Help identifying water pressure issues

Long story, but here goes.
Bought Mako 18LTS with 115 Optimax 2 cycle back in late 2014. The motor had generally run pretty well. I do take it down to the NC coast and get in some fairly shallow water that has some grass and I'm sure I have skimmed the bottom a few times. Last September (2020), I was running at high rpm's and got some tones and the engine dropped down to idle or somewhere in that range. I took it in and was told the problem was the water pump as it had never been replaced. They acted as if it was common to need to replace the water pump every 2-3 years if running inshore down at the coast. So, the water pump/impeller was replaced, I fished some more last fall through this July in Lake Norman in NC (deep clear lake with zero interaction with sand or grass). In prepping for my trip to the coast, I noticed that my water pressure would barely move the needle at idle, and then max out in the 8 to occasionally 10 range. This seemed too low to me along with a fairly week stream, so I took it in to have it checked out. I did mention the popit valve, clean out filters, etc, and the mechanic said something to the nature of "sure, we're aware of all of that and will check it out". So, they did a complete lower unit rebuild (their words, not mine) and assured me it was dialed in. I got it back, and what I'm finding now is that at idle, I'll get 7-8 which is much better than the 1-2 I was getting before. However, as I throttle up, pressure increases all the way to 11-13 up to around 2800 rpms, and then it drops back to 9 or so and will mostly stay there up to full throttle. When I throttle back down, it stays at that 9 or so range until I get to 2500-1800 rpm's or so and it will come back up to around 12 or so, drifting back to 7/8 at idle. The motor has run fine, but it just doesn't seem right to me that the pressure drops back to 9 once I hit around 2,800 rpm's. Seems to me it should keep rising up to full throttle.

As for the telltale, water is coming out fine, but not a hard stream like I seem to remember when it was new. Also, as I get up to higher rpm's, it comes out almost in a spray rather that a tight stream. I can also confirm that the discharge hose just before the exit point while not being crimpled, it is definitely flattened out a bit as it makes a bend. Maybe that has something to do with the spray? I know I've had water hoses before that flatten out a bit without crimping, and they seem to work okay. However in this case, I would think any kind of flattening could cause somewhat of a restriction.

Any thoughts or recommendations are greatly appreciated.

Thank you,
TW

Is this normal behavior or something new?

It's normal for the pressure to jump around some as the thermostat opens and closes, since the amount of water it will bypass varies depending on water temperature. My Suzuki will jump around all over the place, especially in cold water.

The weight of the spring on the bypass should be what will make pressure.

I had a older Yamaha tiller handle outboard that had overheating issues intermittently. Went to digging around in it an found pieces of exploded water pump impeller. I dug a bunch out of it in the time I had it

I put a temp gauge near the dash on my boat and a compatible sending unit on the motor where a plug was just above the #1 spark plug. Those two MUST match within the ohm range for each. Got both from Amazon under $25 total for both. It confirmed the thermostat was stuck open, I replaced the thermostat, now all is well. Seeing flow is good, but knowing the temp is even better.

Both are good information to have

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Originally Posted by Tarheelwx

Long story, but here goes.
Bought Mako 18LTS with 115 Optimax 2 cycle back in late 2014. The motor had generally run pretty well. I do take it down to the NC coast and get in some fairly shallow water that has some grass and I'm sure I have skimmed the bottom a few times. Last September (2020), I was running at high rpm's and got some tones and the engine dropped down to idle or somewhere in that range. I took it in and was told the problem was the water pump as it had never been replaced. They acted as if it was common to need to replace the water pump every 2-3 years if running inshore down at the coast. So, the water pump/impeller was replaced, I fished some more last fall through this July in Lake Norman in NC (deep clear lake with zero interaction with sand or grass). In prepping for my trip to the coast, I noticed that my water pressure would barely move the needle at idle, and then max out in the 8 to occasionally 10 range. This seemed too low to me along with a fairly week stream, so I took it in to have it checked out. I did mention the popit valve, clean out filters, etc, and the mechanic said something to the nature of "sure, we're aware of all of that and will check it out". So, they did a complete lower unit rebuild (their words, not mine) and assured me it was dialed in. I got it back, and what I'm finding now is that at idle, I'll get 7-8 which is much better than the 1-2 I was getting before. However, as I throttle up, pressure increases all the way to 11-13 up to around 2800 rpms, and then it drops back to 9 or so and will mostly stay there up to full throttle. When I throttle back down, it stays at that 9 or so range until I get to 2500-1800 rpm's or so and it will come back up to around 12 or so, drifting back to 7/8 at idle. The motor has run fine, but it just doesn't seem right to me that the pressure drops back to 9 once I hit around 2,800 rpm's. Seems to me it should keep rising up to full throttle.

As for the telltale, water is coming out fine, but not a hard stream like I seem to remember when it was new. Also, as I get up to higher rpm's, it comes out almost in a spray rather that a tight stream. I can also confirm that the discharge hose just before the exit point while not being crimpled, it is definitely flattened out a bit as it makes a bend. Maybe that has something to do with the spray? I know I've had water hoses before that flatten out a bit without crimping, and they seem to work okay. However in this case, I would think any kind of flattening could cause somewhat of a restriction.

Any thoughts or recommendations are greatly appreciated.

Thank you,
TW

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Remember this, as you speed up water is flowing faster around the side of lower unit where your water pickup is, so it’s a little harder for pump to pick up as much water as it does at lower rpm plus your starting to ingest a little air as you speed up. Main thing is that you have good water pressure at that higher speed. Think I would replace that piece of tubing to P tube.

All of this is pretty normal for an Optimax at lower speeds the poppet valves are closed so the water pressure will move up as the motors RPM comes up the water pressure builds up will open the poppet valves and cool the motor down. When slowing down the poppets close the thermostats close to get the motor up to idle temperature. Changing the Optimax water pump can be a yearly thing it is not that they wear out the heat from the Opti exhaust hardens the impeller so it will not pump as well as it should. Typical of a weak water pump on the Opti`s is when idling for an extended period then opening the throttle and get an overheat alarm then goes off after a bit. Normal water pressure on an Opti is around 2-4 pounds of pressure at idle.

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Originally Posted by Bricks

All of this is pretty normal for an Optimax at lower speeds the poppet valves are closed so the water pressure will move up as the motors RPM comes up the water pressure builds up will open the poppet valves and cool the motor down. When slowing down the poppets close the thermostats close to get the motor up to idle temperature. Changing the Optimax water pump can be a yearly thing it is not that they wear out the heat from the Opti exhaust hardens the impeller so it will not pump as well as it should. Typical of a weak water pump on the Opti`s is when idling for an extended period then opening the throttle and get an overheat alarm then goes off after a bit. Normal water pressure on an Opti is around 2-4 pounds of pressure at idle.

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They seem to be complex motors