Low RPM's yeilds 0 water pressure

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LSUtiger

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I have a 120 hp Force on a pontoon. I just got this motor to replace my old Force 90. Replaced impeller, thermostat and temp sensor before taking it out. Motor starts easily and runs great but the water pressure seems low especially at idle. I went through a dead slow zone and the horn went off. I increased the RPMs and the pressure went up and horn off. Running about 10-12 lbs at 3500 RPMs and falls to 0-2 lbs at idle. Does it every time. I've checked the tube seal and water passages and they all seem OK. Any suggestions? Could I have put the impeller in backward?
 
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foodfisher

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Re: Low RPM's yeilds 0 water pressure

My first impeller change forced me to pull the S/S plate off and I found broken pieces of impeller under it blocking water passage
 
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wadeshuey

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Re: Low RPM's yeilds 0 water pressure

I have a 1991 Force 90hp that is doing the exact same thing. Low RPM and start to overheat, High RPM and I'm fine. I have also replaced the impeller that looked like it was still new and didn't need replaced. Does anyone have any other ideas?
 
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john from md

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Re: Low RPM's yeilds 0 water pressure

Did you check the condition of the water pump housings (upper and lower) ? If they are corroded, water will bypass at low speed but will run fine at high speed. Your symptoms lead me to believe this may be your problem.

John
 
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LSUtiger

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Re: Low RPM's yeilds 0 water pressure

john from md said:
Did you check the condition of the water pump housings (upper and lower) ? If they are corroded, water will bypass at low speed but will run fine at high speed. Your symptoms lead me to believe this may be your problem.

John
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John, thanks for the reply. I did check the WP for old impeller parts but not all that close. I have the lower unit down so I'll take the WP apart again and check. Eric
 
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john from md

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Re: Low RPM's yeilds 0 water pressure

Eric,

Two areas to look at. 1. the inside of the impeller housing itself. Look for pitts in the sidwalls and top. Also check the drive shaft seal insure that the inner ring that touches the shaft is still soft and pliable. If not, water will leak from the top and not get to the head.

2. Look at the lower part of the pump under the stainless rub plate. Insure that there is not corrosion destroying the little walls under the plate. If these "walls" are corroded, water can go bypass back to the inlet side of the pump thereby not supplying sufficient volume at low speed.

John
 
L

LSUtiger

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Re: Low RPM's yeilds 0 water pressure

John, I checked the water pump housings and they look very good. Nothing looked suspicious except there is a void of 1/4" in the top of the upper housing around the shaft that looks like it should have a seal of some sort. The top of the impeller seals it at the SS cup side and there is a shaft seal on the outside top, but it just looks funny. Just to check my impeller blade direction, the blades bend to the left left when it is on the shaft and you are looking straight down from the top.
 
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foodfisher

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Re: Low RPM's yeilds 0 water pressure

The impeller blades are bent in the right direction. Looking down, the shaft spins clockwise and the blades trail away.
 
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john from md

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Re: Low RPM's yeilds 0 water pressure

Eric,

I will look at them this afternoon when I get home from work.

John
 
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