[Jeff_H]

I am usually pretty good at diagnosing engine issues, but this one has me perplexed. For several years now I have had intermittent cooling water problems on my Yanmar 3GM 30F. It first showed up about 2 years ago as a less than usual water flow.

I assumed a bottom growth perhaps fouled outside inlet strainer, and so had the diver carefully clean the inlet from the outside of the hull. I then cleaned out the fine strainer on the inside of the boat and back flushed the intake hose and system in case the outside cleaning snapped off some barnacle shells.

I started the engine and all seemed like normal at idle but as I began to motor at normal speeds the water flow stopped entirely. So idled back to the dock. I figured it had to be on the inlet side of the pump since it got worse as the rpms and therefore suction increased. So I disassembled the system from the intake thruhull to the water pump, replaced the impeller, checked the hoses, cleaned out the strainer, back flushed the system, and tightened all of the house clamps and figured that this ought to solve the problem.

It helped a little but the problem was still there, but again only as I throttled up. So I thought that it must be the hose from the strainer to the water pump which was 25 years old, has several tight turns and is pretty long. I figured the higher suction at higher RPM's was sucking the hose closed.

So I went over to WM and bought a new hose. They happened to be out of a long enough length of wire reinforced in the correct size, and so I bought a regular non-wire-reinforced cooling water hose in the correct diameter and a length of larger diameter clear plastic hose, which I slid over the hose at the bends to keep it from sucking shut at the bends. And that worked better, but perhaps with a little less water than the engine had been pumping prior to all of this.

I compromised by running the engine at slightly slower cruising speeds which seemed okay for that season. But then it started again last fall. So once again, I replaced the hose, this time with wire reinforced hose. I still placed the larger diameter clear plastic hose over the new hose at bends and places where abrasion might occur.

As a further precaution I back-flushed the system, replaced the impeller again, and checked all of the hose clamps. And once completed that seemed to work better, but still a little less water at higher RPM's than it had been before all of this started.

This weekend I started up the old Yanmar. It had not been run in about five weeks and had antifreeze in the system. On start up some scale came out with the exhaust water, which was something I had never seen before on this engine, but otherwise it looked like it was pumping water normally at idle. As soon as I increased the RPM's above around 2200 rpm, it seemed to stop pumping water again.

At this point, I have a lot of confidence that the suction side of the system is clear and operational so I am completely baffled. In my mind, if this were a problem on the discharge side of the system, I would expect that a greater water flow would occur at higher RPM's since there is greater pressure to push it out of the system, and that the lack of water would occur at idle.

In my mind the only downstream issue that I am even a little suspicious of is the water inlet at exhaust elbow being blocked and that the higher back-pressure at higher RPM's is somehow keeping the water from coming through the blocked orifice.

Has anyone else ever experienced anything like this? Or does anyone have a theory about what is happening?

Thank you,
Jeff

 

[rugosa]

There's an article in March/April Good Old Boat by Ed Zacko with similar problem, his boiled down to sheared impeller retaining screw in water pump. As RPM increased impeller would freewheel in the water pump. Sorry, don't have link to send you.

 

[single2coil]

Hey Jeff, I have same engine in my 34 C&C. Having mystery over heating also, did much what you have done. I read in one of our sailnet threads that had similar problem. They were in Europe, on a river, if I can recall.
Two things turned out. One was the exhaust elbow. The other was the shaft in the pump, the impeller slides over was slipping or cracked, ? not sure, they replaced the raw water pump, solved the issues. So it turned out to be two issues. I am now getting ready to replace the elbow, no use checking it, too much labor not to replace.

 

[single2coil]

Jeff, that was the article I was referring to. Good Old Boat as Rugosa said.

 

[capecodda]

Jeff,

This is no more than a theory/guess???

There is something loose (impeller blade, scale, something) that when you pressurize the system pushes the loose item against a passage and partially or fully blocks it. The faster you run, the more pressure to seal up the flow. I'd look everyplace the raw water system gets narrow, like the input side of the heat exchanger, exhaust elbow, etc. This guess is based on the scale comment, and a belief that the suction side of the pump is all clear.

When's the last time you took apart heat exchanger(s) and had them cleaned or replaced? Might be worth a shot if it's been a while. Everyplace that the salt water goes is suspect.

Again, a guess. Good luck!

 

[jsaronson]

Jeff,

Same engine here too. I'm assuming there are no leaks and the belt is not slipping. First guess - exhaust elbow. Second guess - foreign object in the system.

Hope you get it solved!
Joel

 

[Sailormon6]

Mine also pumped water intermittently. The water pump is mounted on a bracket plate that is used to adjust the tension of the belt. I finally found that, after years of use, and after the belt tension had been readjusted repeatedly, the bracket had a dimple bent into it, where the tensioning bolt was situated. Every time I adjusted the tension on the belt, it wouldn't stay in adjustment, and the belt would loosen very slightly. This caused the belt to slip, and it would sometimes pump water, and sometimes not. The cause was that dimple in the mounting bracket. It created a "slippery" spot, that allowed the tensioning bolt to slip. I replaced the mounting bracket, and have never had the problem since.

 

[sailak]

Jeff,

Second guess - foreign object in the system.

I also thought of this. Something that idle flow doesn't more, higher rate flow moves it blocking flow somewhere. Admittedly a bit of a long shot.

Odd issue you're having. Good luck.

 

[Jeff_H]

Thank you everyone for the suggestions and kind words. I like the sheered pin on the raw water pump drive. That is a good thought that had not occurred to me. I would hope that I would have noticed something like that when I replaced the impeller, but who knows, I missed simpler things in my old age.

I have checked the belt. That was a good suggestion and was an early suspect and a good one at that. I replaced both belts during the first round of adventures since I had them off and they hadn't been replaced in a dozen years. Sailormon, I had that very same dimple (or slight fold) around the time that I replaced the seals in the raw water pump, roughly 10 years ago. I put the plate in a large vice and flattened it, roughed the surface with course sandpaper, spray painted it, and that seemed to fix the slipping belt problem. Maybe I should check that again just in case.

The exhaust elbow is my next suspect after the slipping drive pin since its been a while since its been cleaned out. It wasn't bad last time, but I'll pull it off and pickle it in vinegar just as a precaution. I don't know why I don't suspect the heat exchanger, but for some reason that does not seem likely.

The fresh water pump does not seem like a likely candidate for a couple reasons beyond the fact that I can't imagine how the fresh water pump failing could affect the water raw water flow at the exhaust. But also, usually when a raw water pump fails there are signs of leakage at the seals, which I did not see, and besides I replaced that perhaps 10 years ago, which was quoted as wickedly expensive until a friend said,"You know I just sold my boat that had the 3GM in it and the buyer was so nasty that I kept my boxes of spare parts. I have a brand new fresh water pump if you want it and it fits." I did and it did.


I will let everyone know how this turns out.

Thank you all again,

Jeff

 

[Faster]

Does this occur hot or cold? wondering if it's something akin to your hot water taps at home 'slowing' as water heats up due to the washers expanding... a long shot/in the dark though.

Sounds like you've done what you can beyond the internal possibilities.. No impeller bits floating around and getting sucked into a blockage at higher flows?

 

[bristol29]

I had a similar problem with a 2GM20F. I finally installed a new mixing elbow that solved the problem. The elbow looked fine upon inspection so I plan to saw in down the middle. I need to know why.

 

[Faster]

I had a similar problem with a 2GM20F. I finally installed a new mixing elbow that solved the problem. The elbow looked fine upon inspection so I plan to saw in down the middle. I need to know why.

Yeah.. someone earlier mentioned increased exhaust backpressure at higher revs.. enough to block a marginal water injection path?? However an impeller pump is pretty close to a positive displacement pump that ought to push through that - it it's healthy, of course.

 

[RonRelyea]

another possibility (just a guess) would be debris just before your heat exchanger ... at lower pressure, the junk just might be getting agitated around, but at higher rpm/water pressure, the junk gets pushed up and blocks your heat exchanger and blocks flow.

 

[CapnRon47]

I check the water entry elbow to the mixing elbow every year around this time. It just collects debris, not sure why it should be partial to higher rpm other than as the flow increases the water forces the particles to block the inner tube.

this is the elbow I am talking about. it enters at the top of the mixing elbow and it is where the water from the heat exchanger leaves to mix with the exhaust.

It sits at the red arrow and has a anti siphon loop between it and the heat exchanger.

Hope you find your answer, let us know when you do.

Ron

 

[THEFRENCHA]

Had same problem with a 3ym
Good flow at low rpm but almost nothing after 2500
Flushed with phosphoric acid Lots of thread on forum
Back to normal and did not need to take anything apart
I would do that before trying anything else because even if mixing elbow is not the main problem in your case ....the flushing will still improve greatly the water circulation

 

[Markwesti]

Mr. H to answer your question yes we have over heated , we have a 2QM Yanmar , heat ex changer . One time it was the through hull , completely salted up . Last time I don't know , but I changed raw water impeller and mixing elbow , Checked all lines for blockage , and the heat ex changer for block blockage , and now we are good in tell the next time HAHA . I suspect your problem is the heat ex changer .

 

[jsaronson]

one other thought - slight air leak?

 

[Jeff_H]

I have to admit there are some very creative diagnosticians here. I am leaning towards the exhaust elbow and blocked passages idea. To add to the discussion, the problem happens whether hot or cold. The only obvious controlling element is RPM. it stops pumping around above 2200 RPM and starts again below it. In gear or not does not change it. Jon, I like your idea about an air leak. I actually thought that might be the case so I took the intake hose off the pump, hooked up a garden hose to it and turned it on. I then closed the seacock. The strainer initially leaked but I was able adjust the lid bolts and that stopped.

I really appreciate every one's thoughts on this problem. There are a lot of good suggestion here.

Jeff

 

[RobGallagher]

I'm going with the exhaust elbow. Second guess is heat exchanger needs a good enema.

If you are not up to removing cleaning/replacing the exhaust elbow just yet, try a good vinegar soak on the raw water side. Let is soak a solid week Then see what comes out the tail pipe. That is most probably not a fix, but it might help in the short term and give you a clue as to the heat exchangers cleanliness. If, by chance, it makes things worse (by loosening scale and clogging things up) then you know you need to start taking the raw water side apart.

Best of luck to you and your exhaust

 

[overbored]

check the exhaust hose/pipe. if there is a restriction in the exhaust it will build up pressure at higher RPM and the water pump pressure will not overcome the exhaust pressure. the raw water pump is not a positive displacement pump, it is a flexible vane pump and the pressure output is determined buy the speed and the stiffness of the vanes. the exhaust elbow can also be carboned up to the point where it causes back pressure into the raw water and stops the flow of water. do you have a waterlock that may be partly plugged. I am sure you checked the impeller key and pump cam are in place. wear in the water pump and pump faceplate, to thick of gasket material and wear on back wall of pump can reduce the pressure that the pump can develop allowing the exhaust pressure to overpower the water pump. Dirty raw water side of heat exchanger can cause a pressure drop across the HtEx, and low water pressure at the exhaust elbow allowing the exhaust to overpower the water pump pressure and stop the flow.