Strange cooling water problem...

[Iain C]

The other week I serviced my engine, and changed the shaft seals and impeller on my water pump (it was missing a vane, which I recovered from the heat exchanger). It's a Johnson unit on a Lombardini engine. I now have a bit of an odd problem.

Usually, if I start the engine, and the boat has either been just motoring (say from club pontoon to swinging mooring etc) everything is fine...the engine starts on tickover and straight away I get a good stream of water both from the exhaust itself and the tell tale. And it doesn't matter if the engine has not run for a couple of weeks either...all is well.

However, if I go sailing (and I wonder if heel has something to do with it) when I start the engine I get nothing. The only way to get her blowing water is to rev the engine quite hard...you see the strainer bowl fill and after a few seconds she's blowing water again. However, I need quite high revs to "prime" the system...starting up on tickover isn't enough.

EG, started the boat on Friday after 2 weeks sat on the mooring...all good. Went to the pontoon for a few hours, started again, all good. Motored out of Portsmouth harbour and then sailed down to Lymington (upwind), and guess what, started to enter the river just a few hours later and no water until I revved hard. Stayed the night, started again this morning and it was fine. It's obviously something to do with heel, or boatspeed when there's no 'vacuum' in the water pipes because the engine is not running.

The only thing I changed when I had the pump apart was one of the brass hose tails on the pump body. I noticed that it wasn't quite the right size for the hose connected to it, and it was also very short, so there had been some bodgery with rescue tape and sealant. I fitted a marginally wider (1mm OD?) tail which was longer and allowed 2 hose clips. I don't know if the bore of the tail was different though.

Could this have affected it? Why? Or is there some kind of bleed screw somewhere I need to open? The boat is fitted with a siphon lock thingy high up above the exhaust level, which in turn has a tell tale pipe which piddles out of the top sides.

Any help gratefully received! Thanks.

 

[Tricia]

Obviously, I don't know the system you have installed and the heights of various components. The only thing I can think off which might cause these problems, off the top of my head, is that if on a certain tack the tell tale allows your system to empty from the pump to the salt water strainer or alternatively creates an air lock around the pump that is not filled from the strainer and doesn't give the pump the head of water it needs to work properly, hence the over revving to build up the head of water again.

A common problem with see through strainers is air leaking around the thread and the seal which can usually be cured with grease, but this doesn't seem to be the problem here as you have no problem after a couple of weeks.

T.

PS could just be the gremlins fking wiv ya!

 

[Tranona]

Air getting in somewhere and most common problems are loose hose clamps, poor seal on water strainer - either lid seating or a crack in housing, and poor pump performance. Last one could be either impeller (but you have changed that) or worn face plate. Anti syphon does not affect intake as it is down stream of the pump.

 

[vyv_cox]

It is quite common for the water to drain from the inlet hose on one tack, especially in boisterous conditions. The pump should always prime itself, to a height of more than two metres. If it doesn't there are many possibilities, some already mentioned:
Air leak in the hose/seacock tail/connection to strainer
Lid of strainer not completely sealed
Wear of the pump front face
Impeller not quite the correct width ( jabsco and Johnson impellers are not always identical)
Pump shaft seals not fitted correctly or worn.

 

[concentrik]

I'll add another cause to Vyv's list - although it's probably rarer than the others. I had endless trouble in a very similar vein last year with the pumped raw water supply dwindling to nothing under certain (unpredictable) circumstances. The overheat warning would sound and the strainer was always found to be empty. I had the pump rebuilt, lowered the strainer etc etc and it still happened. Then I checked the actual flow rate of water round the system (just pulled the hoses off actually) and discovered that the oil cooler inlet tube was blocked with old impeller bits, locking grubscrews etc etc. Once all this was cleared the water flow was restored and fairly gushed from the exhaust outlet. No probs since, touch wood.

What had fooled me was that the flow had been reducing gradually over time. I now have a strainer AFTER the pump and before the engine so I catch any shredded impellor bits before they do any damage. I haven't heard this suggested elsewhere but it seems like a good precaution.

I think I learned that pump function, especially priming, can be fatally impaired by a blockage upstream as well as downstream. So it may be worth ensuring that once the pump is primed, water flow is consistent through the system.

 

[TonyS]

I had the same problem on a new VP 2030. I never found the cause but had the same symptoms. I then fitted a spring assisted SS non return valve after the pump. This kept the pump full of water whatever I was doing, sailing or motoring. I haven't changed an impeller for years. Before it could be twice a season with lost vanes in the system. Cost was less than £10 with hose tails and clips.

 

[Merlin38]

I had a similar problem ie not priming after sailing. The cure was resealing the lid of the Vetus strainer with a new O ring on the centre spindle and also increasing the length of pipe between strainer and pump by a couple of feet by looping it down into the bilge and then back up to the input of the pump. The latter ensured that there was always plenty of sea water immediately available to the pump even if air had been induced before the strainer. A non-return valve would also help but not needed in my case. Wear on the pump face will exacerbate any difficulty in priming and this can be partially repaired by refacing the front cover on emery paper and a flat surface

 

[Iain C]

Thanks all. I've checked the pump face and it's actually bent, let alone worn! I will replace it...time for a speed seal perhaps?

The pump takes what seems like an agonising 15 seconds to prime, however I guess with a fresh/raw cooling system it's less of an issue, however running the pump "dry" is a worry, although I did lube the new impeller.

Ironically as soon as the pump has primed, the water trail from the exhaust is by far the strongest it's ever been!

Will change the face plate and report back...