Inboard Diesel Noob, Impeller question - SailNet Community
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post #1 of 22 Old 08-24-2016 Thread Starter
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Inboard Diesel Noob, Impeller question

Picked up my boat a few weeks ago, have it on the driveway as it needs a bit of work. 1971 Pearson Wanderer with a Universal M3-20 3 cyl 18 hp diesel. I've never owned an inboard, never mind a diesel before. My guess is the motor hasn't been run for anywhere between 3 and 6 years. I managed to get it running, so that's a big relief to me.

I'm just surprised at how little an impeller can draw. I had assumed that I would be able to get it draw water out of a bucket. It pretty much didn't at all and needed to be primed. I'm not sure this will be a problem once the boat is in the water. The waterpump is below the water line so it should self prime. Is this normal?

Also, it seemed that it would stop drawing after a few minutes. Now, I'm not sure if I accidentally let it draw some air. But, is that the case? If it sucks air, then it can vapor lock and burn up the impeller?

MH
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post #2 of 22 Old 08-24-2016
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Re: Inboard Diesel Noob, Impeller question

If the engine hasn't run in years a new impeller would be one of my first things to do - even before attempting to start it. Have you inspected it? If any vanes are missing you need to try to find them because they can end up plugging cooling passes. RWC or FWC?
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post #3 of 22 Old 08-25-2016
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Re: Inboard Diesel Noob, Impeller question

As faster said. change the impeller. before launch. the impeller should be changed at least every 2 years .don't run the engine with a dry impeller

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Re: Inboard Diesel Noob, Impeller question

Ditto the new impeller. Did you clean the sea strainer? Check all clamps and, if the strainer has a gasket, that could be suspect too.

You may have to descale the cooling circuit too.


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post #5 of 22 Old 08-25-2016 Thread Starter
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Re: Inboard Diesel Noob, Impeller question

Sorry, I should have said a bit more. I've had the impeller out and it looked good. It was a little stuck on, but with a clean up, the shaft and impeller are clean and work now. I did a new belt, oil filter and the engine mounted fuel filter (which appeared to be original to the install, the engine was painted at some point and this still had the sticker on it "Change after 50 hrs and then every 100".) I replaced the hose to and from the water pump. I've made sure the cooler flows free. And I've checked the hose from the cooler to the exhaust, all good.
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Re: Inboard Diesel Noob, Impeller question

I dont think some old engines spit as much water as the new ones.

And you wouldnt have had much rpms in the driveway, so old 18hp, at idle, probably doesnt have much through put of water at all.

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post #7 of 22 Old 08-25-2016
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Re: Inboard Diesel Noob, Impeller question

For context; I have recently been working on two client's boats. One with a Universal M3-20, and the other with an M-25. My boat has an M-25 also.

Your engine shipped with either a Sherwood or Oberdorfer pump. The Oberdorfer M202 is the preferred of the two.

You can get the pump to prime faster by adding some liquid soap or glycerin to the pump chamber. I use Dawn.

If you are not seeing the expected flow rate of water while on the hard, check the following;
  • impeller - look closely at the vanes
  • hoses - ensure that there are no kinks or obstructions
  • hose clamps, and barbs - make sure that clamps are tight, and barb fittings are clean
  • heat exchanger - you should have the 2" model. make sure that water and coolant can flow through unobstructed. Some people boil them out and repaint, I simply replace them (~$500).
  • mixing elbow - verify that water can easily flow through the elbow, and into the waterlift muffler.

If you were in the water, I would add to check the seacock, and through hull fitting. The through hull should NOT have a strainer.


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post #8 of 22 Old 08-25-2016
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I'm thinking Mark is probably right. My old Perkins doesn't spit much unless she has some revs on. With the boat out of the water you wouldn't have any head pressure at the seacock, all kinds of air etc. I wouldn't stress too much. You'll be able to tell more when she's in the water, the real test will be to see if she maintains engine temperature under load.
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Re: Inboard Diesel Noob, Impeller question

The rubber compound in a flexible impeller will age, harden, start to crack at the root of each vane, and then start shedding vanes. This is a function of age and not just use. It's unwise to trust one over two years old.
Replace it asap.

Note that effects of entropy work almost as well for a new impeller left on a warehouse shelf for many years as for one in use.

For all of our small engines, impellers like that are best viewed as a Preventative Maintenance item.

Loren
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post #10 of 22 Old 08-25-2016
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Re: Inboard Diesel Noob, Impeller question

The original poster expressed regarding the pumps ability to self-prime and maintain a prime. Given that concern, the owner should inspect the end plate of the pump when it's off to replace the impeller. Leaks between the end plate and the impeller can cause the loss of prime. Scoring in the end plate can be polished out or the end plate can be flipped.
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