Volvo MD22 Heat Exchanger Bleeding

[Kg1]

Hi all, I have a problem which has me stumped; I hope that it rings a bell with someone here.

I recently overhauled the Jabsco RW pump on one of my MD22s (catamaran). Ever since then, it loses prime after the engine is shut down. About 15-20 seconds after shutdown, there is a flow of air backwards from the heat exchanger, which then goes through the pump about half way to the water line level. I originally thought that the air was coming from the pump (seal or gasket), but this is definitely not the case, it comes from the heat exchanger inlet. It all works fine if I give the engine a burst of revs after starting, and once primed the flow is good. There is no loss or change in the coolant. There is no leak of raw water anywhere.

It seems to me that there is a pocket of air which gets trapped in the heat exchanger and then feeds back when the engines stops. Is there a way of bleeding the heat exchanger of air? Or has anyone else experienced this and found another solution?

Thanks in advance,
Kim

 

[spannerman]

I guess you are running with saildrives or shafts, if so then it could be that the heat exchanger is emptying down into the exhaust pot and that is where your 'air' is coming from. Only an idea without seeing the setup.

 

[Kg1]

Thanks Spannerman,
I am running saildrives. The thing I can't understand is that I replaced the RW pump previously on the other engine without any of this problem.

I wonder if there is a blockage in the RW cooling circuit which is not letting the bubble of air through, and when the pressure of the pump stops, it allows the air to flow backwards. The problem I have with this theory is that the engine runs at normal temperature and flow....

 

[spannerman]

Do you have vaccuum break valves in the hose between the raw water pump and the engine, as these are designed to let in air when the engine is turned off, if you have these then maybe the 'good engine' actually has a faulty valve.
I have seen it often on twin stern drive D6/4 installations where one engine gurgles and empties the seawater filter as you watch after turning off the engines.

 

[Kg1]

No vacuum break valve, just a short black hose.

 

[spannerman]

Ok, you either have an air leak which you would expect to leak seawater when the engine is running, or its coming in through the exhaust system.

 

[Kg1]

Thanks,
I suspect that the raw water pump is allowing some back flow past the impeller, which is sucking air from the exhaust through the heat exchanger. It makes sense that since this only happened after I overhauled the pump, it is something to do with the pump.
I replaced the impeller with a Johnson "equivalent", since the Jabsco impellers in Australia are horribly expensive (even by Volvo Penta standards!). However when I compare it with the Jabsco original, it seems to be very slightly (maybe 0.5mm) narrower than the Jabsco, which might be letting water and then air through the pump (past the end plate) due to suction since the pump is about 40 or 50 cm above the waterline. There doesn't seem to be anything else which would prevent air back flow from the exhaust mixing nozzle.
Does this make sense?

 

[spannerman]

I think you have found the answer, a small clearance between the impellor will affect efficiency and/or sealing.