Smoking Yanmar under load

MLampner · Mar 18, 2012

May Help

I'm a relative newbie so an old hand may have better answers, but classically diesels put out black smoke from incomplete combustion, my guess is timing could be off on the valves, injector issues and if the engine has an air cleaner look there as well. A dirty filter can restrict airflow. Also a possible issue can be problems with the return line in the fuel circuit. If the line isn't free running the injector can get overpowered and introduce more fuel than can be burned. This is generally a bigger issue under load as the pump delivers constant pressure but as the prop grabs water and loads up the engine turns slower and can't use all the fuel that your throttle setting is delivering. This last one is rare but can happen.

Last thought compression issues can also cause it if its only happening under load, but this as often as not is a blue rather than black smoke as lube oil get sucked in past failing rings.

Dan Hill · Mar 18, 2012

My first check would be the air intake system also. Restricted air flow due to a clogged air cleaner or a nest from a critter will cause exactly what you describe. Black smoke from poor combustion, oil sheen from unburnt fuel, low power, but runs fine with no load. Very typical. Good Luck!

whatfiero · Mar 18, 2012

yanmar smoking under load

First check your engine valve cover to make sure no one inadvertently cut on one of your decompression levers on .

Next start your engine and remove air filter in a non-dusty environment then check engage tranny in and out of gear forward or reverse which ever pulls ya away from dock add extra lines if necessary see if black smoke still there

Finally run engine 2300 to 2500 in neutral for 30 min till good and hot then engage tranny at idle pour on the throttle till black smoke disappears my take 10 min

If not exaust elbow or muffler clogged from starting engine but not running enough to clear out carbon build up

clean or replace elbow and blow a wet vac out the muffler to check it

robertsapp · Mar 19, 2012

Check your prop and make sure it hasn't turned into a giant oyster bed or barnacle farm during the off season. It doesn't take too many critters stuck to it to ruin its efficiency sufficiently to cause the engine to appear to have no power while emitting black smoke with the transmission in gear. Then pull the exhause elbow and take a look, and check to make sure the intake isn't blocked by something.

You don't say what the engine does if you run up the RPMs with the shaft disengaged. If you still get black smoke, then it's probably an intake restriction or plugged exhaust. If everything is fine, then I'd be taking a look at the prop.

Also, are you absolutely sure it's oil you see on the water? Thick black patches that float on the surface can often be soot rather than oil. You'd expect some soot if the engine is emitting black smoke.

robertsapp · Mar 19, 2012

Ah, sorry, I reread your post, and you did say that there's no smoke when the engine is revved up with the shaft disengaged. I'd be taking a look at that prop. I'll bet there's a bucket load of barnacles on it.

robertsapp · Mar 20, 2012

Well let us know, because we all learn in the process.

Ken13559 · Mar 21, 2012

Dubai, two things I would check 1st. 1)barnacle growth around prop 2)mixing elbow fouling. 90% of engine problem are these items. Been there, done that.

JST123 · Mar 22, 2012

The fact it only happens in gear and and really seems to load up, it sounds like the cutlass bearing is bad.

MLampner · Mar 23, 2012

That is an interesting point. Easily checked if you are somewhere you can either get it short haloed or can get into the water. With the engine off, key put somewhere safe and transmission in neutral see what resistance you get from turning the prop by hand. Double check the engine is off and it's in neutral and if you meet major resistance stop trying to turn it. I doubt you can get leverage to start the engine spinning the prop but some older transmissions don't disengage completely even in neutral.

If it moves fairly freely it's not the bearing if it's clearly out of gear and it seems to take effort or feels irregular cutlas bearing could be at fault. It will also allow checking the prop for barnacles so nothing to lose.

canuck · Mar 23, 2012

As an aside, the worst thing you can do is run your diesel as you are doing. Diesels need to be run at prescribed operating temperatures, under load and at a RPM sufficient to burn the fuel mixture.

If you run it at the dock, at least place it in gear and ensure your mooring lines are adequately tied and run it at least 2000RPm.
Keep in mind that you cannot achieve max engine RPM while tied up even with the tranny in gear.

I think you have carboned up the engine as well and I'd reccommend an oil change.

robertsapp · Mar 26, 2012

Never heard of a seized Cutless bearing but I suppose stranger things have happened. If I wanted to check for such a thing, rather than hauling the boat or getting wet, I'd just unbolt the coupling at the transmission and spin the shaft by hand to see if there's any resistance. That would take all of about 45 seconds on our boat. It might be more difficult on others.

RonRelyea · Mar 26, 2012

I had this same problem and determined it was a fouled prop .... seaweed - to use the generic term --- was able to forward/reverse transmission and shake it off .... I hope your problem is solved this easily

kloudie1 · Mar 26, 2012

Fouled prop is the most likely cause.. A friend's engine was doing that .. When we dove under, the prop looked like a bowling ball made of clamshells.. after some determined chipping everything was fine.. Could be elbow too as has been noted.. both are easy to check.. Good luck
Cutless bearings get more loose as they get bad..

whatfiero · Apr 2, 2012

yanmar smoking under load

Oil will all ways be black as this diesel engine uses the oil to cool the top of the piston I changed my oil 3 times in a week before some one explained that to me.

Tally Ho · Apr 2, 2012

whatfiero said:
Oil will all ways be black as this diesel engine uses the oil to cool the top of the piston I changed my oil 3 times in a week before some one explained that to me.

For what it is worth, the engine mechanic at our local boatyard told me that the oil in Yanmar diesels should be "less black" than others because they run at higher compression....but mine is pretty black, even after an oil change...

Greg