this motor ever saw saltwater, another choke point to inspect is where the exhaust manifolds meet the exhaust risers,.....

Click to expand...

Per previous owner (owned it for 12yrs) boat never saw saltwater, T stat area was clean, just a few pieces of small gravel.
I will check the shop manuel for the exhaust riser and check it out.

Thanks!!

SPeaking of exhaust, I pulled out the brass wingnut's on the bottom of the manifolds.
They were filthy with goo.
had very little water flow coming out.
Stuck a small screw driver in there and they started to flow.
Could the manifolds be jammed up entirely?
Should I take them off and clean them? If so what is best method for cleaning them?

Thinking about pulling the brass wingnuts off again and shooting air up in the manifold.. thoughts?

Ayuh,.... If ya flushed 'em with water, blowin' 'em out with air probably won't change anything,....

Pull the riser off the top, 'n inspect the flatness of the gasket surfaces, 'n see how much crud is in the water ports,....