Raw water pump not priming at idle RPM

[nigel1]

In the last week, I noticed that the Jabsco water pump">raw water pump is not priming when the engine is started and left at idle, it needs the throttle opened to about 1800 rpm before the pump primes.

Shortly prior to this, I carried out an overhaul on the cooling system of the Volvo Pemta TMD 22 engine. Cleaned out the heat exchanger, the mixing elbow, renewed most of the hose's, new freshwater/coolant pump, new stat, replaced the raw water pump impeller, and dismantled, cleaned and re-assembled the vetus anti siphon loop.
When the work was completed, I started the engine to check for leaks, and the pump primed OK at idle rpm.

Boat was not used for a couple of months, until last week when I moved the boat from England to Scotland. On starting the engine, I noticed no water from the exhaust, so gave a quick increase on the revs and the pump then picked up suction.

Then did some checking.

With engine stopped, the water in the suction and discharge hoses drains down (as it should do). On the suction side, the water level in the hose ends up about 6 inches below the suction port of the pump, which I assume is at hull water level.

On starting the engine at idle, I do not see any water coming into the inlet filter, or any rise in the water level in the suction hose.

Increasing the RPM causes a sudden rush of water into the filter, and the pump primes. After this, dropping RPM back to idle, the pump continues to work OK.
With the pump going OK, I dont see any air in the suction hose or discharge hose up to the anti siphon loop. After the anti siphon loop, I do see air in the discharge hose.

Another thing I tried. Started engine, left it at idle, and briefly closed the inlet valve. This caused the water in the suction hose to rise up to the pump suction port.

I have checked all the hose clips, they are tight, no signs of leaks.
Removed cover plate from the pump, impeller looks good, put in new sealing O ring on the pump cover.

Any one have any idea's what could be the problem. Seems odd that this problem has started so suddenly.

Thanks

[Jim Cate]

Nigel, your symptoms are very similar to those I've had when the cover plate in the pump was worn, giving too much clearance between the sides of the impeller and the cover. Associating this with your recent rehab of the cooling system is kinda odd, but in your place I'd have a look at the cover. If it is scored or grooved deeply, try giving the outside of the cover a brisk sanding on a flat surface and then reversing it. This has worked for me in the past.

Good luck... it may well be something else entirely!

Jim

[nigel1]

Thanks Jim

The cover plate is not original, it is one of the replacement covers from Speed Seal, and the impeller sits against a "brass" disc, which in turn sits on a PTFE disc.
See Welcome to Speedseal Safety Covers

But your suggestion has thrown up an idea, maybe the PTFE or the brass disc is worn down and needs to be replaced.

Think I'll give this a try.

Thanks

[nigel1]

I just called Speed seal to order the discs, had a chat with them, and they say the discs should be replaced every couple of years.
Mine are 4 years old, which may well explain the problem.

[Fuss]

There are 2 types of Jabsco impellors, one has 2 small grooves which effectively cause the water to bleed out of the pump. Depending on the circumstances, this may still work ok when replacing a none grooved one.....and sometimes not.
Maybe you could check this.

[HappyMdRSailor]

Betting the theory is spot on... And your new parts work like a charm...

Me??? I like extra work for no reason...

If simple... I'd be temp plumbing the raw out hose to atmosphere just to see if there's a dramatic change in prime, i.e. immediately.... And get a feel for discharge rate...

Or you can simply wait for the parts and likely have the same result...

[nigel1]

Thanks for the replies.
I'll check the impeller type. Pretty sure I noticed the two groves in the impeller.
Disconnecting the discharge hose is no chore, need a big bucket though.
I'll try this when I fit the the new discs. Can have a rate check before and after the discs are fitted.
Going to have to wait a bit, boat is up in Scotland, and I'm heading offshore (for work very soon).
But, I have faith that a solution will be found.
Cheers.

[Zach]

Something else... If you have a raw water strainer that is just at the water line, and not just barely below it, depending on how the plumbing is set up a nick in the o-ring, or a loose hose clamp will vent air into the strainer and lose prime there, until you move enough volume to overcome the air-bleed + head pressure.

Cheers,

Zach

[nigel1]

I checked that one Zach.
When I did the overhaul of the cooling system, I removed the strainer to give it a good clean, and in the process, when I removed the sealing ring, it just fell to pieces.
Replaced with a new one, and have leak tested it, and could find no leaks. Also replaced the small seal which sits between the securing wing nut and the top cover of the strainer.

[Fuss]

Originally Posted by nigel1

Thanks for the replies.
I'll check the impeller type. Pretty sure I noticed the two groves in the impeller.
Disconnecting the discharge hose is no chore, need a big bucket though.
I'll try this when I fit the the new discs. Can have a rate check before and after the discs are fitted.
Going to have to wait a bit, boat is up in Scotland, and I'm heading offshore (for work very soon).
But, I have faith that a solution will be found.
Cheers.

Ok, then if you noticed the 2 grooves then this is more than likely the problem as the water can slowly bleed past the impeller and when dry the suction is not as good due to the 2 grooves.
The jabsco grooved and non grooved impellors both say they are for the same pumps....

[nigel1]

Are there different part numbers for these two types of impellers, grooved and non grooved.
When I checked my stock of used (but good for re-use) impellers, I noticed a slight difference between those direct from Jabsco, and those with a volvo penta name on the impeller.
I used both types over the last few years with problem.
Might be worth a call to Jabsco.

[HappyMdRSailor]

Originally Posted by nigel1

Thanks for the replies.
I'll check the impeller type. Pretty sure I noticed the two groves in the impeller.
Disconnecting the discharge hose is no chore, need a big bucket though.
I'll try this when I fit the the new discs. Can have a rate check before and after the discs are fitted.
Going to have to wait a bit, boat is up in Scotland, and I'm heading offshore (for work very soon).
But, I have faith that a solution will be found.
Cheers.

Good man... You'll need a precisely marked graduated bucket, a stop watch, lab coat, safety glasses, and a video recording setup... I want at least 10 data points per experiment...

Also... an assistant who looks ravashing in a lab coat is a huge bonus...
(not so much if there is no video)

[Fuss]

Originally Posted by nigel1

Are there different part numbers for these two types of impellers, grooved and non grooved.
When I checked my stock of used (but good for re-use) impellers, I noticed a slight difference between those direct from Jabsco, and those with a volvo penta name on the impeller.
I used both types over the last few years with problem.
Might be worth a call to Jabsco.

I can't remember but the list of pumps that the impeller is designed for contained the same pump number on both boxes.

A call to Jabsco would be good but first it would be good to know if your installed impeller has 2 grooves and if you have a spare without, then I would try this as it might be that the person at Jabsco has difficulty to understand what your on about.

[grjfield]

Not wanting to cross threads....but perhaps to plant a 'seed' or 2. Recently read several comments on resale of diesel vs electric thread. Seals for electric are supposedly good for 50000 hours
..vs..water pump seals, discs, water clamps, orings, etc.

So if the diesel starts causing you stress...perhaps consider electric. It may cause other issues....range anxiety.but will definitely solve your diesel maintenance issues!

[Cadence]

Originally Posted by nigel1

In the last week, I noticed that the Jabsco raw water pump is not priming when the engine is started and left at idle, it needs the throttle opened to about 1800 rpm before the pump primes.

Shortly prior to this, I carried out an overhaul on the cooling system of the Volvo Pemta TMD 22 engine. Cleaned out the heat exchanger, the mixing elbow, renewed most of the hose's, new freshwater/coolant pump, new stat, replaced the raw water pump impeller, and dismantled, cleaned and re-assembled the vetus anti siphon loop.
When the work was completed, I started the engine to check for leaks, and the pump primed OK at idle rpm.

Boat was not used for a couple of months, until last week when I moved the boat from England to Scotland. On starting the engine, I noticed no water from the exhaust, so gave a quick increase on the revs and the pump then picked up suction.

Then did some checking.

With engine stopped, the water in the suction and discharge hoses drains down (as it should do). On the suction side, the water level in the hose ends up about 6 inches below the suction port of the pump, which I assume is at hull water level.

On starting the engine at idle, I do not see any water coming into the inlet filter, or any rise in the water level in the suction hose.

Increasing the RPM causes a sudden rush of water into the filter, and the pump primes. After this, dropping RPM back to idle, the pump continues to work OK.
With the pump going OK, I dont see any air in the suction hose or discharge hose up to the anti siphon loop. After the anti siphon loop, I do see air in the discharge hose.

Another thing I tried. Started engine, left it at idle, and briefly closed the inlet valve. This caused the water in the suction hose to rise up to the pump suction port.

I have checked all the hose clips, they are tight, no signs of leaks.
Removed cover plate from the pump, impeller looks good, put in new sealing O ring on the pump cover.

Any one have any idea's what could be the problem. Seems odd that this problem has started so suddenly.

Thanks

It might be time to take a swim and check the intake for growth. Sounds as if you have checked everything else.

Best of luck.