Yanmar 1GM10 Diesel Fuel Out Exhaust and only 2400 RPM

centerline · Oct 16, 2016

Is there any sign of water in the oil?
the very first thing I would do is pull the injectors and have them tested.. its a reasonably easy job to remove them if you can get to them, and its not expensive to have them pressure tested....
a bad injector could cause your symptoms, and if it is an injector and it continues to run that way, the piston will be next to be damaged.

Benny17441 · Oct 16, 2016

Define "recently" as far as when you acquired the boat and when first the engine started to act up. Also indicate if the engine was running well for a length of time. What is the condition of the boat's bottom and prop? Sometimes a combination of causes brings about confusing symptoms. Try and clear individual symptoms like blackish exhaust water could point to a disintegrating impeller which does result in overheating. If you have not replaced the impeller probably will not know its condition. Suggest a replacement. A drop in RPM is usually the result of clogged fuel filters. Same as the impeller suggest replacing. Make sure the air intake tube is unobstructed. I doubt the air filter is still there but make sure there is nothing blocking the intake. Check the condition of the fuel. Old fuel or fuel with a high water content will not burn properly and can cause a drop in RPM. Make sure the boat's bottom or the prop are not fouled, excess drag can cause "overfueling" which results in unburned fuel and usually black smoke (which you do not report). Check for mechanical causes like a failing cutlass bearing which does not allow the propulsion shaft to turn freely which can also lead to overheating. ( does the engine behave drastically different when ran in neutral and when under load?) (check and see if you can turn the shaft by hand and feel for any drag along a single revolution). The injector could be at fault but it being a single cylinder engine it would run real rough and shake violently. I hope this might help.

Davidasailor26 · Oct 17, 2016

eusjim1 said:
first cranking without starting to distribute oil

Might be unrelated to your issues, but generally cranking marine diesels without starting them is not recommended. Cooling water can back up the exhaust and into the cylinders, hydrolocking the engine.

SailormanDan · Oct 17, 2016

I have a 1GM that had issues, getting both the Seloc and Haynes (Chilton?) manuals helped identify causes. You might consider that unburned fuel in your exhaust water and overheating are 2 different issues.
My overheating was caused by "stuff" getting clogged in the water jacket around the cylinder. If you pull your thermostat to ream things out with a bottle brush, make sure you poke straight back in, not just in the direction of the water flow through the hoses. I didn't catch that the first time around. I also replaced the zinc on the engine at the same time, some of the crud cojuld have come from there.
Low RPMs couild also come from clogged filters, one at the top of the cylinder block at the back and one between the engine and the fuel tank. Both use different filters. Possibly also need to bleed the fuel lines, especially after all this.
They are simple bullet-proof engines once you understand them.

Benny17441 · Oct 17, 2016

Garbage in garbage out, what color is the water in the lake? Start the engine and remove the oil fill cap. Then place the palm of your hand about an inch away from the opening. The air that you feel coming out is compression being blown by the piston rings into the crankcase. If you feel a steady and slight flow the engine is OK but if you feel a strong thump with each revolution you likely have significant engine wear. As compression blows into the crankcase as the air/fuel mixture is compressed in the same manner during the intake cycle engine oil can be absorbed into the combustion chamber. Motor oil will not burn as thoroughly as diesel fuel which could account for the color and oil residue at the exhaust mixing elbow. A remedy for strong blow-by is the use of a thicker oil grade or the use of thickening additives. They will result in increased compression and reduced blow-by and can extend the useful life of a tired engine. Regarding the overheating check that the thermostat is opening and closing at near the designated temperatures. Eliminate the possibility of a stuck thermostat that could by-pass the cooling water from the internal engine cooling passages. Sounds to me like you may have three separate but seemingly unrelated issues. Change fuel filters and make sure you have fresh fuel.

NYSail · Oct 19, 2016

And ou are sure the mixing elbow is clean? I replaced mine last year...... the water side was perfectly clear but The exhaust did e was significantly clogged

Capt jgw · Oct 19, 2016

Clogged exhaust elbow is a very common cause of incomplete combustion, and less common clogged air intake. Running at light or no load or at idle can hasten the clogging.

Sailm8 · Oct 19, 2016

I had a 1gm in my old boat and had overheating problems even after replacing the elbow, decalcifying etc. Turned out to be a blown head gasket. Just something to check.

NYSail · Oct 24, 2016

did you recheck elbow first??