Impeller won't suck!

stu9000

stu9000

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imp1_1.jpg

picture is on its side for some reason.
The bottom is pointing left.

I have been doing battle with the engine an I I initially enjoyed and embraced as an important learning experience.
But the usually sweet natured Volvo md7a is sulking for some reason.

I changed the impeller at the start of last season and had no problems.

However the impeller isn't sucking water now.

Fresh impeller added.
New backing plate as the old one was heavily worn.
it is definitely rotating.
The hull to pump pipe isn't blocked.

I have disconnected the hose from the hull fitting and had the hose end in a bucket of water. No signs of sucking up water at all.

Tried new gasket, pouring washing up liquid down the pipe , adding an extra glob of grease all to no effect.

I suspect the inlet connection.
Not the hose that is jubilee clipped to the metal fitting, by the fitting that connects to the pump.
It is likely I disturbed the seal when I moved the hose to fit in a bucket.
I inspected the rubber washer. Seems fine.
I was quite surprised at how basic and loose the fit seemed, but it worked before so I whacked in a load of gasket sealant and put it all back together.
No joy.

I'm not sure where to go from here.

Maybe wrap some ptfe tape around the inlet fitting to get a tighter seal.

Seemed fine last season.
No overheat alarms.
Water coming out of exhaust.

So, I'm assuming I need to find the air leak so it can generate the vacuum needs to prime.

Any suggestions?

Thanks

S
 
stu9000 said:
View attachment 69232

picture is on its side for some reason.
The bottom is pointing left.

I have been doing battle with the engine an I I initially enjoyed and embraced as an important learning experience.
But the usually sweet natured Volvo md7a is sulking for some reason.

I changed the impeller at the start of last season and had no problems.

However the impeller isn't sucking water now.

Fresh impeller added.
New backing plate as the old one was heavily worn.
it is definitely rotating.
The hull to pump pipe isn't blocked.

I have disconnected the hose from the hull fitting and had the hose end in a bucket of water. No signs of sucking up water at all.

Tried new gasket, pouring washing up liquid down the pipe , adding an extra glob of grease all to no effect.

I suspect the inlet connection.
Not the hose that is jubilee clipped to the metal fitting, by the fitting that connects to the pump.
It is likely I disturbed the seal when I moved the hose to fit in a bucket.
I inspected the rubber washer. Seems fine.
I was quite surprised at how basic and loose the fit seemed, but it worked before so I whacked in a load of gasket sealant and put it all back together.
No joy.

I'm not sure where to go from here.

Maybe wrap some ptfe tape around the inlet fitting to get a tighter seal.

Seemed fine last season.
No overheat alarms.
Water coming out of exhaust.

So, I'm assuming I need to find the air leak so it can generate the vacuum needs to prime.

Any suggestions?

Thanks

S
Click to expand...

Do you have a water strainer between the hose you removed from the hull fitting and the pump? If so, maybe the lid on it is not sealing perfectly.

Www.solocoastalsailing.co.uk
 
It's not sucking from a bucket of water though :ambivalence: I suspect it's upline from the pump.

Donald
 
Aja said:
It's not sucking from a bucket of water though :ambivalence: I suspect it's upline from the pump.

Donald
Click to expand...

it's a SkyDrive prop.
The water comes in through a tap on side.
So, no strainer to complicate things.

I'm going to try the ptfe tape to try for a better seal on the water in fitting.
If that doesn't work then maybe it is upstream.
Any suggestions as to how I might diagnose that?

Thanks
S
 
stu9000 said:
it's a SkyDrive prop.
The water comes in through a tap on side.
So, no strainer to complicate things.

I'm going to try the ptfe tape to try for a better seal on the water in fitting.
If that doesn't work then maybe it is upstream.
Any suggestions as to how I might diagnose that?

Thanks
S
Click to expand...

Don't know what a SkyDrive prop is but surprised you say there is no water strainer.
May i suggest that while you have the suction hose in a bucket of water you put a hose from the pump output port directly into that bucket too then start the engine. If water now flows you have a blockage AFTER the pump. If still no flow than at least you are eliminating everything bar the pump.

Www.solocoastalsailing.co.uk
 
Is suspect predictive text strikes again and you have a sail drive which when the boat is inverted becomes a sky drive
The water passage through the leg is notorious for blocking muck and shellfish you need to rod it through from the hole at the bottom at the nose the side grill is in addition.
You also need to take the new front cover plate off and remove all the goo just use the correct gasket with a little light grease.
You do possibly have a backing or wear plate behind the impeller which does wear badly and needs checking
It is possible that you have disturbed the shaft seals when changing the impeller this would cause a lack of suction
Probably best to take the pumpthe of and check it out
 
If the plate was worn then perhaps the body is worn. How old is the pump (in hours) and is there sediment in the water. How pronounced is the heal in the pump body which causes the pump action. Did you thoroughly lubricate the new impeller with wash up liquid or glycerin before first use ? Have you got the vanes the right way ?
 
Looking at the photo, it looks like there sealant of some sort between the faceplate and the pump. Take the faceplate off and get rid of the sealant, use a new paper gasket (as supplied with the impeller kit) and see if that works. The pump relies on a tight seal between the impeller and the faceplate and if you've used sealant on the plate it'll break the plate/impeller seal and won't pump water. If you haven't got a new gasket, use baking parchment to make a new one: use the faceplate as a template and cut it out with nail scissors.
 
Try taking the impeller out and reversing it so the vanes are forced to lay in the other direction. I have noticed that in certain cases when the impeller remains with all the vanes folded in a certain direction without moving for a long time they do not maintain a good airtight seal in order to draw water in. Reversing it sorts that out.
 
A good way to test these things is to check you can't blow through it.
At first sight, that gasket looks very thick, probably preventing the impeller from sealing front and back.
When you've got it so you can't blow through it, the next thing is to avoid any air leaks upstream of it.

Then make sure it is primed. These things are not intended for pumping air, they are rubbish at it, because the rotor vanes are not compressed that much, they just don't generate much pressure or suction with air, the air just expands and contracts in the pump. So:
a) it needs water in it to start
b) until it gets going, it won't pump well against back pressure.

Once the system is primed, a 'fairly worn' pump will often work adequately, but a worn pump is often hard to prime.
 
I've had similar problems with raw water pump priming.

Here's what I did to solve them.

First, remove the impeller and lightly smear waterproof grease around both ends as well as around the blades before refitting. This overcomes any minor gaps in the pump housing that have been mentioned above.

Second, close the seacock and remove the inlet hose which goes from your water strainer to the inlet of the raw water pump.
Get a bucket of water and a measuring jug or some other jug which holds about a litre and can pour accurately. Holding the hose up as high as it will go (hopefully above the height of the water pump) fill the hose with water using the jug. Hold it there and top up the jug.

With the help of an assistant, start the engine at idle speed only, and as the water disappears down the hose keep topping it up using the measuring jug. Keep doing this until the bucket is empty, then stop the engine. Do not get any air locks in the hose.

Refit the hose to the strainer, trying to keep as much water in there as you can and open the seacock. Start the engine and you should find all is well. System will be properly primed and will continue to pump OK now.

Raw water pumps are fine once they are primed, they just aren't very good at pushing water through an empty system. At least, that's been my experience.
 
I've had various impellor "not priming problems". The most mysterious was due to the spindle on the impellor separating from the rubber. So it looked fine when disassembled, the drive shaft turned but once assembled and out of my sight the actual impellor did not turn. More serious was my continual problems with a Volvo where the cooling circuit was longwinded and worst the outlet to the exhaust was a spiral affair that filled with mud which of course tended to bake solid from exhaust heat.

Worn cover plate might reduce efficiency but it should not stop it priming. What's more you can turn cover plate back to front usually and use the unworn side after removing paint.

Suggestion #15 above is good if its just priming. If its failed impellor drive or down the line blockage you suspect then disconnect the outlet of the pump so there is no restriction and try that, just ensure outlet pipe goes into bilges rather than spraying everything with saltwater like I did
 
oldmanofthehills said:
I've had various impellor "not priming problems". The most mysterious was due to the spindle on the impellor separating from the rubber. ...
Click to expand...
I've had that a few times, even with a new impeller, albeit with a 1GM10 - it took ages to identify the first time it happened.Now, I just remove the impeller and check that the spindle doesn't turn inside the nitrile/rubber; if it does, it's dried off, patched with epoxy, and put away as an emergency spare for use in extremis. I've had new impellers last anything from a few days to ten years .. doesn't seem to be any reason for the variation.
 
Plum said:
Don't know what a SkyDrive prop is but surprised you say there is no water strainer.
May i suggest that while you have the suction hose in a bucket of water you put a hose from the pump output port directly into that bucket too then start the engine. If water now flows you have a blockage AFTER the pump. If still no flow than at least you are eliminating everything bar the pump.

Www.solocoastalsailing.co.uk
Click to expand...

good suggestion thanks.

SkyDrive sounds good doesn't it but it was meant to read Sail Drive
 
lw395 said:
A good way to test these things is to check you can't blow through it.
At first sight, that gasket looks very thick, probably preventing the impeller from sealing front and back.
When you've got it so you can't blow through it, the next thing is to avoid any air leaks upstream of it.

Then make sure it is primed. These things are not intended for pumping air, they are rubbish at it, because the rotor vanes are not compressed that much, they just don't generate much pressure or suction with air, the air just expands and contracts in the pump. So:
a) it needs water in it to start
b) until it gets going, it won't pump well against back pressure.

Once the system is primed, a 'fairly worn' pump will often work adequately, but a worn pump is often hard to prime.
Click to expand...

Thanks.ill go easy on the sealant in my next attempt.
I was thinking of ways to pre prime too.
Maybe feed from a bucket that is higher than the pump.
 
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