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Old 15-06-2008, 18:46   #1
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Ever Hear of 'Air-Lock' Trouble In Raw Water Circuit (Yanmar 2gm) ?

Hi folks,

I ran my new-to-me 25 year-old 2gm for approx 50-60 hours last year. Normally it worked well, and ran cool (it is raw water cooled). On about 3 occasions, the raw water flow stopped (as discovered by quickly pulling off the hot hose from the engine to the exhaust elbow). There was no water coming out of this hose, but the flow returned fine, after just revving the engine quickly - it then ran fine for hours, etc.

I have replaced the impeller, and my auto mechanic brother took the pump apart. There was a bit of scoring on the inside of the pump cover, but he said nothing to worry about. I could see rebuilding / replacing the pump if I had consistently low water / high running temps, etc - but I am baffled by the intermittent trouble.

The engine starts immediately, and runs well otherwise. I believe it to be relatively low hours, for it's age!

Any ideas on what could cause such an intermittent problem???
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Old 15-06-2008, 19:39   #2
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Could it be an anti siphon loop which is not working properly somewhere in there?
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Old 15-06-2008, 21:17   #3
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Other problems arrive from objects plugging the intake. Garbage,like plastic bags and even seaweed are the most common.

Another cause could be that your intake line is sucking air from somewhere, like a loose hose clamp above the waterline.

Or, your outlet line could have a restriction which could even be in the cooling system itself. Revving the engine puts more pressure/suction on the line getting it going again.

Being raw water cooled I'd lean more towards the restriction side of things, but without actually being there it's all an educated guess!.
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Old 15-06-2008, 21:35   #4
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what did your old impeller look like was it missing vanes? If so where did they go?
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Old 16-06-2008, 00:51   #5
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water cooling

Remove & check the thermastat for operation in hot water.Also the water path from this point into the block with a stiff wire. I found a blockage from salt in one of my engines. Get a service manual if you intend to do your own work as every thing is covered & the internals are better understood. (The impeller has a repution of becomming unbonded from the shaft in the 1GM pumps but I do not know if it is a problem on the 2GM.)
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Old 16-06-2008, 05:05   #6
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Thanks for all of the helpful input.

The old impeller was fine, no broken vanes. Just changed it "in case"!

The PO may have had impeller troubles / broken vanes in previous years, not sure!

There is no antisiphon loop in the line. I shut the seacock when the engine is not running, so I don't feel I require a loop.

re: obstuctions to the intake - I agree a plastic bag, etc could account for an occasional blockage. However, I just started the engine for the 1st time this year (on the hard, running from a clean bucket of water), and it did it again. Started with good water flow, then stopped!!!! After revving it up, and putting the hose back on, it ran for 10 minutes with lots of water out the exhaust.

All of the hose / clamps seem OK / tight on the intake side, and I had the exhaust elbow off late last season - very clean/ no obstuctions. I also soaked it in muritatic acid (weakened), and circulated the acid through the block.

Tested the thermostat last year - opened and closed fine.

Re: the thermostat - As I understand it, I should have raw water pumping the whole time, whether it circulates through the block, or bypasses it, to then go out the wet exhaust.

I do have a service manual, and am getting pretty familiar with this engine!


What bothers me is the intermittent nature of this loss of water. It may not be an airlock, as i suggested - but whatever it is only happens once in a while, and revving the engine seems to get the pump to overpower the problem. If I have excessive salt buildup, or other restrictions, I would think I wouyld see reduced water flow / overheating all of the time.

Any other thoughts???
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Old 16-06-2008, 05:07   #7
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How about the exhaust elbo?
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Old 16-06-2008, 06:58   #8
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Do you have a strainer on the intake? The way my old raw water volvo worked, and I believe most others are similiar, is that when the thermostat is closed the raw water passes through a very small orifice in the thermostat housing. This is what builds the backpressure in the coolant system, and why removing the thermostat is bad long term. When the thermostat opens, it has more room to flow than just the orifice and you get higher coolant flow through the engine. Any debris in the coolant is most likely to get caught in that small orifice in the t-stat housing, shutting down flow until the thermostat opens up. Sometimes the debris will work it's way out, often the thermostat housing needs to be pulled and the orifice cleared. This is one possible cause of the intermittent flow, and why revving the engine up(heating it up till thermostat opens and stays open)might make the flow start.
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Old 16-06-2008, 08:16   #9
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Def- Checked (and cleaned) exhaust elbow last year - was not blocked at all anyway.

Fish - Not sure if that is how mine works. If so, in addition to the small hole, there is a water bypass hose that, I believe, lets the raw water bypass the block when the thermostat is closed (and therefore not calling for cooling water). Therefore I should still have the same rate of water flow, to the exhaust elbow, regardless of whether the thermostat is closed or not - I believe!!!

Keep the coming - I do appreciate the input!!!
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Old 16-06-2008, 08:20   #10
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Sometimes, the waterpump hub de-bonds from the rubber impellor. You can't tell it's bad. Try another new impellor.........?
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Old 16-06-2008, 08:23   #11
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Joli - I can try that. Would that not keep it from pumping well all the time though? Or could that cause intermittent trouble?
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Old 16-06-2008, 18:45   #12
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Have you aken off the pipe from the pump to the therostat housing to check for salt build up or checked for a kink at the bends? may be there is an old blade hiding there.

Regards Bill
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Old 16-06-2008, 20:20   #13
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Joli - I can try that. Would that not keep it from pumping well all the time though? Or could that cause intermittent trouble?
Sometimes it will pump and sometimes it will simply spin the bush. Not sure why it does it but my brother recently went through it.
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Old 16-06-2008, 21:39   #14
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Quote:
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Sometimes it will pump and sometimes it will simply spin the bush. Not sure why it does it but my brother recently went through it.
Note:
Some impellers have a cross pin, others have a star bushing.
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Old 16-06-2008, 21:42   #15
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I had a similar problem that was intermitent but I'm not sure that it applies to your situation. My raw water strainer was about 16" above the waterline. One of the gaskets on the strainer went bad and there was an air leak. The engine worked fine when it was primed but wouldn't draw water when not primed. I replaced the gasket on the raw water strainer and moved the strainer below the waterline so that priming would never become an issue.

Have yo0u checked to see that the hose clamps have not cut a hole in the hose. I had this problem with my fuel intake line. It caused the engine to loose prime.
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