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Old 17-06-2008, 05:03   #16
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Thanks for all of the input!!

I will be at the boat in a couple of days. Wil check the impeller and all hoses, as suggested. I believe I have enough hose to cut a bit off, and put the clamps on again. Of course, i can buy new hose if not!

Re: the strainer, it is a brand new Perko, and the old one was a cheap small plastic one. It may be slightly above water line. I will have to check. The boat is not in the water yet, but I will try to have it low enough to fill / prime from the water pressure from the seacock.
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Old 21-06-2008, 19:08   #17
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Checked impeller (great condition).
Checked hose from pump- water leaving there.
Cheked hoses for blockages and loose clamps- all good.

Checked thermostat - worked fine.
Checked hose from thermostat - no / very little water until thermostat opens.

After looking closely at the thermostat, I believe I was incorrect in my assumption that a large voume of water would bypass the thermostat, when it is not open, and go to the exhaust elbow. The bypass hole on the thermostat is very small, and would not allow much water to pass, until such time as it opens, and allows much more water to pass from the block.

I started the engine and ran for about 20 minutes with lots of water volume, even at idle - ONCE OPERATING TEMP IS REACHED, and the thermostat opens often. There was vrey little water exiting the exhaust until the thermostat opened, and continued to open, quite frequently I imagine, to maintain a constant temp.

Does this sound normal??? Am I missing anything??
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Old 22-06-2008, 07:00   #18
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There WILL be less flow until the thermostat opens up, but whether or not it's the right amount can really only be determined by someone looking at it. Maybe post up a youtube video of the flow out your exhaust before the thermostat opens. Enough old salts on here they will probably be able to tell you whether it's sufficient.

It's been a while since I had my salt water cooled engine, but I sort of remember a blast of water out the exhaust every 2-3 seconds until it heated up, then it was more continuous.
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Old 22-06-2008, 07:13   #19
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Don't know if your pump is belt driven but check the belt for wear and tightness if it is and also if there is no slippage of the pulley on the shaft.
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Old 22-06-2008, 10:11   #20
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This may seem pretty elemental, but it's worth a try: Remove the hose on the OUTPUT side of the raw water pump. Blow hard. See if anything comes out the end that goes into the exhaust elbow input. It won't work on the input side of the pump because of the seal of the vanes, but you should be able to move some water or air in the engine block galleries. Also, leading from the seawater strainer to the raw water pump, does the hose rise above seawater level at any point? Can it be lowered? If the issue is the need for an anti siphon (engine below waterline with raw water cooling) then install a standard hose barbed siphon break (as used in head installations) between the OUTPUT side of the water pump and the seawater intake to the engine. I use a siphon break downstream of my heat exchanger OUTPUT and the exhaust elbow. I then have an 1/8" copper tube running from the top of the siphon break through the side of the centerboard trunk (you can also run it out the transom if stern mounted engine). When I do my engine checks, I look to see if the stream of water shooting out looks good and strong, and isn't too warm.

I just went back and reviewed your posts and realized you had confirmed the hole in the thermostat. Try removing the thermostat and performing the blow test if you are getting no output. It sure sounds like a blockage in the guts of the cooling jacket, and the only test, short of pulling off big chunks of engine, is to blow something through it. When I did that to my old Volvo, seawater cooled engine, I found great gobs of rust and sludge, and ???. I had to acid etch, probe, scrape and yell at the engine before converting it to a freshwater cooling system. The happiest day was tossing the Volvo into the dumpster and installing a Yanmar 3GM30F.
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Old 25-06-2008, 18:10   #21
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Roy - Thanks for the advice. I haven't tried blowing through the hose. However, I have taken the hose off the exhaust elbow, as well as the hose of the output side of the pump. When I pour water down the block (exhaust elbow hose) it does immediatley run out of the bottom (where the pump hose connnects).

As I think I mentioned earlier, I ran it for 15 - 20 minutes last weekend, and it runs fine / did not overheat - as well as running it for hours at a time last year, under load, without overheating. So, I do not think I have a permanent blockage in either the block, or the hoses - but am baffled as to why I have lost my water flow about 3 times last year, and it seemed to start again when I took off the exhast elbow hose, which was dry, revved the negine up, which got the water flowing again, and quickly put the hose back on. I will be putting the boat in the water in a few days, so I will check the height of the raw water strainer - will likey open seacock and make sure the strainer fills itself, with the pressure form the seacock. If not I will lower it. The topw of the stariner is about 6" above the seacock, which should be still well below the waterline.
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Old 25-06-2008, 19:23   #22
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Have you replaced the impeller? If the seawater strainer, the seawater impeller, and all connecting hose is below water level, you should have plenty of flow to the exhaust elbow, having confirmed that the jacket galleries are open to flow. Also, was the flow stoppage occuring at the onset of engine operation? I believe you mentioned it, but that could be a simple thermostat issue. Possibly, you could purchase a new one and keep the old one as a spare (or spare of a spare).
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Old 26-06-2008, 01:22   #23
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Any chance of the exhaust system having an intermittent constriction. I am wondering if backpressure through the raw water outlet is impeding the priming of the raw water pump.

Loose exhaust baffles in the can, carbon build up etc. etc.
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Old 26-06-2008, 11:52   #24
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Thanks for all the input!!

Have changed the impeller.

The thing is I do have lots of flow to the exhaust elbow - once the thermostat opens, but I believe the very small bypass hole in the thermostat may account for the littlke / no flow to the elbow for the 1st couple minutes.

I put new exhaust hose in last year, and cleaned out the rubber silencer - I do NOT have a waterlift muffler. As mentioned before, usually I can motor for hours without trouble, so maybe I do have a sticking thermostat, that fails occasionally - I have tested it in hot water - looks clean, and opens fine - but maybe it sticks once in a while!!! I will order a new one.


I will test more, once in the water next week - easier than running from a bucket!!

Thanks again!!
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Old 23-07-2008, 14:04   #25
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northeaster, it has been a couple of months since your last post. Did you resolve the problem? i had a 25 yr raw water cooled Universal. The flow thru the block was insefficient to push up the hose coming from the thermostat housing to the mixing tube in the exhaust pipe coming from the exhaust manifold. Even after using a gallon of the acid treatment! new hoses, new pump, new waterlock muffler since the lack of water melted the inlet plastic tube. When it was working hardly any water would come out the exhaust since the 2 bleed holes in the thermostat are about 1/16".
I traded the boat in for a 2 year old boat!!
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Old 23-07-2008, 14:12   #26
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CatnBill- Thanks for the input!

I have had the boat in the water for only 3 weeks now, but so far so good!!
I have motored approx 10 - 15 hours at different speeds and the engine has worked well - no overheating / water loss at all.

I will have to wait and see if it happens again. I still feel it is more of an airlock or something causing the pump to not work as well at times. Of course, if the pump were brand new shape, it may overcome any issue. There have been other threads on similar symtoms, except the other people generally say it happens after healing alot while sailing .

My water strainer is below sea level, so the pump gets water "pushed" right into it's intake. Maybe this explains why it "airlocked" once while feeding from a bucket, as it would have to pull the water up the hose!
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Old 23-07-2008, 14:40   #27
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There was no strainer in my boat. The pump was about 3 inches above the water level so it had to lift the water initialy to start the flow.
The day that it "went south", water was coming out the exhuast along with steam. After 2 hours at 85% power the temp went up to about 175 degs then the impeller spun on its bushing and the temp topped the gauge. After a new pump, with the thermostat out it would not push any water up the mixer feed hose.
keep an eye on the temp, it should not go above 155 deg with the 142 deg thermostat.
I now have a Yanmar fresh water cooled with a temp gauge that I installed to supplement the "idiot light/alarm".
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Old 23-07-2008, 16:56   #28
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CapnBill - I installed a temp guage as well, as I wanted more info than just the buzzer. Unfortunately, my gauge starts at 130 degrees, and goes up from there. My raw water cooled engine only runs at 130, so the needle really doesn't move until there is a problem. However, it was still usefull to see the actual temperature when it did go up last year. As soon as I pulled the hose off the exhaust elbow, revved the engine a couple times - returning the water flow, I could watch the temp go down right away.
Of course, I have been told since that it may have been better to shut it down and let it cool off, instead of re-establishing the cold water flow immediately to the block. Since it is not an aluminum block, I thinh the chances of cracking it like that are fairly low, as it was only up around the normal operating temp of a fresh water cooled engine anyway.
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Old 22-08-2011, 09:05   #29
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Re: Ever hear of "air-lock" / trouble in raw water circuit (Yanmar 2gm)?

for possible solution, see here:
Solved! Overheating Yanmar 3GM30 due to Johnson raw water pump not priming. - Yachting and Boating World Forums
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Old 26-08-2012, 16:15   #30
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Re: Ever Hear of 'Air-Lock' Trouble In Raw Water Circuit (Yanmar 2gm) ?

I had a similar problem with the raw water pump on my 2GM motor, fresh water cooled. Due to excessive heeling when sailing air would get into the water intake line. Often the system would air lock. To avoid this problem I would close the thru-hull for cooling water when sailing to keep air out. (This was a bother.) Later the raw water pump shaft seal failed and I replaced the pump. Strangly, this also cured the air lock problem. I conclude that although the old pump housing and impeller looked fine it was actually leaking enough that it would pump water but not air. Hope this helps you. ET
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