Mercury Outboard 80hp 1988

by Tony
(Elizabeth, Wv USA)

Hello to all, I've got a weird issue with my 1988 80hp Mercury Outboard. The engine has very low hrs on it and runs great. A month ago, I installed a new water pump impeller, the old one was suffering from what I would call low water pressure, 5 or 6 psi at idle and 7 to 8 at wide open throttle. After double checking my work, I hooked a set of ear muff and water to the engine and fired it up.

The new pump worked great, pee tube was showing good pressure and my PSI gauge was showing 9 to 10 psi at idle. So I shut it down and called it good. Today I took the boat out on the Ohio and everything seemed to be good. I noticed at idle to 2000rpm everything was good but over 2000 rpm's the water pressure gauge stated pulsation from 14 to 15 psi to 6 psi, back and forth like the ticking of a clock.

The engine wasn't getting hot or acting weird, so I went ahead and ran it back to the dock and brought it home. Running it with muffs and a water hose it was doing the same thing. I was about to chalk it up to a faulty or clogged water pressure gauge, when I noticed the pee tube was pulsating the same way. it always has a stream or water but you can tell, the high pressure and pulsation to low pressure. Anyone have any idea's what's going on?