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Discussion Starter #1 (Edited)
https://cvws.icloud-content.com/B/A...dDocs&p=42&s=X8HOVjN_dqiMHxUvbZEBxD4XkGs&cd=i
I have a 10 foot Zodiac driven by a 10 hp Yamaha short shaft. The water pump works beautifully until the boat goes on plane. When fully on plane the telltale stops showing water and will not flow water until I come off plane. Once off plane the water flow is very hot for a few seconds indicating that no water was moving through the engine. It quickly cools thereafter. Any ideas on why this might be? The intakes seem fully submerged on the outboard. I appreciate any help as it has cost me a few impellers whenever a new driver is in charge.
 

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Do you have a set of muffs? If you have more than one intake then maybe one of them is plugged? You could put the muffs on each side to see if there is a difference. Just a guess.
 

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Discussion Starter #3
Do you have a set of muffs? If you have more than one intake then maybe one of them is plugged? You could put the muffs on each side to see if there is a difference. Just a guess.
Like the ones to flush out the outboard? I did pull the screens off and have rebuilt the water pump and inspected the passageways. The motor works great except when going real fast.
 

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Discussion Starter #5
Maybe your motor is mounted too high and when you're on plane the intakes for the motors lift out of the water?
Did those photos come through? It seems deep enough but I'm no expert.
 

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Discussion Starter #7
Check the thermostat?

Sent from my SM-G960U using Tapatalk
I did hand check the water and the water is cold until it shuts off. And the responsiveness of the return to flow indicates that it is some kind of priming issue with the pump. I would also think that a thermostat would behave opposite, like shutting down when there is much water flow, not shutting down when it is hot. But i will tink about that and check if nothing else comes up.
 

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Maybe your motor is mounted too high and when you're on plane the intakes for the motors lift out of the water?
Did those photos come through? It seems deep enough but I'm no expert.
They didn't make it through. There's often two intakes. One by the prop and one just above the cavitation plate. If that upper one is out you could be sucking air.
What happens when you rev it up on idle?
 

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Discussion Starter #9
They didn't make it through. There's often two intakes. One by the prop and one just above the cavitation plate. If that upper one is out you could be sucking air.
What happens when you rev it up on idle?
Reving it at idle produces no problems.
 

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Is thermostat installed correctly? Mine installs only one way.

Yank it out. Put it in a pot of water on the stove. Heat it up. See if it's opening/closing. One less thing to worry about.
 

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One other idea I had. The impeller is keyed to the shaft. Perhaps the key came out or whoever worked on it last forgot to install the key. Or the impeller separated from the ring. At low RPMs the impeller grabs the shaft. When you crank up the RPMs the shaft just spins inside the impeller. Impeller has to fight the increased water pressure on plane. If the thermostat is fine you should yank the lower unit next and check the impeller.
 

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The thermostat does not need to be open for water to flow out indicator, it's simply an indication of water pressure, if the thermostat were stuck shut it should still "pee."

Those intakes definitely look like they are submerged at all times. You say other boat drivers have cost you impellers, what do they look like when they failed? Is the "hub" spinning as Joeer77 questioned or are the pump vanes damaged from lack of water in the housing?
 

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Discussion Starter #14
One other idea I had. The impeller is keyed to the shaft. Perhaps the key came out or whoever worked on it last forgot to install the key. Or the impeller separated from the ring. At low RPMs the impeller grabs the shaft. When you crank up the RPMs the shaft just spins inside the impeller. Impeller has to fight the increased water pressure on plane. If the thermostat is fine you should yank the lower unit next and check the impeller.
I tried reving the engine when not moving and the impeller is very strong at high RPMs. I am also fairly meticulous at rebuilding it since I have done it several times and everything is in order on each rebuild. Thanks for the input!
 

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Discussion Starter #15
The thermostat does not need to be open for water to flow out indicator, it's simply an indication of water pressure, if the thermostat were stuck shut it should still "pee."

Those intakes definitely look like they are submerged at all times. You say other boat drivers have cost you impellers, what do they look like when they failed? Is the "hub" spinning as Joeer77 questioned or are the pump vanes damaged from lack of water in the housing?
I have burnt through several as at first, I thought it was just a failed impeller. I would replace it test run the motor, and think everything is good. I would then take it out and travel at high speed and it would fail. Many times I thought my work was the problem as it failed for an unknown reason. It was hard to diagnose as the only time there is a problem is when traveling very fast. When traveling at 20 MPS in a 10-foot dinghy, the last place you are looking is around the motor at the water outlet as you don't want to crash into anything. So a few water pumps failed while I was out the helm. It was about my 3rd on where I went out and had a friend driver while I laid back on the transom.

When I pull out the water pump it is mostly suffering from vanes that have the tips melted off and melted rubber filling the outflow port on the water pump housing. I have worked diligently to recover all the pieces and replace any damaged parts.

I own about 8 outboards and have done about 30-40 pumps in the last several years and feel my skills are up to doing a proper job. When idling and at most speeds the pump works well. When coming out of the "hole" the high RPM engine and low speed boat pump water just fine. Only when really going fast do I suffer failure.

I am thinking about positioning some gopro under water shooting at high frame rate and running over them to try to find the problem. I am still hopeful someone here might have and idea and I appreciate all suggestions. Thanks!
 

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Not to insult you at all but have you verified you are using the right replacement?

I've seen pumps at work melt PVC pipe because the flow was so restricted that the friction essentially heated the water to boiling. I don't know how this might relate. Just information.
 

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Discussion Starter #17
Not to insult you at all but have you verified you are using the right replacement?

I've seen pumps at work melt PVC pipe because the flow was so restricted that the friction essentially heated the water to boiling. I don't know how this might relate. Just information.
I would not mind being insulted if it helped me towards a solution. I believe I am using the correct pump and use the proper serial number lookup when replacing it. Also, after having done many many water pumps I am decent at recognizing a pump that is properly fit.

Thanks for the input.
 

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Just thinking outside the box but, try adjusting the tilt one hole as in your pictures it appears the motor is tilted in and when on plane it might be creating a cavitation bubble around the intakes and not allowing water in until you slow down and the bubble dissipates. Cheap try! Good luck.
 

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That last picture looks like it is telling you the problem. Look at the bubbles coming off the strakes on your hull. I believe your hull is pushing air right into your intake at speed - just plain bad luck that your hull design and motor depth work out that way. You need to get that intake into 'clean' water without any air cavitation off the hull.

Buy a Bay Kit to extend your motor down another 5 inches. Install (another) new water pump to clean up any previous damage. Problem will be solved.
 

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That last picture looks like it is telling you the problem. Look at the bubbles coming off the strakes on your hull. I believe your hull is pushing air right into your intake at speed - just plain bad luck that your hull design and motor depth work out that way. You need to get that intake into 'clean' water without any air cavitation off the hull.

Buy a Bay Kit to extend your motor down another 5 inches. Problem will be solved.
^^^
This.

From this picture it looks like the cavitation plate is out of the water. I did a tiny bit of research and saw this article from BoatUSA:

https://www.boats.com/how-to/the-ou...peed-with-outboard-engine-height-adjustments/

It looks like the water intake may be sucking some air when you are on plane running full out.

Hope this helps you figure it out, CopperMan.
 
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