MD5A Water impeller not self-priming

[divonic]

I have run out of ideas, I have read lots of threads without seeing my particular problem, so I'm turning to the community ...

1980 Volvo MD5A, raw-water cooled, flexible impeller water pump. Seacock at top of Saildrive 110S, new transparent hoses, 18" hose from seacock to pump, 12" hose from pump to engine block. Pump is above sea-level. No raw water filter (that's a different issue, but the Saildrive has a grating and it's quite deep).

Last season, our first year, I replaced the impeller using the Volvo Kit 3586496 that includes the paper gasket for the plate. It would not self prime, i.e. no water when the engine started after launch. I primed it 'manually' by taking the hose off the engine block, sucking water (yes, with my mouth) to above the pump and replacing the hose. Then it worked fine while the engine was running. Same problem next time the engine was started. I had kept the old impeller because it was in perfect condition, so I compared the two and could not see any difference between them. I put the old impeller back in, and after initial priming as above it worked fine - so I shrugged and went sailing a lot.

This season, same sequence, same problem with two differences - sometimes it sucks the water up and sometimes it doesn't - and I'm talking about after a few hours sailing, or sitting in the marina ... and the old impeller was showing signs of wear so I had binned it (pity about that in retrospect).

I've got smarter about the manual priming - I used to do it with engine running, meaning I got a faceful until I jammed the hose back onto the engine - now I suck it up with engine off, close the seacock, start the engine and open the seacock. There's never a problem once it's running through.

If I close the seacock and try to suck I get nothing. If I suck up the water, close the seacock and leave it, the water stays put (transparent hose).

So in my analysis there's no blockage and no leak.

Q1. I shouldn't be able to suck water through the pump if it's not rotating, should I? It needs a strong suck but it does come through. You'd think there must be a gap in the pump, but I checked and the impeller face is aligned with the front of the pump where the gasket & plate screw on. So what's happening?

Q2. a flexible impeller pump is supposed to be self-priming, right? So it should not matter if it’s sitting with air in it (it only has to lift water about 4 or 5”)?

Q3. would you expect the water to fall back through the pump to sea-level when the engine is switched off – or given there’s a water-trap on the exhaust, should it stay put because there’s a seal stopping air getting into the cooling system through the exhaust (the only way in)?

Thanks in anticipation for any ideas – it’s driving me crazy. I have a workaround, but not for when I need the engine in a hurry, and I’m fed up with being a contortionist in the engine compartment – she’s a Jaguar 25 and there’s not a lot of space.

 

[eagleswing]

well i do believe that one can suck water through a raw water impeller pump, though i hve never tried to do so. most of the raw water impeller style pumps i am familiar with are not self priming and require location below waterline. that being said, if your raw water pump is above water line, you could alter the priming procedure as follows: put a tee in the supply line. to the pump. run a long hose off the. top of the tee, with the end more easily accessible to you than your present system of crawling near the raw waterpump hose. at the top of this hose , have a stopper. and a funnel. use the. funnel to fill water into the hose to prime the pump. when the hose. is. full of water, cap off the upper. end. with a. plug. mount the hose. with a support. bracket well above waterline where it is easily accessed by you, and be sure to cap the priming hose after each prime. mounting its uper. end. securely well above water line wil ensure your. raw. water fill. doesnt end. up spiling all over and starving the raw water pump of water.


i had. to use. this system to prime. an. airconditioner. water supply that lost prime. every time. we. went sailing. i fastened the upper end of. my priming. hose. into a. bracket on. my bulkhead wall easily accessible to fill the hose. with water. then cap. it off. keeping the priming hose support. bracket far above. water. line. will. ensure. that. you. never lose. water. out. of. the priming. hose...

 

[DerrickHigton]

After no problem for many years, my MD7 stopped priming after my boat was put on the hard and dried out. Wear on the cover plate was providing a leakage path; a new cover solved the problem.

 

[scottie]

Try turning the front cover back to front

 

[vyv_cox]

As has been suggested, the usual cause of this problem is wear of the front plate, but don't forget that wear also takes place at the back of the casing. Impeller type pumps are self priming but priming will not take place if there is an air gap inside the pump at the sides of the impeller. If the plate can be reversed, some can and some can't, then try that. If it cannot be reversed you can grind it back to flat using wet and dry papers on a glass sheet, or buy another.

Another possible cause of excessive clearance is your paper gasket. If the back face is worn a new plate may not fix the problem, try fitting it with some gasket goo instead of the paper.

 

[Topcat47]

I have a strainer mounted above the waterline. When the engine stops, the line from the strainer to the pump remains full of water, even when the pipe from the seacock to the strainer is empty so the pump will always prime.

 

[Modulation]

Had a similar problem earlier this year with a Yanmar and went through all the steps you've been through. New impeller, gaskets cover plate - no difference. Mine turned out to be a worn pump body - apparently some Johnsons use very soft metal. Very difficult to see unless you can get a light into the pump body and see if it shines past the impeller vanes - mine did.
New one £380 so I cannibalised on from a scrap engine I had. No problem since.

 

[Trundlebug]

Just smear all the faces of the impeller, including both ends with waterproof grease before fitting.

I've had exactly the same problem as you, and although worn end plates were also an issue, the grease trick solves the initial priming problem.

 

[sailorman]

Just smear all the faces of the impeller, including both ends with waterproof grease before fitting.

I've had exactly the same problem as you, and although worn end plates were also an issue, the grease trick solves the initial priming problem.

one should never be able to suck the water passed the impeller pump

 

[exfinnsailor]

Its the wrong impeller. Changed mine the other year. Brought a proper YANMAR impeller with the correct part number etc. Fitted it no water. Tried priming it still no water. After an hour of trying put the old one back in. Brilliant . Put the new one in .. Nothing .. Took it out and compared it with the old one .. Same .. Tried again .. Same result .. Compared them again and the centre boss was slightly more recessed on the one that worked .. Bought NEW Johnson Impeller from Volvo Dealer .. Away it went ..

To look at they are near enough identical one was a Jabasco and the other a Johnson ....

 

[sailorman]

http://www.ittflowcontrol.com/files/artdoc4851.pdf

 

[pvb]

No raw water filter (that's a different issue, but the Saildrive has a grating and it's quite deep).

I think if you were to fit a raw water filter above the pump, your problem would disappear.

 

[Tranona]

I think if you were to fit a raw water filter above the pump, your problem would disappear.

Whilst a good idea to fit the strainer, not sure it will cure the current problem. As several others have noted, it is almost certainly a worn pump or incorrect impeller. If it won't prime as it is, no hope of it lifting water even higher above the waterline. Only benefit would be that is is easier to prime by pouring water in the strainer with the seacock closed - but that does not solve the basic problem.

 

[forelle541]

I have just had the same problem on a johnson 35 Pump, it was just eating impellers and not priming. It turned out to be the seals not seating on the shaft and letting air past, when pumping water the pressure pushed the seal against the shaft so no leaks not a dribble to give them away. New seals and it sucks water straight away.

 

[pvb]

Whilst a good idea to fit the strainer, not sure it will cure the current problem. As several others have noted, it is almost certainly a worn pump or incorrect impeller. If it won't prime as it is, no hope of it lifting water even higher above the waterline.

If it works once it's primed, a raw water filter will ensure there's water above it in the inlet pipe to prime it.