Mercury 850 water pump/pressue

[katsandsuds]

Guys, I have a 1975 merc 850. I started it up in the yard tonight and the water out the p-hole had a weak stream. I pulled the tube coming out of the block down to the p-hole, and there is good water flow out of the block, but if I put my finger over it, I can stop the flow easily. Fished a wire thru the tube and thru the p-hole, and feels clear. Blew into the tube out the p-hole, and the air goes thru, but there is pressue like it may be partially clogged. I will have to pull the tube and the p-hole fitting out to make sure they are clear.

My question is how much pressure should the water be under coming out of the block to the p-hole tube? Should I be able to stop it with my finger? I have been thinking of putting a water pressure gauge on the engine, would this tube be the best place, or is there a better place to put the sending unit for the gauge? The water pump/ impeller has less then 20 hours on it, first thing that was done after I bought the boat. Thanks

 

[Silvertip]

Re: Mercury 850 water pump/pressue

There is no sending unit for a water pressure gauge. You are measuring water pressure which is exactly the same thing your speedometer does. You can put a "T" fitting in the tell tale line and run that line to the gauge. That's all that's needed. There is no magic number for water pressure since every engine is different. The idea is to install a new water pump impeller and then not the water pressure reading at idle, mid range, and wide open. Any deviation from those numbers at a later date means the impeller needs attention. That said, the engine probably has about 5 PSI at idle and about 15 PSI wide open. Yes, you can stop it easily with your finger especially at idle.

 

[katsandsuds]

Re: Mercury 850 water pump/pressue

Fantastic, thanks.

 

[Silvertip]

Re: Mercury 850 water pump/pressue

Here is how it looks on my 75 HP triple.