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Inboard Diesel Noob, Impeller question

2K views 21 replies 13 participants last post by  marvout 
#1 ·
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
 
#2 ·
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?
 
#5 ·
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.
 
#7 ·
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 2¼" 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.
 
#21 ·
#8 ·
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.
 
#9 ·
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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#10 ·
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.
 
#11 ·
Before I ran the engine, I had the impeller out. It seemed in good shape, perhaps a bit stiff from age. All housing surfaces were cleaned. All surfaces were smooth and there were no scratches. The impeller seems to fit properly snug in the housing.
No, I was not expecting that I would use/trust this impeller once on the water. I managed to get all the other service parts for the boat from my local auto parts store, the filters and hoses and belt. I expect that before I launch next spring I'll have a nice new impeller in place. You can all sleep well. <grin>
I do have exhaust questions, but I'll post that to a new thread.
 
#13 ·
I change the impeller every second oil change. They are that unreliable. However for little used boats replacement should probably be on calendar schedule. I would think annually. The impeller is easily the most unreliable piece of a boat and close to the top in criticality, and gives no clues or warning to its pending demise.

Not that an engine should be considered necessary, of course, but when one does choose to rely on it it must be reliable.
 
#14 · (Edited)
The Oberdorfer N202M- should pump 6 GPM at 1700 RPM, 9 GPM at 2500 RPM, and 12 GPM at 3500 RPM (which you should not hit, as the engine should not rev at over 3200 RPM)

The N202M-15 uses 3/8" fittings, and the N202M-16 uses 1/2" fittings. Universal used the -15
 
#16 · (Edited)
Data in my preceeding post was excerpted from the Oberdorfer spec sheet, which also includes this performance chart.

Here, knock yourself out...


On reading it it does not seem to agree with the printed specs (@1750 RPM I see 3 GPM. I don't see 12 GPM at all)
 
#17 ·
Pumps run at 1/2 the engine RPM, or something. I think mine was on the the injection pump shaft...or camshaft...or some idler gear...when I last looked. Probably varies. They live longer at reduced RPM.
 
#19 ·
Getting the pump to draw when you're on the hard can be difficult, and you don't want it taking too long for water to get to the impeller.

My solution: I drilled a hole in the bottom of a bucket, and then installed a 90 degree hose barb in it. The barb is attached to a three foot length of garden hose. On the end of the garden hose is a barb that will fit into the hose that runs from my raw water seacock to the engine.

I just disconnect the hose from the seacock, stick the garden hose barb in it, and set the bucket down someplace a little higher than the mid point of the engine. I fill the bucket with water, then start the engine. This guarantees the pump is going to prime, as gravity assists it. I run a hose to the bucket to keep it filled. You can run the engine as long as you continue to replenish the bucket.

The barbs are just fitting from the local Home Depot. Run you five bucks. You'll need the bucket and hose at the end of each season to winterize the system (depending on where you live). Just fill the bucket with the pink anti-freeze and run a few gallons through it.
 
#20 ·
The pump should have no trouble self priming itself. The previous comment about a scored cover is worth checking out. I had a Universal 5411 with a Sherwood pump and had that issue. Removing the cover plate and filing it flat fixed it. You need a good seal between the side of the impeller and the cover plate to be able to draw a slight vacuum to get the pump to prime.
 
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