Water Pump Issues

[Highroller62]

Hello to all,

First time poster but long time lurker. I have an old Mercury 80hp three cylinder with oil injection and power tilt and trim. I purchased it from the original owner. The boat is a tracker TX17 tournament pro. The boat and motor are in excellent condition. According to the original owner I purchased it from it only has around 100 hours on it. I've used the boat a couple times before I resided to do some standard maintenance. so one of the items was replacing the water pump impeller. after replacing the impeller I have good pressure at the pee tube and the gauge read around 8psi at idle and over 2000 rpms reads just shy 15 psi. I notice anything over 2000rpm the water pressure would bounce back and forth steady like a clock down to 5psi to 15psi like tick, tick, tick of a clock. Not random, but like it has a pressure meter. I have disassembled the system and double checked my work and everything seems to be correct, but can't seem to figure out what's causing the pulsation. Does the model Mercury have a water pressure regulator. If not does anyone have any idea's!

Thank You!

 

[Dave1027]

Google "Poppit valve".

 

[Texasmark]

You may be seeing your thermostat opening and closing. The stat controls flow below about 2500 RPMs (published number). Above that, water pressure forces the poppet off its seat allowing for full flow through the powerhead. The stat would cause variations. The poppet would be a solid function if the same engine speed was maintained....aka amount of steady water pressure.
I have a thermostat on my engine and at low speeds it reads stat pellet stamped temp. Once I get on plane....aka enough RPMs the poppet opens, the gauge falls back to the water ambient owing to the fact that the poppet has sufficient volume to totally bypass the stat.

On my engine the stat and poppet are both under the same cover mounted on the rear water jacket cover of the engine block.

 

[Highroller62]

You may be seeing your thermostat opening and closing. The stat controls flow below about 2500 RPMs (published number). Above that, water pressure forces the poppet off its seat allowing for full flow through the powerhead. The stat would cause variations. The poppet would be a solid function if the same engine speed was maintained....aka amount of steady water pressure.
I have a thermostat on my engine and at low speeds it reads stat pellet stamped temp. Once I get on plane....aka enough RPMs the poppet opens, the gauge falls back to the water ambient owing to the fact that the poppet has sufficient volume to totally bypass the stat.

On my engine the stat and poppet are both under the same cover mounted on the rear water jacket cover of the engine block.

I understand this, what I don't understand is, let say your on muffs, my controls have a rev tap, (rev the motor without being in gear) run it up to say a steady 2500 rpm's and the peeflow and water pressure gauge both in sync with each other pulsates, low pressure/high pressure and so on back and forth. Only when you are at idle does the stream and gauge remain at a constant pressure.

 

[Highroller62]

Forgot to video the pee tube spray, but it's doing the same as the gauge. Never loses water flow but pulsates from a strong stream to a weaker stream.View attachment VID_20210822_172433879~2.mp4

 

[Texasmark]

I don't know. Maybe Dave is onto it. A weak spring on the poppet might be causing the problem....doubt the stat would do that due to the different mechanical design of the two valves. Maybe the long term sitting up caused the spring to weaken, or the rubber seal may have softened and when the pressure builds to a point it releases, then resets. I no of no water pressure regulator on any outboard....or inboard for that matter.

 

[Highroller62]

Well, before I started to throw money at it, Itried a couple of things and think I solved the problem. It seems as if my ear muffs where not sealing well. Putting pressure on the muffs seal, the water pressure issue went away. I verified this by taking the boat down to the ramp and stickinjg it in the water, with no pressure issues. So it seems as if the pump was sucking air causing the popping valve to open and close rapidly. Please remember this! I chased the issue for two week and luckally, I found it and it didn't cost anything but some time and a new set of muffs.. Thanks to all that offered there help!