Re: how do i hook up a water pressure gauge, mercury 85hp 850
I went thru the same learning process a couple years ago with my 1971 Merc 800. Tried teeing into the telltale with the same results, no reading.
I then installed a "T" fitting into the port on the waterjacket cover located between cyls 2 and 3. There is presently a 1/8" NPT plug installed there from the factory. On one side of the "T", I connected the pressure gauge and on the other side of the "T" I connected a temperature sender.
With a new impeller installed, I get about 3-4 PSI at idle, and at WOT I read 11-13 PSI. I also have a temperature gauge but these motors do not have a thermostat so they run REALLY cool and the gauge needle rarely moves off the peg.