Sea strainer problem

[Anthonyexmouth]

Curious problem with my sea strainer. Had an overheating problem with my little sabb, thought it was the pump so I stripped out down and installed a service kit bit still had intermittent problem. Now I've noticed that the problem seems to be the sea strainer. Seems to be an air lock that requires me to take the top of, the water starts following and then the engine starts pumping water ok. Nothing changed, 50 yr old boat/engine and never had this problem before. When I open the top of the strainer it's half empty and then immediately fills. Boat is on a drying mooring and has been for years.

 

[sarabande]

Sounds almost like a blockage between the strainer and the inlet, with air thenbeing sucked in through the lid gasket as the engine turns. Vaseline the O ring and see what happens.

 

[Anthonyexmouth]

No blockage, can see straight down. No air leak otherwise after the tide comes in the strainer would be full of water, I have to take the lid of and then the water rushes in and fills it, then when I start the engine all is well but if I don't do that is like there's an air lock and no suction from the pump. Only noticed as I have a clear lid. It's really weird. 40 years working on boats and never seen this.

 

[DavidJ]

I know you have serviced the pump but on a jabsco pump the back plate can wear so that you can loose pressure. Maybe in your case the pressure (or vacuum) is enough when there is water in the strainer but not quite enough to get the water up there.

 

[Anthonyexmouth]

It's a diaphragm pump on the sabb. But why would the strainer be empty until the lid is removed, that showed there's no air leak. If there was the water pressure would let it fill as the tide comes in.

 

[DavidJ]

How about going back to the check valve. It’s the only way I can see of sealing air from the system
Hmmmm I’m rethinking this. The check valve will stop air escaping and your strainer refilling so it’s working. Until the pump starts. Is the pump electric or mechanical. If electric then it’s whatever signals it to start.
Hmmm thinking again
If you disconnected the pipe from the discharge end, does the strainer fill?

 

[Anthonyexmouth]

Checking the discharge end is my next task, bloody awkward getting in there, BUT taking the water outlet hose off the exhaust manifold doesn't fill the strainer and as there are no blockages so taking the end off the outlet from the pump SHOULDN'T make a difference, but it might so I'll check that.

Pump is mechanical.

 

[Anthonyexmouth]

Weird update, hoping someone has an idea.
Put the pilot on one of my deep water moorings the other day, came out today and the bloody sea strainer was empty. Had to take the lid off to fill it to get the water pumping. How can it empty when it's on a deep water moorings?

 

[sarabande]

Above the waterline and an air leak from the lid ?

 

[Anthonyexmouth]

Above the waterline and an air leak from the lid ?

It's below the waterline. When I take the lid off water rushes in and overflows.

 

[Portofino]

Over heats cracked something in the block and heat exchanger so gas is finding it’s way back and like blowing tanks on a sub the gas is bowing water back out of the strainer .
After it sits and cools down .

The gas can’t come in through the raw water inlet under the boat , it can’t enter via unsealed lid as you say it’s below the WL so if so should seep water out , and it is not you say .
So where does the gas come from it’s gotta be from the motor itself …..unfortunately.

 

[Anthonyexmouth]

When the engine is running it's fine, it's a very simple cooling system, no heat exchanger. The route is strainer, pump, block, exhaust. Water pressure should keep the strainer full when the engine isn't running.

 

[Portofino]

When the engine is running it's fine, it's a very simple cooling system, no heat exchanger. The route is strainer, pump, block, exhaust. Water pressure should keep the strainer full when the engine isn't running.

All the more likely if raw water cooled for gases to track back and blow out the strainer.
Pump keeps it full running .Pump off = gas pressure / heat soak blows back .

As said the gas / air ain’t coming up through the boat hull if it’s under the WL .

 

[Anthonyexmouth]

My point is, there's less resistance for any gas to escape from the exhaust, rather than overcoming the water pressure and pushing it out of the strainer and it's odd that water pressure isn't filling the strainer up again. Water ways are obviously clear as the engine runs fine once the strainer has been filled by taking the top off and allowing it to fill. The top gets screwed back on and run all day without problem.

 

[Portofino]

My point is, there's less resistance for any gas to escape from the exhaust, rather than overcoming the water pressure and pushing it out of the strainer and it's odd that water pressure isn't filling the strainer up again. Water ways are obviously clear as the engine runs fine once the strainer has been filled by taking the top off and allowing it to fill. The top gets screwed back on and run all day without problem.

Yes in un over heated block as intended .

But you report in your opening post this occurred after a “overheat “.
Thats the significant change .
Somethings changed . Weird cracks connecting passages or spillways etc that once were unconnected .
Might not be vis external obviously as the gas / fluid would be seen escaping.

Head off , Boroscope the cylinder(s) to investigate further .
First run the exhaust straight out no water lock , if it’s got one ( you have not said ? ) in an easy eliminatory way .Thinking elimination of the heat causing some internal rearranging of the baffles and thus resisting gas escape = forces it back and blows out the strainer .

Figure out what over heat could have damaged.