Yanmar 4JH sea water cooling problem

[Talulah]

Engine issues this weekend so any suggestions of cause appreciated.
Set off fine across Channel Friday night. Motored for about 4 hours before the wind kicked in. Sailed the rest of the way.
Outside of Cherbourg Harbour restarted the engine but no water out the exhaust.
Removed sea water strainer. Fine. Removed engine impellor cover. Impellor looked fine but noticed hiss of air excape as the cover was loosened, prior to water flowing out
With the engine off sailed in to the marina and just restarted the engine at the last moment to berth. Still no water out of the exhaust.
Once alongside removed impellor cover again. Water flowed in. Started engine. Disconnected pipe on outlet side of heat exchanger. Water came out and all was well.
Restarted engine Sunday morning. No issues. All was well.
Sunday afternoon after sailing across Channel entered Portsmouth harbour and again no water out of the exhaust.
Pulled pipe off heat exchanger outlet side, no water coming out.

Engine is a Yanmar 4JH4E 8 years old.
I anticipate pulling the Sea Water cooling apart but any suggestions beforehand?
Many thanks.

 

[Tranona]

Check that the seal on the top of the water strainer (assuming it is above water type) is sound as this is a common source of air getting in. Change impeller and check cover plate for wear on face. Check inlet seacock for any obstructions and all hose clamps for security.

 

[Talulah]

Check that the seal on the top of the water strainer (assuming it is above water type) is sound as this is a common source of air getting in. Change impeller and check cover plate for wear on face. Check inlet seacock for any obstructions and all hose clamps for security.

Thanks for the response.
When I remove the sea water strainer cover the water flows overs the top. Plus taking out the strainer means I've pretty well ruled out sea water strainer itself.
Equally disconnecting the inlet hose to the sea water pump provides a decent flow. So this rules out strainer and seacock.
However, the inlet hose was a clear pipe reinforced with twisted wire. It is no longer clear so you can't see the water flow. This blackening of the inside suggests heated water backflowing towards the strainer.
I wonder if discolouration is from heated water being passed back down through this pipe due to 'steaming' occuring in the heat exchanger.
Equally, is the air escaping from loosening the impellor cover as a result of 'steaming' in the heat exchanger as opposed to air being drawn in? The thing is I wouldn't expect any air to be escaping under pressure as the flow from the sea water pump onwards should be unpressurised other than the head of water.
The pump assembly was new about 2 years ago so I'm not expecting any issues due to wear. The impellor was changed a week ago just prior to this so that does suggest a link. But it could be the new impellor has formed a better seal preventing the pressurised air from escaping back towards the inlet side.
What is confusing is if the heat exchanger is blocked why is water flow good once I've removed the air at the impellor. Maybe flow is insufficient to cool the engine enough. Steam builds up and once the steam has back flowed to the impellor the impellor becomes ineffective.

 

[vyv_cox]

I don't think the steaming theory is credible unless possibly the exhaust manifold is partially blocked with salts.

After sailing for a time in boisterous conditions I suggest that the water in your strainer runs back into the sea on the appropriate tack. It may then take some time for the pump to reprime, although with a pump in as new condition it should happen eventually. I have a check valve next to my skin fitting to prevent this, which used to be a problem with my installation. However, I now have a Vetus strainer installed as instructions, 15 cm above the water line and no longer have the problem.

 

[Talulah]

I don't think the steaming theory is credible unless possibly the exhaust manifold is partially blocked with salts.

After sailing for a time in boisterous conditions I suggest that the water in your strainer runs back into the sea on the appropriate tack. It may then take some time for the pump to reprime, although with a pump in as new condition it should happen eventually. I have a check valve next to my skin fitting to prevent this, which used to be a problem with my installation. However, I now have a Vetus strainer installed as instructions, 15 cm above the water line and no longer have the problem.

Understood. I'll go down and investigate later in the week. The spare mixing elbow is in the car.
I'll report back with any findings.
Interesting to read about your check valve.

 

[chewbacca]

Does sound too much of a coincidence that it has occurred after fitting a new impellor.If by any chance you still have the old impellor handy it might be worth throwing it in.It might help narrow it down a bit.
Regards

 

[Talulah]

I've pulled off the heat exchanger.
My latest theory is that the sea water has been bypassing the heat exchanger.
The sea water enters at the bottom. It is then meant to travel the length of the heat exchanger and return on itself in the top half where it then goes into the mixing elbow via the antisyphon tube.
If the sea water in the heat exchanger itself is not getting refreshed it will eventually start to steam and I think this may be why the sea water pump has pressurised air coming out of it when I loosen the cover.
You can see the end of the heat exchanger casting has quite a bit of corrosion around it. The end cover is warped. i.e. if you sit it on a flat surface it rocks instead of sitting flat. Hence the warping has led to the seal getting pushed aside and the sea water able to bypass the heat exchanger. That's my theory anyway but I'm open to suggestions.
I need to replace the end cap but I suspect they are not available seperately. Probably only available as a complete heat exchanger for about £2.5k
I have some thick silcone sheet that I may try first to make new seals.
The fine tubes of the heat exchanger were remarkably clear although I did pass some wire down to check.

 

[djc]

Sounds like you've checked the sea water system and found no fault as yet , just an thought / idea but a friend with same engine set up had a problem of sea water would be flowing normally on low revs but after increasing to high revs after a while the sea water cooling flow would stop and the engine would start over heating .
This turned out to be his exhaust inner hose had a faulty flap of rubber which when on high revs would flap down and block cooling water pressurize and over heat engine. Replaced exhaust hose and perfect , Just a thought ?.
Secondly I have a Yamnar 4JH 2E the impeller faces inwards and I struggle to remove and refit impellers just wondered if you use a proper impeller puller tool to remove your old impellers .

 

[seaangler23]

Is there any way on that engine to by pass the water pump and stick a hose from mains water supply on while along side? be carefull to make sure the engine has started before turning the tap on so the water in the exhaust doesn't back up into the cylinders, i do this regular as it's only a small pump on mine and I've cleared a few blockages this way, also will show you if it's the pump to blame and washes a bit of salt out as a bonus

 

[Talulah]

Many thanks to all for the suggestion. The sea water pump faces outwards on this engine so it's really easy to get at.
I did some more investigation this weekend and despite my earlier thoughts it does look like air is getting sucked in through the top of the sea water strainer. (As suggested by Tranona.)
The sea water strainer looks like this and is mounted directly on to the hull. The top of the sea water strainer is below the water line. (Mine is slightly larger than this one.)

I noticed that as time went on the top section of the sea water strainer would fill with air.
I would blead this out by slackening the yellow thumb nuts, retighten and all would be well.
I would have to regularly bleed the air out of the top of the strainer.
(This is with the boat in normal conditions.)
So it suggests the seal is allowing air to be sucked in (albeit very slowly and only when the engine is running) but isn't allowing the air to escape.

 

[seaangler23]

Try a bead of silicone around it and tighten it up and go for a run if that doesn't happen again there's your problem