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Old 30-10-2010, 13:47   #1
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Water Pump Problem - I Can't Figure this Out . . .

We have two Volvo MD2030 engines on our boat. One of them eats raw water impellers at a rate of one every 50-100hrs. The impeller on the other engine lasts for years.

I have replaced the pump completely on the one engine and it still eats impellers at the same rate. This has been going on for 2 years and it is getting expensive to spend $30 every 4-10 weeks.

I have not yet switched pumps between the two engines.

Anyone have any ideas on why one engine eats impellers in two separate pumps (one brand new)?

Mark
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Old 30-10-2010, 14:11   #2
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Old 30-10-2010, 14:34   #3
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colemj

I don't know how the pumps are set up on your engines, or how they are driven, off the cam, off the crank, etc.
Could it be an alignment problem of some sort.
On my Westerbeke 46, for instance, the pump is driven by a tenon on the end of the crankshaft, mating with a open mortice type on the pump shaft.
When installing a new pump, the nuts go on snug, and the engine is rolled over to align the male and female parts, then you tighten them.
If you don't do this, the seals and bearing are gone in a season.
Hope this helps.
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Old 30-10-2010, 15:03   #4
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When you changed the water pump did you change the other cover of the impeller?

The cover can get a grove on the inside of it where the surface of the impeller sweeps against it. After a while that surface becomes pitted. Could a burr on the pitted surface be causing it?
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Old 30-10-2010, 15:37   #5
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When you changed the water pump did you change the other cover of the impeller?

The cover can get a grove on the inside of it where the surface of the impeller sweeps against it. After a while that surface becomes pitted. Could a burr on the pitted surface be causing it?
Ah, I forgot to mention that. I put on a speedseal impeller cover. After a year with the problem, I switched the cover with that from the other engine. After another year with the problem, I put the original cover back on. Now the problem continues.

There is no problem with the other engine using the speedseal cover or the original cover from the engine that eats impellers.

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Old 30-10-2010, 16:26   #6
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Sounds to me like you have a restriction in the line somewhere, either up stream or down. But if the pump is cavitating it will eat up the impellers. And since you replaced the pump it can't be the cam plate inside.
Are the pulley's on both motors the same size?
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Old 30-10-2010, 16:46   #7
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I had a similar problem once.

It turned out I had sucked up a small piece of wire, not sure from where or how.

When the motor ran it was sucked enough to just poke in and when the engine was off it snuck back up the inlet where it was conveniently jammed,.

Replaced the hose containing the offending wire after 9 impellers and bingo all good.

Good luck. At least you have 2 motors, we only had one!
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Old 30-10-2010, 16:50   #8
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A downstream obstruction will overload the impeller blades. It is probably a good idea to check the entire system for bits of old impeller. They can lodge in hoses, heat exchangers etc and will cause the problem you describe. Also check that the water outlet into the exhaust elbow is clean.
Regards, Richard.
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Old 30-10-2010, 16:58   #9
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Check the flow rate difference between the two motors. You can do this by catching the water as it runs out the exhaust. Then you'll know if you have a restriction.
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Old 30-10-2010, 17:20   #10
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Just a wild guess: when you shut down the engines, does one of them bounce back, like it rotates a quater turn backwards before it stops? If so, that will put a big strain on the impeller.
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Old 30-10-2010, 17:28   #11
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I would test for:

- misalignment, or
- pressure issues (on either side).

I am not sure of the second thing, but I believe a clogged line can in the long run kill a rubber impeller (?).

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Old 30-10-2010, 18:01   #12
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Just a wild guess: when you shut down the engines, does one of them bounce back, like it rotates a quater turn backwards before it stops? If so, that will put a big strain on the impeller.
Yeah that too!
Are they both RH motors. On Cats they usually are.
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Old 30-10-2010, 19:10   #13
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My gueess

is cavitation like delmarrey mentioned. This can be checked by putting a pressure guage on the suction and discharge of each pump. Run them and see what you get for pressures. The pressures of each pump should be similar.

If one of the pumps is turning faster than the other, it CAN cause cavitation too.

I'm not ruling out alignment issues, but if alignment is an issue, the mechanical seal sould fail pretty quickly. I'd also look at the internal casing of the pump, if you have a cavitation problem, you should see signs of in inside the casing.

Just my opinion.

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Old 30-10-2010, 22:59   #14
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Right hand-left hand?
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Old 31-10-2010, 00:24   #15
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Right hand-left hand?
You know? Rotation!
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