Need Raw Water Pump Advice

[chris5977]

My boat's engine is overheating because the the water pump is not working. I am certain that it has an adequate supply of water. I even replaced the raw water strainer. I am also certain that the problem is not downstream because I pulled the hose off the heat exchanger and not a drop of water comes out. It all started when I replaced the impeller. I knew it was due because I had not done that since I got the boat. A couple of teeth were missing. I cleared out the broken parts of the impeller. During the process shaft that turns the impeller came out of the engine. I just stuck it back in an it fit fine. I ran the engine with the cover off and can see that the impeller is spinning. But when I put the cover back on it won't move any water.

I did some research and they say to pull the entire pump and align it if the shaft comes off. I will try this but it just doesn't make any sense. Either it fits or doesn't. Just seeing if anyone has insight into what is going on.

Universal Diesel Engine Owners Manual - Sea Water Impeller Replacement - Marine Diesel Direct / TOAD Marine Supply

[chrisjs]

This may sound ridiculous, but are you sure the seacock inlet is really open??
With the pump cover off, impeller exposed, open the seacock gently and make sure water begins to flow out of the pump face plate area. Obviously, this assumes your pump is below outside sea level. Either way you should see water flowing into the strainer with the seacock open. Assuming water is coming in but just not getting to the pump, you may have a blockage between the inlet and the pump (eg kinked hose) or your pump face plate may be worn to the extent that you are not able to suck water up from sea level into your pump. Check for scoring on the inside of the pump face plate. Depending on the design you may just be able to reverse the face plate to now match the unworn exterior surface to the impeller. Lastly if all else fails, try to prime the pump (face plate back on) from above by pouring water into the manifold or tubing just down stream after the pump. Good luck!! It is always the simple things that screw us in the end!!

[chris5977]

Good advice on priming the pump. I will try that.

I am certain that I have a good supply of water. I put my mouth on the supply hose and sucked in seawater. It seems like the impeller is not creating a vacuum. I did try rubbing it with dish soap.

[chrisjs]

I would definitely check/ reverse the impeller cover plate if possible. It does not take much "leakage" or impeller bypass for these pumps to not self prime

[Chasing Summer]

Did you remember the woodruff key on the impeller shaft?

[Steve W]

Not much help, but I can tell you that on my Universal M-18, if the raw water pump shaft is not all the way in the slot on the shaft that drives it, there is no way the cover would go on over the impeller, so it sounds like you are good there. Next thing I would try is priming it, though mine has never needed that and the thru-hull is well below the raw water pump. Third thing is confirm that the hose after the raw water pump is clear and not kinked or blocked and finally, I have heard of some impellers that have separated from their hubs and the impeller will turn with no load, but will not pump when the housing is sealed up. Hang in there, its probably some small thing causing the problem and driving you nuts. Oh, the key is in the keyway, right?

[chris5977]

The woodruff key is in and the impeller spins, but I should check to make sure.

Does shaft alignment really matter when replacing the impeller? The pump has such low tolerances it doesn't seem to have an opportunity to get out of line.

[Chasing Summer]

How much time you got to fiddle with it? Are you in a good supply port? make a clear plastic cover so you can see what's going on inside.
I agree with previous post. The rubber can break down with age and the hub comes loose. You can't spin it by hand but with water load it doesn't spin.

[chris5977]

Time is a big issue because I live in Seattle and the boat is 75 miles away in Anacortes. I need to get the boat ready for Memorial Day weekend! You guys have given me enough tips that I think I'm going to hold off on buying a new pump and give it another round of trouble shooting.

And again to stress. There is absolutely no blockage anywhere in the seawater system. I have checked every inch of it and isolated the problem to the pump. So it seems that the problem is one of two things:

1. The impeller is not spinning under load (unlikely, because it is in there really tight and I watched it spin with the motor on and faceplate off).

2. The impeller housing is damaged preventing a vacuum from being created (then I do need to get a new pump).

I put some pics of the old impeller which has since been replaced.