tell tale channel clogged 1966 Merc 500

[Bowjamer]

has new impeller -clogged before hose to outlet pee hole . Where clog? can I install a temp gauge and see if its cooling enough ?

 

[racerone]

You might find a pipe plug near spark plugs.----A water pressure gauge could be fitted.----And install a new impeller every 5 years.