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Old 12-04-2015, 13:20   #1
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MD7A water pump not priming

Hi,

My boat has had intermittent issues with water cooling since I bought it a couple of years ago.

The engine is a Volvo MD7A 13hp engine with sea water cooling. The water comes in from the saildrive leg, up to the water pump, then round a siphon loop before going into the engine. The water pump sits about 6 inches above the effective water level in the inlet hose. Once the pump is primed the water flow if fine - occasionally it will prime by giving the engine a rev, but more usually I need to fill the inlet hose then re-attach to the pump.

I have replaced the inlet hose, the short copper pipe that push fits into the pump, and the o-ring, and experimented with sealant and putty around these joins with little effect.

I have also checked the impeller which is intact, turns with the engine and does not seem to turn independently of the shaft (have heard the inner barrel of the impeller can sometimes detach from the rest). I have removed the outlet hose from the siphon loop and taken the inlet hose off the saildrive leg and into a bucket of water so both hoses are loose, but still no water through.

Next I am going to try sanding the face of the pump housing and the pump cover, then putting in a new impeller and gasket.

Should I be trying anything else? And if all else fails is there a practical way to manually prime the line? Was wondering if an inline pump like the fuel primer bulbs would be worth trying?

Many Thanks,

Neil
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Old 12-04-2015, 13:47   #2
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Re: MD7A water pump not priming

I have a standard transmission, not a sail drive.

I would look for air leaks in the incoming line. Devilishly hard to find.

I hate the push in connectors. Do you have good o rings? They should be square in cross section.

Also look at the other end of that copper line. I had similar problems to you and finally was told I had the wrong washer in the transmission end of that line, that fixed it.
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Old 12-04-2015, 14:02   #3
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Re: MD7A water pump not priming

Check the gasket on the raw water strainer
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Old 13-04-2015, 04:45   #4
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Re: MD7A water pump not priming

Tighten the tolerances in the water pump as you have described by sanding the body of the pump and the face plate, or replace the face plate with a new one. You should also replace the cam in the pump as it's probably worn out. Then a new impeller.

Also you should relocate the siphon loop from where it is. Place it between the thermostat housing and the exhaust elbow. That way there will be less tendency for air to get back into the pump.

DougR
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Old 13-04-2015, 08:56   #5
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Re: MD7A water pump not priming

Another vote for checking and replacing the gaskets on the raw water strainer.

There is an upper and lower gasket. The originals were probably cork, over time they deteriorate and break the seal. Get some cork casketing material from an auto parts store and cut new gaskets. This will be the cheapest boat repair you will ever do.

Also, consider getting a Speed Seal, it makes changing the impeller much easier. It is a replacement cover for the pump that uses thumbscrews.

Welcome to Speedseal
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Old 13-04-2015, 09:36   #6
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Re: MD7A water pump not priming

If its anything like my MD 17, the pump inlet is a hose barb that is pushed into the pump with the black rubber seals. Like the other copper pipe connections. There should be a retaining mechanism off the pump cover plate screws. Make sure that barb and seal is good. How old is your inlet hose? Could it be collapsing? Run a hose from a bucket to the pump inlet and another hose from the pump outlet into a bucket. That will tell you if the pump is working. If so, you have other problems. Plugged mixing elbow? Water has to exit before a pump will move water. The copper tubes are ok as long as the seals are good the mechanisms are in place to keep them attached. Also i had to move my pickup to a through hull as the saildrive tube became blocked with "stuff". Put super lube on your impeller and housing since you might be running dry until you figure it out. Impellers dont last long without water....
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Old 13-04-2015, 16:13   #7
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Re: MD7A water pump not priming

not sure what you mean by the impeller is intact. if you just pulled the cover and it looked to be in one piece that might not indicate it is in good condition. it could be stiff or have taken a "set". how long has that impeller been in there, is it the same one that was in there when you got the boat a couple years ago? if it has been in there a while I would change it first before trying to chase down other potential problems
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Old 13-04-2015, 16:21   #8
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Re: MD7A water pump not priming

Quote:
how long has that impeller been in there, is it the same one that was in there when you got the boat a couple years ago? if it has been in there a while I would change it first before trying to chase down other potential problems
I agree. +1
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Old 13-04-2015, 16:42   #9
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Re: MD7A water pump not priming

+1 for reviewing siphon placement.

I cannot say for raw water (sic!), but on our split system the siphon is just before exhaust.

b.
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Old 13-04-2015, 16:46   #10
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Re: MD7A water pump not priming

Another idea: possibly partly clogged engine water conducts may back pressure making priming difficult.

Our pump is below sea level and never ever requires priming. It is effectively always 'wet'.

Is your pump above sea level?

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Old 13-04-2015, 21:13   #11
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Re: MD7A water pump not priming

I installed a "T" and valve near the imput of the pump with the problem and hooked a pressure line (either fresh or salt) from another system. just open the valve, prime the pump and close the valve. provlem solved. I have valves on the out put side of my airconditioning pumps to vent them after taking some air under the boat in rough seas. Open the valve for a short time to let the air bubbles out before starting the freezer or airconditioning pump.
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Old 14-04-2015, 05:55   #12
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Re: MD7A water pump not priming

Guys,

The MD7 has an oddball feed to the water pump. On the regular transmission units it is a copper pipe from the transmission to the pump inlet. The pipe pushes into the pump and has a largish square section washer. The pipe has a ring and there is (or should be) a clip on the water pump, fitting over the cover/speed seal, that holds it in place.

Any kind of rubber fitting will not work, it will leak, unless you have the retainer clips. The copper tube MAY have sufficient stiffness to hold itself in place, if you use a very good sealant.

It is an odd and not very good fitting. Imagine a hose barb where you push copper pipe into the barb.
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Old 14-04-2015, 06:30   #13
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Re: MD7A water pump not priming

In both the picture and the parts diagram (link) the triangular retainers are missing.

Sea Water Pump: 861009 - category details

milky oil in a volvo md7 - Page 2 - SailNet Community
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Old 14-04-2015, 12:47   #14
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Re: MD7A water pump not priming

Thanks for all the replies.

I've ground the housing and plate, and replaced the cam and impeller (last one was new when I got the boat) - still no water.

Have made some progress though. A bit of soapy water and a lot of blowing and there's definitely a small air leak from the inlet side, almost certainly from where the copper pipe push fits into the pump. The copper pipe has already been replaced, with a new o-ring, and the retaining clip is located properly - so annoying that this seems the cause.

Tomorrow I will remove the pipe again and check where it locates into for crud. Might then just try and put it back together with some sealant.
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Old 14-04-2015, 13:24   #15
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Re: MD7A water pump not priming

Just an idea. Try toilet bowl wax. It's good stuff to have around to plug holes and it's cheap. And it's not to hard to seal with if you need to take the pump apart and put it back together.

Can't vouch it would work In this application, it might. I think I've done that myself. Damn Alzheimer's!

Also, I have searched Home Depot/Lowes and found some sink o-rings that work pretty well for the copper pipe. Can't recall the part number but they are hidden in the the faucet section.
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