Check for any bluing of the exhaust bend where the bypass water injects into the exhaust bend. If any discolouration then check the bypass hose is not restricting the bypass flow to the exhaust on both sides of the thermostat housing.
The bypass allows flow through the system while the thermostat is still closed during warming up.
The pump throughput is OK at 10 litres per minute.
With the engine
at normal running temp I measure the flow through the engine with a bucket and stop watch at the exhaust. The engine needs to be at about 2000 rpm
and the exhaust temp should be 70 to 75 degrees C with a flow of about 8 litres per minute.
I check mine at the marina with the mooring
lines attached and in forward drive at 2000 rpm
so it is easy to lean over and catch the flow and I can hold my hand in the exhaust water flow (just)