Priming Raw Water Pump?

[Play'N Hooky Too]

So last week I was checking the anodes on the raw water pumps. When I removed the brass plug on the starboard engine pump the anode, or what was left of it, came untreaded and was stuck in the opening. While I was trying to get a grip on it with some needle nose pliers to remove it, it wound up falling down into the pump housing. No problem, I needed to service the pump anyway.

So today I pulled off the intake housing, retrieved the chunk of anode, cleaned out a little fouling inside the housing, and put the housing back on with a new gasket and a new anode. I decided while I was there, I might as well just go ahead and change out the impeller also. Popped the cover, pulled out the old impeller, and noted that it had an odd wear pattern. Upon closer inspection I noticed that at some point, someone had installed the cam plate backwards such that the teeth on the plate didn't line up with the bars on the intake and discharge ports.

Finally got the screw broke loose, turned the plate around, installed a new impeller, and put the cover back on with a new gasket. No problem. I fired up the engine to check for leaks and noticed that I didn't see any evidence of flow coming through the sea strainer. So shut off the engine, confirmed that the sea cock was open, and thought maybe I put the impeller in with the fins bent the wrong way. I pulled the cover back off the pump and it was dry. I sprayed some soap into the pump housing for lubrication and with the cover off, I cranked the engine. Fins are facing the right way....check. I put the cover back on making sure to apply a little extra torque on the screws in case maybe I didn't get the cover tight enough the first time. I cranked the engine and again, no noticeable flow through the strainer.

I spent the next hour or so checking all the connections, removing and re-installing hoses and fittings. Still no flow whenever I fired up the engine. Finally, I pulled the anode plug and filled the raw water pump with water to see if I could tell where it might be leaking around any of the fittings. No leaks. I put the anode plug back in and fired up the engine with the pump "primed", and now I can see all kinds of flow coming through the strainer.

The thing is, the pump should be self-priming right? If it can't generate enough suction to lift water less than a foot, is it going to be able to supply enough flow to keep the engine cool under load? What would be the likely culprit? Is the pump housing it's self just shot?

The engine is a DD 8V71TI, 1976 vintage.

Sorry for the long story.

Thanks
-Alan

 

[SEVEN]

If memory serves me there is a nut on the top of the pump. Remove it and get a funnel and fill it until it overflows. Put the nut back on and fire up the motor. You have a vapor lock on your hands. Easy fix.

 

[Play'N Hooky Too]

But I was under the impression that the pump should be self-priming ...ie: positive displacement. That being the case, it should pull a vacuum even when it's dry. Obviously running it dry for any length of time would burn up the impeller, but until that point it should be able to generate enough suction to lift a 10-inch column of water. Am I mistaken? I don't recall having to prime the pumps the previous times that I replaced the impellers.

 

[SEVEN]

Sometimes it does and others it doesn’t.

 

[whoover]

Personally the only time I have had this issue it is because I have an air leak at the strainer, which doesn't sound like your issue. When you start it again in a few days, if there is no water there has to be an air leak. I takes very little of an air leak to cause me problems. I soap the hell out of the impeller when putting it in, helps sliding it in and maybe creating the necessary vacuum to start the flow of water.

Walt Hoover

 

[Pascal]

Air leak. Could be at the strainer although you would see a drip but likely at a hose.

Raw water impeller pumps are self priming. The only time i ve had an issue was on a 3412 where there was a debris blockage at the fuel cooler which in the 3412 is before the pump. Stupid design

 

[Captain Ralph]

, and thought maybe I put the impeller in with the fins bent the wrong way.

Impeller vains will correct their direction when it starts running.
If you used the original impeller, it's wear pattern is now different from your movement of the cam.
If you put a new impeller in, Was it old of stiff? Was it ice cold? Water Ice cold? These conditions will keep an impeller from setting and priming by itself at idle.
Lube it up well and rev the engine a couple of times. Then the impeller will load, pump and hold priming water for next start.

Use a flat file and clean up the gasket surface of the pump also. Ensuring all is flat and where screw drivers were used. These little burs can keep the cover from sealing on the pump.
This would be a good time also to ensure the cam end is flat also and not holding the cover off also.
And last, ensure the cover goes back on matching the same wear pattern. You can see on the cover where the cam is.

Interesting thing, I always thought the cams single mounting screw was placed to keep it from getting installed in the wrong position.

 

[Play'N Hooky Too]

Thanks for the replies gentlemen. Since I don't see any external leaks, I'm wondering if it might be the cover and/or the wear plate. Seems that enough wear on either of these would prevent the pump from developing enough suction to lift water when the pump is dry due to air short circuiting around the ends of the impeller. Since water can't "short circuit" as fast as air, the pump will move water once it is full (probably just not as efficiently as it should though). Does this sound plausible?

It seems to be working for now, so I guess I'll just move on to the next thing. Probably will go ahead and order a new wear plate and cover... you can never have too many boat parts.

Thanks
-Alan

Capt. Ralph, we must have been typing at the same time. I'll check through that list of things you recommended. I think for the most part I've eliminated most of them though. I was able to flip the cam plate around so it seems that the screw hole is symmetrical, at least on the pump I have. It is also flush with the outside of the pump housing.

 

[bigbill]

on my 1977 46 sf with 8v71ti;s after a haul out or impeller replacement i would have to rev the engine a little to get it to start pumping, and this is after doing all of the above mentioned ideas