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Old 07-04-2017, 10:17 AM   #1
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Premature Raw Water Impeller Failure

This past weekend I experienced an engine over-temp situation and had to return to the dock on one engine. Looking at impeller integrity first, I removed the cover and saw that the vanes at the face of the impeller were intact so I did not think that the impeller had failed. Next I pulled the entire pump (not difficult) thinking that the pump had failed. After the pump was off I removed the impeller and found that the vanes on the internal half were gone. I could not see that when the pump was on the engine as the pump was on the outboard side of this engine (Lehman 120, starboard side). Given that I replaced the impeller in March, 2017 and it had just 10 hours use I am perplexed as to why this impeller failed so soon. I checked the part numbers. I had used the correct impeller.

So, anyone have any ideas on what could have caused the premature failure? Is there a way to mess up the installation such that early failure is likely? The cover plate is a Speed Seal.
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Old 07-04-2017, 10:30 AM   #2
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How old was the impeller? Between sitting in a store for maybe years and possibly in your inventory for some time, the rubber may have deteriorated from age. I try to avoid spare impellers sitting in inventory for more than a couple of years. If the bore of the pump looks smooth, the bearings turn smoothly, the fork looks good, I would buy a new impeller and see if that solves the problem.

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Old 07-04-2017, 10:46 AM   #3
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Only wild guesses on my part....if the pump dries out between uses, vanes can stick...but I would guess they had to be factory flawed too.

Total factory flaw maybe.

Very old stock before you got it

Wild guesses.... as even a several year old installed impeller usually does better than that.
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Old 07-04-2017, 11:12 AM   #4
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Just wild a$$ guesses --

Any chance the impeller was run dry? Either after install or later into the 10 hours. Did you use glycerine or something else to prelube new impeller? Debris lurking in inlet, like an old vane or a cover seal screw? Is the speed seal cover plate new too? Was the old impeller stamped number and physically compared size a perfect match for new one?
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Old 07-04-2017, 11:56 AM   #5
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Quote:
Originally Posted by sunchaser View Post
Just wild a$$ guesses --

Any chance the impeller was run dry? Either after install or later into the 10 hours. Did you use glycerin or something else to prelube new impeller? Debris lurking in inlet, like an old vane or a cover seal screw? Is the speed seal cover plate new too? Was the old impeller stamped number and physically compared size a perfect match for new one?
I may have run it dry for a couple of minutes but I had lubed it with soap beforehand. But, not certain it was the starboard side that I forgot to open the sea-cock. The old and new impellers were of the same numbers and are physically identical. Speed Seal cover plate is not new but only had about 200 hours use. I'm thinking you are correct about the "run dry" as the culprit because the pump look just fine.

Thanks guys for the prompt come-back.
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Old 07-05-2017, 07:39 AM   #6
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I had a 54 hp Yanmar in my sailboat. I purchased spare raw water impellers from Depco. The correct Johnson part number was on each impeller.
Here's the thing.
The johnson impellers purchased from Depco would not even last 100 hours. The Johnson impellers purchased from Yanmar Dealer would be in perfect shape after 500 hours. Same part number on the impeller.
I never figured out why. But now I only buy my impellers from the engine manufacturer dealer.
Life lesson.
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Old 07-05-2017, 09:56 AM   #7
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Small debris that got in the intake (sand?) or a plastic bag that covers the intake for a few minutes can cause the pump to run dry for a little while without you noticing it.

As others have pointed out, just wild guesses. The important thing is that you managed to get back to the dock without any harm to your engine.
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Old 07-05-2017, 05:24 PM   #8
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Something that works for me is to apply a finger of waterproof grease inside the housing, the face plate and the end plate when installing rubber impellers. Doing this protects the impeller at initial startup and increases suction.
I've been doing this since the early 60s and can't remember any impeller failure in my life. All engines big and small, gas, diesel. I pull my impellers once or twice a year (depending on use), inspect, grease and reinstall. Current boat impellers went over 5 years. 2 Detroits, Onan and Perkins. Still stored as emergency spares.
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Old 07-05-2017, 07:57 PM   #9
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I would think about the lifetime of an impeller running dry at way less than a few minutes. Under a normal start if you're near the waterline, dry time is nil.
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Old 07-05-2017, 08:04 PM   #10
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I had a fairly new impeller before I pulled the boat out for bottom job . I never thought about it drying out . Boat was out about 4 months and two of those it was pretty hot out . The impeller failed not long after relaunch . Five of the eight vanes broke off .
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Old 07-05-2017, 11:11 PM   #11
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The one time I had a similar experience was on an outdrive, out of the water, wearing the water hose ear muffs. The muffs slipped. I notice the slipped muffs within a couple of minutes of starting the engine, but that was enough time to tear of most of the impeller vanes.
If it is dry, the vanes will tear off. Life lesson!
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Old 07-05-2017, 11:29 PM   #12
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I bought a new impeller to install, pulled the old impeller, and as I was comparing the old and new impeller, they were identical EXCEPT, the new impeller had cracks on three of the vanes. This was a new manufacturers brand impeller. I check them closely before I leave the parts place now.
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Old 07-06-2017, 03:04 AM   #13
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From these experiences,there is a real risk any spare impeller kept onboard may not stay in usable condition.Would it be worth unpacking the spare impeller, checking its condition, greasing it with a rubber grease(would dielectric be ok?)and resealing it into its packing?
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