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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #1 (Edited)
I noticed yesterday that after motoring (at about 2500 rpm) for about 30-40 minutes to get out into the Chesapeake bay and out of the path of all the traffic. I dropped the engine down to an idle (about 600 rpm) and locked the steering since I was single handing, so that I could try to temporarily maintain a heading into the wind while I raised the main.

I was fighting with some sticking plastic mainsail slugs, which caused the operation of getting the sail up to be a little longer than expected. I'm guessing that the engine was idling for about 5-10 minutes, when the overheating alarm went off. Since it's only got a dummy light (right now), I don't know what the temp was before or after. I also didn't think at the time to look and see if water was coming out of the exhaust while the alarm was going off. But I did put my hand on the oil pan after the motor was shut down to see how hot it was, and it was warm to the touch, but not scorching.

I initially assumed that maybe I had done something wrong when I installed the new impeller a few days before. So I sailed for a couple hours to let it cool down and restarted without the alarm. Afterward I motored back about the same amount of time without the alarm. And it's also worth noting that before I got my rigging inspection, I motored it for about 2-1/2 hours from the marina where I got it to my slip without any overheating issues.

So here's my assumption (feel free to correct me)... With the motor running at its lowest rate of 600 rpm after being run long enough to warm it up, not enough water was being moved through the engine to keep it cool.

My question is this: Is this normal for a circa 1979 Yanmar 2QM15? Or is this an early sign that the water pump is marginal and not pushing as much water as it should? (remember, it's already got a new impeller).
 

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That's not normal. Beyond that I am unable to point to a likely culprit other than suggest that there is not enough water being pushed through the system. But you had already reached that conclusion.

Your engine is direct water cooled, right? I mean, you don't have heat exchanger with anti-freeze in it. So let's assume that the inside of your cooling system is all corroded and has some unwanted growth in it.

I'd do this:
Take the hose loose where it brings water from the thru-hull to the water pump. Run a garden hose from the dock to the hose you just took loose, and attach it securely. A hardware store can sell you a fitting, or in my case I just use a garden style spray fitting and hold to two hoses together by hand. It's only for a few seconds anyway.
Then open the thru-hull fitting and squirt the pressured water into the hose for a minute or so. What you're doing, of course is back flushing your water intake system and clearing it of any debris.

But that still leaves the inside of the water jacket in the engine as a likely source of inefficiency. Right off hand I don't know how to clear that part; maybe someone else has done it.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #3 (Edited)
Yes, it's raw water cooled, and the hose has an upward arch in it before it comes back down to the exhaust exit down by the water line.

Should it look like a full on water hose coming out of the exhaust, or just a small constant stream with occasionally spitting every few seconds...?
 

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Yes, it's raw water cooled, and the hose has an upward arch in it before it comes back down to the exhaust exit down by the water line.

Should it look like a full on water hose coming out of the exhaust, or just a small constant stream with occasionally spitting every few seconds...?
No, not the exhaust. The other way, so that the water goes out the thru-hull. You'll do it with the engine off.

I'd be afraid to blast water through the engine because of the exhaust elbow arrangement; remember, at a certain point in the engine cycle the exhaust valve is open and I'd be afraid water would find its way into the cylinder. That can happen very easily and has happened to me. I had to take the injectors out, dry things out, re-assemble, prime, bleed and re-start. It was a PITA, so let's avoid that mistake if we can.

But, now, it could be that the inside of the engine water jacket itself is all muddy and fouled up. If that's the case then the procedure we're talking about will have little effect. I'd try the safe, simple way first and if that doesn't fix the problem, well...we'll go from there. But I kinda feel like you're going to solve the problem by doing what we're talking about (let's call it a thru-hull reverse flush). The reason I think so is because we're blowing out the part of the system that always stays wet and thus provides a nice place for things to grow. Or lodge. I found a teeny weeny catfish in my air conditioner thru-hull once. The part of the cooling system that is above the water line dries out when you're not running the engine, right? So it'd be hard for something to grow.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #5
Should it look like a full on water hose coming out of the exhaust, or just a small constant stream with occasionally spitting every few seconds...?
No I meant... should the water stream look like my description above when the boat is running normally.

(The flushing exercise is a whole different topic).
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #6 (Edited)
I found this particular advice on another website about troubleshooting raw water Yanmar cooling issues - Yanmar Cooling:

To clean the seawater loop, start the engine and pour a couple of gallons of distilled white vinegar into the seawater strainer. Shut the engine down as soon as the last of the vinegar is poured in. Let it sit for 2 to 3 days, then start the engine and flush it out.
For that matter, I would think you could use something like CLR, which is safe for plumbing and it's made for dissolving mineral deposits in a lot less time than 3 days.

But since I don't have any idea what the water jacket looks like or how small or how few the capillaries in it are (thinking of a small car radiator). I'm wondering if loosening such deposits could send them further into the system and create an even bigger problem...?

---

Also found another product (the replacement for the old Marsolve) that claims to be a heavy duty marine decalcifier - Rydlyme Marine
 

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Tartan 27' owner
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Be careful with using products other then vinegar for flushing the the engine cooling system. Some products will harm the rubber hoses etc.
Vinegar is a naturally occurring substance that is completely harmless to the bay. It is a low strength acid that wont harm the hoses but will need some time to work its magic. You can always do multiple flushes but leaving it in the engine for some period of time gives it time to work.
If you try this make sure to check your exhaust after you start it up with the raw water open. You should see plenty of crud coming out.
Good luck.
 

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If your Yanmar is raw water cooled the hight temp alarm sounds off at around 140 degree's. This keeps evaporation to a min. and helps eliminate salt build up in the cooling chambers.
Rather than hooking up pressurize hoses, if you remove the thermostat and the rear zinc it will allow you some access to clean out the galley ways.
When you replaced the impeller did you resurface the cover plate. Take some 100 grit sandpaper laying on a flat block and work the cover till it's smooth. You can also flip the cover plate and use either side. This turned out to be my problem with an over heating issue. Something else to check might be the mixing elbow.
Bill,
 

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600 RPM is rather low for Yanmar 2QM15. I cannot verify it right away, I believe that proper idling speed is about 1000 RPM, I read it somewhere. At least it is how I keep my idling speed. Are you sure it wasn't low oil pressure alarm? Try to adjust the idling speed and see that happens.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #10
When you replaced the impeller did you resurface the cover plate.
Bill,
When I removed the cover plate and scrubbed it with a plastic 3M pad to get off TONS of crud and corrosion, the resulting shiny plate showed a light circular groove where the brass ring at the center of the impeller would ride. So I actually did flip the plate to the smooth side before reinstalling.

When I replaced the zinc at the water pump inlet, it wasn't really corroded, but it did have quite a bit of scale build up on it that crumbled off with light pressure. I've been unsuccessful thus far in removing the zinc that is at the back of the engine on the cylinder head. (It's frozen in place). I'm thinking I may need to remove the entire plate that it screws into, in order to get the zinc out. And if it's this frozen, I don't know if the previous owner tightened it down too much the last time it was changed. Or if it was just never changed at all. (Won't know until I get it out).

But I think I'm going to give the 3 day white vinegar flush a try and see what comes out of the engine.
 

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No I meant... should the water stream look like my description above when the boat is running normally.

(The flushing exercise is a whole different topic).
Yep, the exhaust water just sort of spits out; it doesn't run steadily.

You're getting some good info from this thread.

But I've got a question/observation/comment;
With a direct sea water cooled engine, wouldn't it be true that the inside of the engine dries out between runs?
I mean, when you shut the engine down the water drains out of the engine, back through the water pump and out the thru-hull down to the level of the water line. Then the still warm engine evaporates everything that is left leaving a dry environment inside the water jacket.

The only way this wouldn't be true, it seems to me, is if the thru-hull were closed immediately upon shut down; that would trap the water (or vinegar) in the system.

Since the water pump is below the water line it would always be wet, and therefore get all gummy.

Am I wrong?......

PS.....I found the water line level on the inside of my boat by attaching a length of clear hose to a thru-hull and watching how far the water level rose.
 

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Loose belt??
 

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1979 C&C 30 Mk I - 2QM15
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172 Posts
Discussion Starter #13
But I've got a question/observation/comment;
With a direct sea water cooled engine, wouldn't it be true that the inside of the engine dries out between runs?
I mean, when you shut the engine down the water drains out of the engine, back through the water pump and out the thru-hull down to the level of the water line. Then the still warm engine evaporates everything that is left leaving a dry environment inside the water jacket.

The only way this wouldn't be true, it seems to me, is if the thru-hull were closed immediately upon shut down; that would trap the water (or vinegar) in the system.
Feel free to correct my logic if I'm wrong... But my plan is to shut off the intake thru hull valve. Then I'll fill the above-water-line strainer basket with vinegar and start the engine. I'll continue to pour until the last of the 2 gallons of vinegar goes into the strainer basket. At which point I'll immediately shut off the engine. And since the exhaust host has a rise in it that's above-the-water-line too, the engine water jacket, the pump, and both the intake and exhaust hoses should be full of the trapped vinegar.
 

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Feel free to correct my logic if I'm wrong... But my plan is to shut off the intake thru hull valve. Then I'll fill the above-water-line strainer basket with vinegar and start the engine. I'll continue to pour until the last of the 2 gallons of vinegar goes into the strainer basket. At which point I'll immediately shut off the engine. And since the exhaust host has a rise in it that's above-the-water-line too, the engine water jacket, the pump, and both the intake and exhaust hoses should be full of the trapped vinegar.
That's exactly how I would do it.
But are you sure the strainer basket is above the water line? I would have thought the strainer would be below the water line so that the pump stays primed. Just a thought/question. Doesn't change the procedure one bit.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #16
The strainer basket is mounted to the transverse bulkhead between the cockpit and the cabin and the top of the basket is only about 3 inches below the cockpit seats. So yeah, it's definitely a couple feet above the water line.
 

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The strainer basket is mounted to the transverse bulkhead between the cockpit and the cabin and the top of the basket is only about 3 inches below the cockpit seats. So yeah, it's definitely a couple feet above the water line.
You know, if that basket seal was not perfect then it may draw in a little air, losing its prime.

Next time, before you start the engine, look and see if the strainer is full of water. If it isn't, then maybe air has leaked in and the impeller needs to spin pretty hard to prime things up. Then, when you idle back down it starts sucking air again.

Just a thought......a cheap one, too!:)
 

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Since the engine isn't overheating at normal speed and power, I doubt the vinegar flush is the solution. If the engine internals were scaled, then the overheating would occur at normal power, not at idle. I suspect its the raw water pump that is losing suction at low RPM, perhaps due to wear and excessive clearances.

By the way, oxalic acid is a better choice than CLR or vingear for dissolving and flushing the cooling system. It's relatively safe and it will work in minutes, not days.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #19
How high are strainer baskets for raw water cooled engines "usually" mounted in relationship to the water line?

In any case, the Yanmar is full of vinegar and being "pickled" as we speak. So 3 days from now I'll report in and tell you how much (if any) gunk comes out. And I'll also note how full the strainer is at idle (600-700 rpm), at 10k rpm, and at 20k rpm. But the strainer looks relatively new, so the O-ring for the lid is in good shape and appears to give it a positive seal.
 

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1979 C&C 30 Mk I - 2QM15
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Discussion Starter #20 (Edited)
Since the engine isn't overheating at normal speed and power, I doubt the vinegar flush is the solution. If the engine internals were scaled, then the overheating would occur at normal power, not at idle. I suspect its the raw water pump that is losing suction at low RPM, perhaps due to wear and excessive clearances.

By the way, oxalic acid is a better choice than CLR or vingear for dissolving and flushing the cooling system. It's relatively safe and it will work in minutes, not days.
My original first thought was that the water pump (even with a new impeller) may be in a marginal state and not moving enough water at idle. So that'll probably be the first thing I rebuild/replace if soaking in vinegar doesn't clear any visible crud out of the cooling system.

But as I said earlier in the thread. When I pulled out the zinc closest to the water pump, which was probably replaced only a year or so ago. It had a layer of lime scale all the way around it that crumbled off when I grabbed it. So this would seem to indicate that there's at least some level of lime scale build up inside the water jacket too.

Anyway... I figured for the price of only $3.50 worth of vinegar and 5 minutes to pour it into the engine, it was an easy exercise and worth a try.
 
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