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Old 07-23-2018, 03:19 PM   #1
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New Cummins Genset chewing through impellers

Hi All,
Our Swift 44 is not quite a year old and I have replaced the genset impeller twice now.
They are cracking and shedding their vanes. The waterflow is fine and there are no blockages.
Anybody else having this problem as I heard that Cummins have an issue with this.
Cummins 9.5 MDKDM

Cheers,
Clive.
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Old 07-23-2018, 03:39 PM   #2
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Contact Onan/Cummins. There may be a defect in the pump that causes this. Impeller ought to last a couple of years or a few hundred hours.

Check in manual and see when replacement is recommended. If it is shorter than this interval (calendar time or run hour wise), you should have a valid warranty claim.
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Old 07-23-2018, 03:40 PM   #3
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Is this the 3 or 4 cylinder kubota motor with the direct drive water pump? If so, mine which is 16 years old, tends to go through them also.

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Old 07-23-2018, 03:42 PM   #4
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I doubt the water pump has anything to do with being a Cummins.
Surely it would be jabsco, Sherwood, oberdorfer or some other brand.
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Old 07-23-2018, 04:14 PM   #5
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Thanks guys,
Service is 500 hours or yearly and Cummings here said that they had not heard of anything although I heard that they have an issue where the pump is allowed to run dry.

I am just seeing if other Swift 44 owners have had the same issue to see if there is a problem with install/genset or just me.
2 impellers in less than a year seems excessive - 190 hours to date.

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I doubt the water pump has anything to do with being a Cummins.
Surely it would be jabsco, Sherwood, oberdorfer or some other brand.
Sorry you are right, I should have said that the Sherwood pump on the Cummins genset is chewing through impellers.
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Old 07-23-2018, 04:41 PM   #6
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Occasionally my sea strainer empties a bit and I get a dry start on the Genny.

If yours empties every time, that could be hard on impellers.
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Old 07-23-2018, 04:53 PM   #7
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Occasionally my sea strainer empties a bit and I get a dry start on the Genny.

If yours empties every time, that could be hard on impellers.
+1. Check and see if you are getting a small air leak that is letting the hoses or strainer drain out before you are starting.
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Old 07-23-2018, 06:53 PM   #8
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Has impeller failed right after start? Or after running a long time? Right after start implies airbound pump.

Has gennie sat for a long time between runs? That is hard on impellers too.

A good hint is when you crank it up, put hand on pump cover plate. If it gets warm, it has not primed.
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Old 07-23-2018, 07:34 PM   #9
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Hey some great tips guys, thanks.
I keep an eye on the strainer and it always has water in it.
The first one went after about 60 hours. I put this down to the genset having been in storage for a while before being fitting into the boat.
Now a 2nd one has started to crack I am suspicious. We use the boat a lot and it's never not been run for less than 3 weeks.

The cracks are on the base of the vanes. I am glad I got to this one before the vanes came off as it was a pain having to take the heat exchanger apart to find the bits!
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Old 07-23-2018, 07:45 PM   #10
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Is it possible the "new" impeller was old stock?
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Old 07-23-2018, 07:59 PM   #11
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Cracks are usually from heat caused by running dry. Eventually it causes vanes to break off. When you install the impeller, take a finger full of waterproof grease and lube the inside housing, shaft, anvil, and cover plate. On the first startup, the grease seals the impeller to the housing much better causing greater suction. (Directions in old manuals that way.) My impellers on the generators go 500+ hours and are still in good shape. I inspect and re-grease 1-2 time a year. DD mains go 2500 hours. Anywhere in the raw water plumbing before or after the pump will get cooler to the touch if water is being pumped. You can also install a tee in the line with a thermometer.
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Old 07-23-2018, 08:08 PM   #12
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You`d be unlucky both the original and the replacement impellers were deteriorated old stock, but it`s possible. It`s like both impellers were affected by the same cause.

Checked the pump housing for anything untoward? Is the strainer clean? Could the original impeller have been a similar but incorrect part,and the replacement was bought using the same part number?
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Old 07-23-2018, 08:35 PM   #13
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I think, as most have pointed out that the issue is to do with the pump running dry. When I changed the impeller very little water was present.
I'll see if it is draining out somewhere and report back.
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Old 07-23-2018, 08:44 PM   #14
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If the impeller is installed with the vanes in the wrong rotation they tend to wear out very quickly. I’ve seen mechanics install them backwards only to have the owner repeat the same mistake unknowingly. Don’t assume the vanes will flip themselves into the correct rotation.
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Old 07-24-2018, 01:08 AM   #15
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If the impeller is installed with the vanes in the wrong rotation they tend to wear out very quickly. Iíve seen mechanics install them backwards only to have the owner repeat the same mistake unknowingly. Donít assume the vanes will flip themselves into the correct rotation.
Doesn't matter.
I had heard this and sat there at last change and manually spun it one way and the other again and again ant the blades instantly change direction.
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Old 07-24-2018, 07:00 AM   #16
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Confirm there are no blockages in the auction side. I had a mercruiser bravo stern drive that ate impellers. I finally traced the problem to a water inlet pinched shut in the transom by aluminum corrosion. This caused the pump to run dry at anything over idle speeds
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Old 07-24-2018, 07:35 AM   #17
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On your Swift Trawler, is the raw water pump above or below the water line? If it's above, then it will always start dry and run dry until it pulls in water, and that's a likely cause of your issue.


One trick I have heard is to create a loop in the raw water intake hose to trap a pool of water in the intake hose and pump so it gets a wet start rather than a dry start.
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Old 07-24-2018, 08:02 AM   #18
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Quote:
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On your Swift Trawler, is the raw water pump above or below the water line? If it's above, then it will always start dry and run dry until it pulls in water, and that's a likely cause of your issue.


One trick I have heard is to create a loop in the raw water intake hose to trap a pool of water in the intake hose and pump so it gets a wet start rather than a dry start.
+1

Pardon the dust as this pic was during my refit and repower.

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John Deere has the transmission cooler (black vertical cylinder) before the raw water pump. After the pump, the raw water travels up over the engine to the heat exchanger. After shutdown, the raw water pump sits in a low point of the raw water loop with the pump full of water.

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Old 07-24-2018, 09:35 AM   #19
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Quote:
Originally Posted by twistedtree View Post
On your Swift Trawler, is the raw water pump above or below the water line? If it's above, then it will always start dry and run dry until it pulls in water, and that's a likely cause of your issue.


One trick I have heard is to create a loop in the raw water intake hose to trap a pool of water in the intake hose and pump so it gets a wet start rather than a dry start.

I have heard of this before and in fact we had this problem with our last boat. The way we got around it was to apply plenty of silicon grease (Dow Corning Compound 4) to the impeller when fitting and more importantly turn the seacock off after shutting the genset down. Don't open it until you are ready to start it the next time. This keeps water in the seawater pump and prevents the dry starts that ruin the impeller. To stop myself starting it with the seacock turned off I put a notice on the battery switch for the genset that had to be removed before I could turn it on.
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Old 07-24-2018, 10:22 AM   #20
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How long does it take for water to appear at the exhaust outlet? A few seconds or immediate is good, plus ten seconds bad. Station an observer at the exhaust outlet to ascertain delay time, if any, when the go button is activated.
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