KAD42 keeps shredding impeller

BruceK

BruceK

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I'd be very grateful if someone could help me determine why my starboard impeller keeps shredding.

Last year before lift out everything ran fine. Engine temps were good. Boat was on hard for 4 months now over winter. On drop in took her for a spin and everything fine. Changed impeller and checked she was pumping. Boat stood for two days and next outing she had shredded the impeller within a couple miles. Changed impeller and checked she was pumping. Ran boat and temps fine. Boat stood for 2 days and she shredded the impeller within 2 miles on next trip.

I have managed to get all impeller bits out on both occasions. When I take the raw water feed off water gushes out so no blockages between leg and pump. Impeller sits 1.5 inch above waterline so does not have to lift water far. I have removed the hose downstream from the basket and a fair amount of water gushed out then stopped. (yes it scalded my hand trying to catch any wayward impeller bits that might have got past the basket...doh!)
Now the unscientific bit. On the downstream hose I can both blow and suck reasonably freely with the sound of gurgling water from the ?transom? (difficult to be sure when working alone) I wouldn't have thought this possible with my 290 legs that vent below the cavitation plate. However there are no leaks in the cooling system.
The legs have been off for a Volvo dealer service and replaced by them, when running the engine there is no unusual exhaust note.
Pump housing looks good with some wear but nothing unusual and the impeller makes a good tight fit all round.
When opening the basket the basket cap isn't sticking on requiring lifting off with the aid of a pair of pliers which indicates some loss in vacuum.
 
Maybe a silly question but have you fitted a genuine impeller if not this could be your problem.
non genuine sometimes are a bit to tight on faceplate.
 
Yes. Genuine impeller from Volvo. But both did seem to be a bit tight on face plate tbh. The port engine is just fine with it though.
 
What is immediately downstream of the pump?
is it a cooler?
Strip that cooler and clean out any impeller bits. If there is any blockage in the raw water system it will cause the pump to be starved of water which will cause the impeller to overheat. The pump should remain cold with the engine running.
If there is no blockage the pump may not be pumping efficiently. You can buy a new raw water pump or , for less money a pump rebuild kit.
Alao check the raw water strainer. When you remove the screw on lid is it difficult to remove the plastic disc - there should be a vacuum created by the pump which is holding the disc in place which has to be broken ? If the disc is easily removed it may be the disc is not be making a seal - or the pump may be worn. If the water is not being held in the strainer by the seal created bu the pump and the strainer disc/seal the pump may be dry on engine start and may not initially pump water - which may trash the impeller causing further pump inefficiency .
 
There are no observable blockages. Water flows freely from the leg to pump.
One thing I didnt mention is the engine does not overheat until on the plane. It has pumped well always while testing which has either been at idle or displacement speeds. Having the engine overheat on the plane and under load seems to have an obvious answer but could it be that when on the plane the intake hose is clear of the water where it would connect to the drive and sucking air. I.e it has cracked over winter and not noticed by engineer or it has decoupled from the drive. Or am I reaching here. Would hate to lift the boat to find thats an improbability
 
Given the clue that it doesnt happen until on the plane, I'd check the nipple where the inlet hose attaches to the leg.
If this breaks,or hose becomes detatched then the hose is under water at displacement speed and water pickup is fine. Once on the plane however, it's above the waterlevel so just sucks air... result in no cooling water and shredded impellor.
 
tico said:
Given the clue that it doesnt happen until on the plane, I'd check the nipple where the inlet hose attaches to the leg.
If this breaks,or hose becomes detatched then the hose is under water at displacement speed and water pickup is fine. Once on the plane however, it's above the waterlevel so just sucks air... result in no cooling water and shredded impellor.
Click to expand...

+++111 !
 
Fundamental, but is the deflector plate (the one that flips the impeller blades in good condition? we have them break up and shred the rubber works, another trick at least on the old jabsco pump, is that you can reverse the front plate if the inside is showing grooves, and the back plate plate can also be a problem, this can be replaced after removing the deflector.
 
Thank you for all the replies. Had a mate dive down and inspect the raw water feed pipe from the drive to the transom. All appears to be good. With the leg raised I blew down from the pump and the only bubbles escaping were from the drive intake vents.
So, scuppered I have ordered all new hoses and a new pump in case it is the pump or a hose is collapsing or blocking with any growth when under vacuum pressure because it is not doing it under positive pressure. After that I'm all out of ideas.
 
Hi, I had the same problem a couple of years ago with a KAD43. In the end it turned out to be a small imperfection in
the pump. As soon as we changed the waterpump the problem was cured. Cost me four impellers though, regards Nick
 
Thanks. I have ordered new pumps and hoses. I will fit them this weekend and report back. I hope it's that simple because I am all out of ideas.
 
Don't really know anything about VP but could it be to do with which way the impeller is rotated when fitted?
(I know with Merc that you have to ensure you fit it in a clockwise direction)
 
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