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?