Yanmar low RPM under load

[jmurray109]

I have a Yanmar 3QM30H and recently while in gear it will only go up to 1900 RPM. It also kicks out extra black unburnt fuel from the exhaust. While in neutral the motor RPM goes up to it normal level.
I have pulled off the mixing elbow (2 years old) and it still is in good shape. I removed the injectors and had them professional cleaned. I visibly inspected the exhaust manifold, while still attached to the motor. I could not see anything that stood out. Also while under load, I manually disengaged the compression from each cylinder. There was no change in motor sound on the middle cylinder. The other 2 you could hear the change in the motor.
So that brings me to the next question. What is it. The only thing else I thing I could do is to remove the exhaust manifold and inspected it better. Other than than, is the middle cylinder done. Any assistance would be appreciated.

[Wotname]

Originally Posted by jmurray109

I have a Yanmar 3QM30H and recently while in gear it will only go up to 1900 RPM. It also kicks out extra black unburnt fuel from the exhaust. While in neutral the motor RPM goes up to it normal level.
I have pulled off the mixing elbow (2 years old) and it still is in good shape. I removed the injectors and had them professional cleaned. I visibly inspected the exhaust manifold, while still attached to the motor. I could not see anything that stood out. Also while under load, I manually disengaged the compression from each cylinder. There was no change in motor sound on the middle cylinder. The other 2 you could hear the change in the motor.
So that brings me to the next question. What is it. The only thing else I thing I could do is to remove the exhaust manifold and inspected it better. Other than than, is the middle cylinder done. Any assistance would be appreciated.

Check valve lash (tappets) on middle cylinder - actually check all of them.
Does sound like middle cylinder is not good. Valve burnt?
Compression check?

Have you checked that the prop and bottom is clean?

[a64pilot]

OK, after you check valve adjustment, just for kicks try loosening each injector line at idle, once they leak, that cylinder should cut, just in case somehow your decompression lever is messed up.
But it certainly sounds as if you have a dead cylinder.

One of the three cylinder Yanmars is bad about breaking pistons, I don't know which one

[Wotname]

Originally Posted by a64pilot

OK, after you check valve adjustment, just for kicks try loosening each injector line at idle, once they leak, that cylinder should cut, just in case somehow your decompression lever is messed up.
But it certainly sounds as if you have a dead cylinder.

One of the three cylinder Yanmars is bad about breaking pistons, I don't know which one

Good tip about the decompression lever!

It was early 3GM30's and IIRC, it was number 3 (furtherest from flywheel). Supposedly had a bad batch of pistons but why it was only number 3 eludes me. Might be wrong of course .

[jmurray109]

Bottom clean and nothing on the prop. I am not a motor head and I am volunteering my time for a youth sailing group. Trying to fix something and I am way over my head. I am hoping it is not the cylinder, and hoping to find a clogged exhaust. Thanks

[Wotname]

OK, well first try the easiest, check the tappet clearance (valve lash) on the middle cylinder. It should be about 8 thou (0.008" or 0.2mm). If that is OK, then try A64's idea (post#3). After that, it's a compression test or call a mechanic.

[RaymondR]

Heavy black smoke and lack of engine revs is a classic sign of engine overload.

[a64pilot]

Originally Posted by RaymondR

Heavy black smoke and lack of engine revs is a classic sign of engine overload.

It is, because it’s excess burned fuel, reason is an overloaded engine doesn’t pull in as much air but the pump is injecting the right amount of fuel, remember Diesels unlike spark ignition engines don’t have a carburetor that meters fuel and air together.
So a dead cylinder will be excess fuel, cause it’s not burning it and can show similar symptoms.

Hopefully It’s a too tight valve, and hopefully it hasn’t been burnt.

[CaptainJohn49]

Had a bad air cleaner and it blew black smoke under load. I had inspected it and it looked slightly dirty but not clogged. Boy was I wrong. Replaced it and the engine ran fine. Usually the problem is simple. Just an idea.

CaptainJohn49

[Taipe]

That's a good approach, starting with the simplest and least complicated first. Factory trained Yanmar mechanics commonly say that if they can't find external causes for low rpm under load (i.e. dirty fuel, dirty fuel filter, dirty air filter, dirty injectors, plugged exhaust) they will then usually find that the head is carboned up because the owner is afraid of over-revving the engine. This particularly applies to those of us who mostly use the motor just to come in and out of a marina or an anchorage.

[Mike Banks]

OK--Stating the obvious--check your propeller for damage or obstruction such as rope etc. If the engine is straining and throwing out black smoke--you run a risk of damaging both engine and gearbox.

[Jerry Woodward]

I question how you know the mixing elbow is in good shape? You can't really see up into the elbow and it could very well have a constriction that you can't see. I had similar issues with my 2GM20F engines although not to that extreme. The elbows looked pristine from the outside and I couldn't see any obvious problem, but replaced them anyway. Solved the problem. There is a reason why they call it the achilles elbow.

[jmurray109]

Originally Posted by Jerry Woodward

I question how you know the mixing elbow is in good shape? You can't really see up into the elbow and it could very well have a constriction that you can't see. I had similar issues with my 2GM20F engines although not to that extreme. The elbows looked pristine from the outside and I couldn't see any obvious problem, but replaced them anyway. Solved the problem. There is a reason why they call it the achilles elbow.

I removed the elbow, not sure if you would even call it an elbow because it is almost straight. This engine has 2 types of mixing elbows one that is almost 180 and one that has a slight bend. I could see everything and it was clean. I had replaced this elbow approximately 2 years ago, and it was still in good shape.

[jmurray109]

Originally Posted by CaptainJohn49

Had a bad air cleaner and it blew black smoke under load. I had inspected it and it looked slightly dirty but not clogged. Boy was I wrong. Replaced it and the engine ran fine. Usually the problem is simple. Just an idea.

CaptainJohn49

Took the air cleaner off and ran engine without. Still same issue.

[ATC]

Wow, the second time today, your exhaust mixing elbow is 'caking up' with carbon. This causes restriction in the exhaust and you have unburnt fuel, a reduction in RPM's and horsepower.