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Yanmar 3GM 30 Cooling Water Mystery

24K views 43 replies 23 participants last post by  Capt Len 
#1 ·
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
 
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#34 ·
Len,

That is just what I was thinking after I took the elbow off and saw that the weld was completely missing. The water inlet to the elbow is only a couple inches from the flange and I was thinking that I was lucky that I never had hydro lock.

The only thing that keeps the water from going back into the exhaust manifold is that the gasses escaping from the engine turning over would blow the water back toward the exhaust hose which then slopes downward towards the water lift muffler.

Still and all, it was a relief to find a potential source of the problem while I was counting my lucky stars that it never overheated or got hydro locked.

Frankly, all things considered, it all went about as well as it could have. I actually snapped the head off of one of the retaining bolts (the other three came out like they had been put in just yesterday). I was really afraid that it was boring bar and tap tool time. But with the flange off there was a pretty long stub of the bolt exposed. I was able to put liquid wrench on both sides of the bolt and then it turned out with a vise grip like it was meant to work that way.

The other thing that surprised me was that before yesterday I was certain that I had taken this mixing elbow off and replaced the exhaust piping. But when I actually saw the elbow, I realized that I had not taken it off and that the hose that I replaced ran from the muffler to the thru-hull and not from the elbow to the muffler. The elbow and hose to the muffler all looked to be original to the 1996 engine.

Jeff
 
#33 ·
Ouch! So at higher wxhaust pressure the (now free floating) internal part of the elbow would be forced into a position to block water entry from the mixing port.

Great case!

While solved, another thing to consider with similar scenarios is a WELL lubed seacock vibrating closed at higher rpm. Had that start to happen on my sea trial last week. So strange since the seacocks are usually so stiff!
 
#36 ·
The engine is back together with a new mixing elbow and a new length of exhaust hose. I ran the engine in the slip and all seems pretty good no matter what rpm. Of course the real test will come underway.

I will probably pickle the system just in case.

Thanks folks,
Jeff
 
#38 ·
Why not disconnect the various hose and simply see where the problem is? Yes, a little bit of water will squirt around, which you might control with another hose to the bilge. There will not be an overheat problem in the few seconds of running to observe the flow.

On the inlet side of the pump suck water from a bucket thru a short piece of hose.

Disconnect the hose from the heat exchanger outlet. Run the engine. See what happens, etc.
 
#41 ·
Why not disconnect the various hose and simply see where the problem is? Yes, a little bit of water will squirt around, which you might control with another hose to the bilge. There will not be an overheat problem in the few seconds of running to observe the flow.

On the inlet side of the pump suck water from a bucket thru a short piece of hose.

Disconnect the hose from the heat exchanger outlet. Run the engine. See what happens, etc.
Good suggestions. Most of that was already done before I posted the original comments. I actually have a 'Y' valve immediately behind the inlet seacock which lets me winterize my engine easily. By removing the hose from the water pump I could flush water through the strainer and out of the hull. By using the Yvalve I could suck water from a bucket and so was able to check water flow both ways and through the pump. I also disconnected the hose at the discharge end of the heat exchanger and let the water run into a bucket

By the way, its not just water that leaks, but very loud and noxious diesel exhaust when you do that, so I did not do that very long or at a high RPM.

All of that checked out fine, which is why I was mystified that with all of that seeming to function normally the water flow would stop when the engine was revved up. I had not expected to find the core of the mixing valve loose within the valve where it could shift and block the flow.

A couple other insights from this project that I would like to pass along. Yanmar claims that the mixing elbow gaskets are not reusable. Looking at how they are made, I think that they are correct so popping the mixing elbow off means having a new gasket available.

Yanmar publishes that the ID for the exhaust hose is 5 cm in diameter. That is slightly less than 2" diameter and more than 1 7/8" hose. I chose to go the 2" diameter route feeling to old to wrestle the "too small size" onto the muffler in the confined space. That led to some nervous moments tightening the hose clamps about whether it would every completely tighten up. I did have the sense to go with Tee bar type hose clamps as a 'just in case' that I was glad that I had done even if they are pricey.

Jeff
 
#42 ·
I had a 'slightly oversized' hose for my (newly built) mixing elbow and had the same concerns, Jeff. I used a hitemp gasket sealer 'just in case'..
 
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