Yanmar 2gm20 - runs perfect but black smokes when manuavering

[frlrubett]

Yanmar 20GM20, engine starts perfectly, runs flawlessly. Regularly serviced and looked after - unknown engine hours.

Problem only happens when in a marina when engaging hard astern to stop the boat. Black smoke and a brown/black deposit in the water. Deposit does not have rainbow coloration to it and does not really 'bloom' out when released. When out of gear, no issues, when on passage no issues even when pushing into tide.

Have had a look at other issues on this forum, am thinking about the following trouble shooting...

Air Filter - removing and running to see if that solves it.

Exhaust Elbow - removing & checking for blockages/deposit inside.

Injectors - run a diesel adative cleaner.


Failing this, leakdown pressure test and remove injectors and get them cleaned?

Any other suggestions?

 

[Rappey]

Using full throttle in reverse but more gradual when going forwards? Could just be unburnt fuel and throwing out soot.. many diesel engines throw out black soot when instant full throttle.

 

[robmcg]

Fouled prop, damaged prop, pitch of prop wrong ?

 

[Bilgediver]

How much do you have tovease back on the speed control to stop the smoke? Does the engine smoke if you push the speed control to full ahead and does it clear quickly?

 

[frlrubett]

How much do you have tovease back on the speed control to stop the smoke? Does the engine smoke if you push the speed control to full ahead and does it clear quickly?

Yes clears quickly. If doing a u-turn in marina. It would smoke when atern/forward is first engaged but then when engine is backed off it stops immediately.

 

[frlrubett]

Using full throttle in reverse but more gradual when going forwards? Could just be unburnt fuel and throwing out soot.. many diesel engines throw out black soot when instant full throttle.

It does sound like this, some unhelpful pr1ck was watching me manauver and decided to comment about it, I had not really noticed it before.

 

[Tranona]

Nothing to worry about. When you go into reverse quickly the engine is temporarily overloaded and unable to burn all the fuel. The governor quickly adjusts the fuel delivery and the black smoke disappears.

 

[Dellquay13]

My 1gm10 does this too, but otherwise it’s fine. Sudden full astern in a marina is about the only time you hit high revs from tickover, and the only time you are in a position to notice it. As a test I tried full ahead from tickover while sailing and it does it then as well, but not as easy to spot.
I think it is just suddenly blowing out a rich sutty exhaust cloud that has gathered in the exhaust or water trap, as it soon clears.

One fellow berth holder once asked me if I was burning coal instead of diesel.

 

[frlrubett]

Thanks for this.

 

[DownWest]

Agree, overfueling to get the revs quickly.

 

[LittleSister]

Your 'throttle' (misnomer alert!) lever only sets the revs you want to achieve, it does not directly control the amount of fuel going into the engine, and doesn't affect the amount of air going into the engine at all. The governor provides enough fuel for the engine to reach the revs commanded. The amount of air going in will depend on the revs at which the engine is spinning.

When you are at tickover and command high revs the governor will provide lots of fuel, but at low revs the engine is not drawing in enough air to burn that much fuel - hence the smoke. As the engine speed increases it will draw in more air, a greater proportion of the fuel will be burnt and smoke will decrease. As the engine reaches the revs you have set the governor will limit the amount of fuel to only that required to maintain that speed (which will depend on the load, if any, you are placing in the engine).

If you progressively move the control lever, rather than switch it straight from tickover to a high revs setting, this should reduce the amount of smoke produced (especially if you have an engine with a heavy flywheel and hence slow to change speed), and will likely increase the engine revs just as quickly (because only fuel that has enough air to burn is doing the work), but this might not be practical in a marina manoeuvring situation, particularly if your engine control lever is at ankle level while you need to be keenly looking around, and perhaps attending to ropes etc.