Yanmar 3GMF can't achieve running RPM

[datasailor2]

The 3GMF has been a great motor over many seasons in my 1983 Hunter 34. She has had lots of love and tender touching during my tenure (third owner) and currently has under 2k hours (according to the gauge). She starts and runs ok at lower RPM (750 idle > 19K). No issues when I leave the dock and motor away from the marina. No sign of any noticeable exhaust smoke. However, when I push RPM up higher (25k > 27k) to reach cruising speed, within 5 or 10 minutes the engine begins to have a subtle change in noise and then I see blueish exhaust smoke. I back off the throttle and engine appears to run normally again (blueish smoke disappears). I immediately suspect cooling issue and checked the raw water fresh water heat exchange by hand. The engine did not seem to be too hot and the temp sensor did not alarm (tested that and it does work). I thought at first that there was possibly a cooling system obstruction issue that only causes problem when the engine is at working load, since I did seem to feel that the cooling water discharge (splash) was not as robust as I felt it has been (one of those gut feelings). Now I have gone though the cooling system stage by stage and find no obvious reduction in cooling water (raw or fresh) flow. I read about the cause of blue smoke but I am not convinced that engine is due for some serious Ring and Piston work. I had a boat neighbor that mentioned similar issue where his motor ran find at lower RPM but would start to run rough at higher RPM and he could not achieve normal running RPM. He eventually discovered his issue was this muffler. Apparently the internal baffles had corroded and collapsed cause exhaust obstruction and increase manifold pressure. I have decided to take this opportunity to replace the exhaust header and elbow riser. Just wondering what the community thinks of this issue and most likely causes.

 

[shemandr]

You should not take my opinion as expert or anything close. But to me blue smoke means burning oil. Are you tracking your oil level? I'll await responses from others who know more about engines for my own education.

 

[datasailor2]

Thanks shemandr for your reply regarding Oil. Yes you are correct normally blue exhaust = oil in combustion. That said, I change oil yearly and check level prior to every outing and have never seen any lost of oil. As a matter of fact I have never had to add oil between yearly exchange. Also the blue smoke only appears within several minutes of running at normal running RPM (not excessive) and dissipates when RPM is lowered. During low RPM operation or at starting there is no sign of any smoke from exhaust. If cylinders were glazed, rings worn or valve seals leaking I would think that blue exhaust would be evident at all times during operation and I would see a noticeable drop in Oil level over time. So I guess the question I am attempting to figure out is whether or not a partially blocked exhaust flow, which would cause increase in exhaust manifold pressure when engine RPM increases, could cause this condition of oil getting into combustion in some way.

 

[Michael Davis]

Blocked exhaust or dirty fuel filters would be my first guesses

 

[NYSail]

When was the last time you changed your mixing elbow......

Good Luck

 

[kappykaplan]

When was the last time you changed your mixing elbow......

 

[Benny17441]

Concentrate on solving why the engine is not reaching RPM and forget about the bluish smoke; that should fix itself. The great majority of engine problems are fuel related. Check for fuel starvation at high RPM, blockage on the fuel tank's pick up tube, clogged fuel filters, failing fuel lift pump or poor fuel condition,. To eliminate some basics, adjust the engine valves to specs and check the intake air tube for any obstructions. Is there a loss of power? What happens if you do not reduce RPM, will it smoke more or change color to gray or black? Not saying it could not be the exhaust mixing elbow or any other obstruction in the exhaust system but check the most probable and least expensive things first.

 

[Jerry Clark H356 SV Persi]

In addition to the mixing elbow, check your prop for growth and/ or fowling. If you have an adjustable pitch prop that is worn it can cause similar power issues if worn gears cause it to become overpitched. If it’s fixed pitch and Fowler with barnacles or growth, similar results.

 

[Noodat Lady]

You haven't reinsulated or otherwise modified the engine cover? Unlikely in a sailboat I know, but air starvation would also give those symptoms.

 

[Maine Sail]

Sounds over propped. This is one of the most common sizing issues we see on boats.. The exhaust elbow could also be at play, but if you can't reach max rated RPM + 100 (see below), and never could, this is an indication the engine is over-propped.

Running these engines over propped can also lead to plugging the exhaust elbow sooner and actually voids the warranty on new engines.. Being over propped is kind of like trying to drive a Toyota Tercel, up Pikes Peak, in 4th gear.....

 

[Allan12210]

Check the fuel pick up tube for a screen. The tube needs to be pulled from the tank. Look inside. If still there, remove it. That caused us grief once. It will load up and clear. Your filters are plenty of protection

 

[Jeff]

I had issues a year ago with my 3GM30F. Started and ran fine to 2000 RPM. It
would actually lose RPM when the throttle was pushed farther. Turned out to
be a minor leak with the bleeding screw in the primary filter. The threads had
stripped and would no longer seal. Bought a new fuel filter assembly and my
problem was resolved. Good luck with your problem.

 

[chuckwayne]

I vote for the exhaust elbow - just changed mine after similar issues - totally coked up, before the water injection - si I was getting plenty of flow. BTW, look at the aftermarket stainless steel assemblies - less expensive and longer lasting that the Yanmar parts

 

[datasailor2]

Thank you all for your input and good advice. I am confident that fuel system is OK. I have found that exhaust/cooling elbow had sign of expansion corrosion that was almost breaking through the external wall of the elbow with potential leaking of exhaust into engine space and cabin (see attached jpg). Flushing the cooling port with water did not show any sign of obstruction or communication between the exhaust and water ports and totally irrelevant at this point since I obviously have to replaced the manifold extension and cooling elbow and, while at it, I pulled muffler out for inspection. The muffler had no sign of corrosion or internal collapse so I returned it to service. I installed new header extension and cooling elbow and started her up. I was able to see some improvement in exhaust cooling water splash, so there must have been some level of obstruction in the old unit. There was no sign of blue or dark exhaust when starting and warming up. I can easily rev the RPM up to manufacturer's recommended 36+K while in neutral without any significant change in exhaust color. However, when I engage forward gear the RPM can not exceed 27k. This leads me to the next great advice mentioned above regarding prop pitch. I did have 3 blade installed several years ago and really found great improvement in back-down and tight space maneuvering. Since the prop and shaft were installed by professional who at the time assured me the pitch was appropriate for my engine I didn't give it a second thought, since I felt I appeared to be achieving hull speed at around 25K and since some literature by marine diesel owners seem to say normal cruising RPM was typically in the range of 25-27k I never seemed to have purpose to push it harder. I'm planning to haul the boat out to get the bottom repainted, and will see if prop is properly matched at that time. Thanks again everyone for your insight.