# Yanmar 2QM15 Overheating at Idle...



## backcreeksailor (Mar 9, 2009)

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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## TSOJOURNER (Dec 16, 1999)

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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## backcreeksailor (Mar 9, 2009)

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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## TSOJOURNER (Dec 16, 1999)

backcreeksailor said:


> 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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## backcreeksailor (Mar 9, 2009)

> 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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## backcreeksailor (Mar 9, 2009)

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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## CalebD (Jan 11, 2008)

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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## billangiep (Dec 10, 2003)

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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## CrazyRu (May 10, 2007)

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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## backcreeksailor (Mar 9, 2009)

billangiep said:


> 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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## TSOJOURNER (Dec 16, 1999)

backcreeksailor said:


> 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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## timebandit (Sep 18, 2002)

Loose belt??


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## backcreeksailor (Mar 9, 2009)

Selkirk said:


> 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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## backcreeksailor (Mar 9, 2009)

timebandit said:


> Loose belt??


Nope... I tightened the water pump and alternator belts when I changed the impeller.


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## TSOJOURNER (Dec 16, 1999)

backcreeksailor said:


> 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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## backcreeksailor (Mar 9, 2009)

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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## TSOJOURNER (Dec 16, 1999)

backcreeksailor said:


> 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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## JimsCAL (May 23, 2007)

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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## backcreeksailor (Mar 9, 2009)

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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## backcreeksailor (Mar 9, 2009)

JimsCAL said:


> 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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## TSOJOURNER (Dec 16, 1999)

I'm not a boat guru, but I did own and maintain my O'Day 322 with a Yanmar 2GM20 for about a dozen years.

My strainer was located below the water line. I sort of figured they were all like that, but I can see where, as a safety measure, someone would want to loop it above the water line.

Regardless, I think you're hot on the trail of the problem now.....


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## JimsCAL (May 23, 2007)

backcreeksailor said:


> 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.


If you think you have scale buildup, try oxalic acid. It will work MUCH better than vinegar. And a box of crystals sufficient to make a gallon or two of liquid is only about $5 in the local hardware store. Circulating the acid with a portable pump is the way to go, then flush with fresh water.


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## backcreeksailor (Mar 9, 2009)

JimsCAL said:


> If you think you have scale buildup, try oxalic acid. It will work MUCH better than vinegar. And a box of crystals sufficient to make a gallon or two of liquid is only about $5 in the local hardware store. Circulating the acid with a portable pump is the way to go, then flush with fresh water.


I've read about oxalic acid and I was just a little leery about putting something that strong into a 30 yr old engine, mainly because I didn't want it eating away at rubber seals and paper gaskets that could in some cases be as old as the engine.

Is the main advantage of using oxalic acid just the time it takes to work? If I'm not in a hurry and I have a few days to let it do its thing, won't using a weaker acid like vinegar dissolve the same amount of scale build up?

If not... What kind of pump would you use? How would you connect it? And how would you close off the exhaust system so that it could circulate the acid without pumping it out?


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## JimsCAL (May 23, 2007)

I have used oxalic acid many times on my 27 year old raw-water-cooled Universal 5411 and have not had any problems. If you do a Google search for "Atomic four acid flush" or something similar you will get dozens of hits, many with detailed instructions. Other choices are muriatic acid (too strong in my opinion) or Marsolve (good product but you'll have to order it and it's 5 times the price of oxalic acid). I put in an electric washdown pump in parallel with my engine's raw-water pump to facilitate winterization and also for the periodic acid flushing. Just about any pump will work, even one of those cheap plastic one that mount in an electric drill.


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## backcreeksailor (Mar 9, 2009)

Today ended the 3-1/2 day pickling process with distilled white vinegar. The exhaust pumped out a white milky discharge for a good 1 to 2 minutes before the exhaust water started coming out clear again. So I'm assuming that at least some of that was dissolved lime scale. 

I did notice that when the strainer lid was just snugged vs. cranked down tight, that the exhaust water flow was noticeably less. So we'll see this weekend if that was the fix for the overheating at idle or not.


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## TSOJOURNER (Dec 16, 1999)

backcreeksailor said:


> Today ended the 3-1/2 day pickling process with distilled white vinegar. The exhaust pumped out a white milky discharge for a good 1 to 2 minutes before the exhaust water started coming out clear again. So I'm assuming that at least some of that was dissolved lime scale.
> 
> I did notice that when the strainer lid was just snugged vs. cranked down tight, that the exhaust water flow was noticeably less. So we'll see this weekend if that was the fix for the overheating at idle or not.


 In 1963 I was a newly trained electronic technician, US Navy. I had spent a full year going to school - that's 8 hours a day, 5 days a week, for a year - and was all fired up and ready to go to work.
I reported aboard my first assignment and found the senior ET, as we were called.
Imagine my dismay when all he gave me was a flashlight and a nickel!
The nickel, he explained was to be used to open up the cabinets so that I could see the guts to all the radars, radios, loran, etc.....And the flashlight? That was to be used to observe. Don't touch anything, he said, just look. And then wake up the duty technician and tell him what the symptoms are.

I have never forgotten the importance of keen observation when diagnosing any problem. Start with the symptoms.


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## Mipcar (Nov 8, 2008)

backcreeksailor said:


> Today ended the 3-1/2 day pickling process with distilled white vinegar. The exhaust pumped out a white milky discharge for a good 1 to 2 minutes before the exhaust water started coming out clear again. So I'm assuming that at least some of that was dissolved lime scale.
> 
> I did notice that when the strainer lid was just snugged vs. cranked down tight, that the exhaust water flow was noticeably less. So we'll see this weekend if that was the fix for the overheating at idle or not.


I posted a reply in your other query about the engine anode. So I'll just be brief here, if your not sure when /if the rear anode was ever last changed then there is a good chance that it has broken down to the extent it is flaking of large chunks and clogging the waterways in the head. You may be getting overheating at low rpm as the pump is not producing enough pressure to overcome the blockages.

Mychael


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## dreuge (Sep 18, 2009)

Hi,

I just came across this post, and I have a problem with my 2QM15 in my 1979 C&C overheating (or alarming) when I restart the engine within an hour of shutting it off. The alarm usually shuts off after 30-90 seconds, but this past weekend as I was motoring into the harbor, the alarm came on after 5 minutes of engine use. The local water temp was 92F, so I expect this exacerbated the problem.

I am planning on doing a vinegar soak next weekend. What was the outcome of your vinegar cleaning? Did it solve the overheating problem?



backcreeksailor said:


> Today ended the 3-1/2 day pickling process with distilled white vinegar. The exhaust pumped out a white milky discharge for a good 1 to 2 minutes before the exhaust water started coming out clear again. So I'm assuming that at least some of that was dissolved lime scale.
> 
> I did notice that when the strainer lid was just snugged vs. cranked down tight, that the exhaust water flow was noticeably less. So we'll see this weekend if that was the fix for the overheating at idle or not.


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## dreuge (Sep 18, 2009)

Hi,

Here is a follow up. I managed to "fix" my problem last year (see above) by flipping the impeller plate over so that it now had a flat surface against the rubber impeller, I redid all hose connections with a gasket dressing(form-a-gasket #2) at the hose clamps ends, and pickled the engine twice with vinegar for a week. After all of this, I had better exhaust water flow and the engine alarm no longer initially alarmed while motoring. I still had the problem that after shutting off the engine and restarting within a short time, the alarm would go off for about 30-45 sec, but then would be fine. All was well for about a year.

This past June, I sailed across the big bend part of the Gulf and back (~180 NM each way). I motored for the last three hours on the way home and experienced no engine problems. Three weeks later, I returned to the boat only to find the engine seized solidly. It was a seized piston due to a failed head gasket which let salt water into the cylinder. 

I just finished rebuilding my 2QM15, but in the process of pulling the cylinder liners I discovered the real culprit, salt scale build-up in the engine which was preventing proper engine cooling. See "www.flickr.com/photos/dreuge/sets/72157631067396554/" for pictures.

During the rebuild, I cleaned out the scale very well, and the rebuilt engine runs great and cool. While pickling with vinegar did seem to help a bit, I plan to yearly spend the few dollars and use Barnacle Buster. 

SO if you have an engine whose temperature alarm buzzes when restarting a warm/hot engine, You likely have a scale build-up problem! Get the concentrated barnacle Buster and recycle it through you engine. It's a lot cheaper than a rebuild.


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## jsaronson (Dec 13, 2011)

The sea water strainer could be full of jelly fish, or who knows what. If the thru hull has an external strainer, it could have growth on it.


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## dreuge (Sep 18, 2009)

jsaronson said:


> The sea water strainer could be full of jelly fish, or who knows what. If the thru hull has an external strainer, it could have growth on it.


Yes, strainer and water in-flow is a first check. But since the alarm would only buzz after an engine shutdown followed by a restart and never during an initial start up, then it would highly unlikely be an intake flow problem. Also note that the buzzer would stop after 30-45 seconds after the engine restart.


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## RichH (Jul 10, 2000)

I dont have the specs for the 2QM15 but for the 2QM20, the raw water output at the exhaust exit is 3 gallons per minute at 2000 rpm. Translating that to 1000 rpm would be 1.5 gallons per minute and 1 gpm at 700 rpm (normal idle speed).

Suggest you get a measured bucket and stopwatch and measure the actual output overboard discharge, especially at 1000 and idle speed. 
If the idle speed output discharge is significantly LESS than 1gpm then the posibilities are":
• Improper replacement impeller ... an impeller that is too NARROW and is allowing leakage internally at the SIDES of the vanes.
• Worn 'side walls' or cover plate - allowing leakage IN the pump (from the high pressure side to the low pressure side of the vanes) 

Since you recently 'replaced' the impeller the odds on favorite would be a too narrow vaned impeller was used. The new impeller is 'bypassing' at the sides of the vanes. 
Other possibility is that you used a new and TOO THICK gasket for the side plate .. which has positioned the cover plate to impeller side distance too far away from the vanes. The impeller sides of the vanes should 'just touch' the side plate surface of the pump housing and the cover plate. If not 'just touching' you dont form a seal. 

Why overheating at idle and not at full rpm? .... at Full rpm the viscosity of the water and the mass of turbulent water is still making a seal at the sides of the vanes. At idle, the water IN the pump is less turbulent and thus a greater chance to 'break the liquid seal' between the vane sides and the pump. 

Hope this helps


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## dreuge (Sep 18, 2009)

I agree with you regarding the impeller seal. That is why I flipped over my water pump backing plate last year. And that fixed the problem of over heating when running.

I think you misread my recent post. My long lasting problem had nothing to do with low versus high RPM over heating, but restarting an engine a few minutes after it was shutdown(I would even let it idle 5 min before shutting down). The problem was due to a lot of scale blockage in the cylinder block (See the photo with cylinder liner removed). The engine has been rebuilt and the water jacket in the block & head cleaned out of all the scale blockage. The engine runs perfectly now. 

But thank you for your reply, because measuring the water output now is a good point. I'll post what I get for the 2QM15 for other to compare. A while back when I asked how much exhaust flow should I expect, I was told that it should "burp" out.


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## fryewe (Dec 4, 2004)

The 2QM15 raw water pump is rated at 400 liters per hour at 2000 RPM per the service manual.


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## RichH (Jul 10, 2000)

Sorry to misread. 
Vinegar doesnt have the needed acidity/potency to remove carbonate scale, ditto oxalic acid .... unless you are willing to wait days and days. 
Muriatic (Hydrochloric) acid will readily attack the cast iron once it penetrates the calcium carbonate scale. 
Marsolve is an inhibited mix of Hydrochloric, etc. - the inhibiters lessen the destruction of the cast iron.

Use Marsolve, *RydLyme*, or other inhibited commercial boiler descalers ... if you dont want to 'dissolve' the cast iron.
http://www.rydlyme.com/home.html


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## L124C (Oct 4, 2007)

I have a 2GM20 about the same vintage. After extended idling it sometimes does the same thing. As soon as I increase the RPM's the buzzer quits. Has does it for years, and now I just hear it as a reminder that diesels don't like to idle. While I flush it with fresh water after every use, the PO's didn't for 20 years and God knows what the insides look like. I'm not sure I want to "dissolve" anything! They say raw water diesels never get hot enough anyway, so I'm not going to worry about it. If it overheated under power, that would be another issue. It runs great, and if I ever needed to go in for a major repair, I would deal with it then. 
BTW, I usually cruise at 3000 RPM. A friend had the same motor and hated it. He thought it was underpowered and had maintenance issues. Yanmar told him the motor is a high speed diesel and that he was not using enough RPM. The motor doesn't even achieve it's higher HP rating until 2800 RPM. He worked it harder, fell in love with it and never had any more maintenance issues! Check the HP/RPM ratios for your motor.


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