Blue smoke

[FlyingDutchman]

In my boat is the Volvo Penta MD22A (59 hp) engine.
The engine is 11 years old and has done 750 hours.
The boat is a 24' displacement mobo.

When the engine is started cold there is blue smoke coming from the exhaust.
It is not that bad, but it smokes.
As the engine warms up and is under load, the smoke disappears, but when it idles the smoke comes back again.

The oil consumption is near to nothing. I have done 70 hours this season and the level has maybe dropped one mm on the dipstick.

Normal cruising rpm is between 1400 and 1800 rpm, the maximum rpm is 3600.
Starting is excellent (less than a second) cold and warm.

What could be the cause of the blue smoke?
Normally, one would say oil consumption, but this engine does not consume any.
Also, the fuel economy is very good at about 1,4 liter diesel per hour.
The injectors have never been serviced or taken out, but bad injectors cause black smoke?

Any ideas highly appreciated!

 

[adrianm]

As far as I know blue-white smoke is ok at idle.

What you don't want is blue smoke under load as that signifies oil being burnt.

 

[twisterkai]

1800 rpm cruising with a max of 3600 rpm is rather low and so the engine probably isn't working hard enough. The smoke is probably being caused by glazing of the bores (so the rings can't seal as well as they should). Diesel engines should be run under load at approx 70% of max rpm which would equate to about 2500 rpm for you - assuming your prop is correctly matched to the boat. If the glazing isn't too bad then giving the engine a good long thrash may give some good results, otherwise its a strip down, pistons out and glaze bust & possibly new rings (but not necessarily).

 

[miket]

Possibly technically correct but totally OTT I would have thoght.
Most diesels smoke on start up and off load to some degree.
If you don't have any other problems, leave it alone.

 

[FlyingDutchman]

Thanks for your responses!
I don't have any other problems, it is just the smoke that puzzles me.
If the bores would be glazed causing the rings not to seal properly it would IMHO lead to either oil consumption (not the case) or poor compression leading to difficult starting (not the case either).
I agree that 1800 rpm (5,5 knots) cruising is rather low, but at 2500 rpm (7 knots) there is a big wash.
Occasionally I let it run at 2500-3000 rpm to give it a trash, but maybe I should do that more often?

 

[Andrew_Fanner]

Big numbers of Thames (and other) river based mobos have this issue. I leave well alone, check the oil level often, change yearly and get to "faster" bits when possible.

 

[[2068]]

Seconded. The occasional thrashing is good for you.

 

[blueglass]

[ QUOTE ]
Big numbers of Thames (and other) river based mobos have this issue. I leave well alone, check the oil level often, change yearly and get to "faster" bits when possible.

[/ QUOTE ]
That's right. had some concerns about this on the river last season - now all at sea and other than initial start up exhaust is clear as a bell. Engines (Perkins Sabre 225gTi) clearly prefer to be worked.
Your smoke is no big deal and completely par for the course. Just get your revs up a bit more when wash permits.

 

[FlyingDutchman]

Thanks again for your comments.
I think I will try to speed up a little whenever possible.
It's just that if you are used to 1800 rpm, 2500-3000 rpm sounds as if I am abusing the engine!

 

[Andrew_Fanner]

Usual "Thames tickover" is about 1200 which giver about 4.5 knots against the stream above Teddington. Get below Wandsworth and kick in the turbo to be greeted with a gout of smoke as all the clag comes out. If you arrange to punch a big flood you can do this to a limited extent in the 8 knot zone with a bit of savvy.

Some twin engine types give it a full neutral turn as the tankies put it. In a wide and deep bit naturally

 

[Bilgediver]

THis blue smoke could be poor combustion caused by dirty injectors. Is there also a trace of oil in the exhaust water.

Do not take the static oil level as a measure of zero oil consumtion. Your fuel pumps are mounted internally and there is always fuel leakage from these which goes into the sump so as you lose lube oil you are also gaining fuel dilution. This is one reason the oil should be changed as often as Volvo suggests as if too much dilution and worn rings it is not unknown for yacht diesels to run away even if the fuel lever is at stop, running on the lube fuel mixture leaking past the rings !!!!!!!!!

You will smell the diesel in the oil if you have dilution but bear in mind it is normal to and extent in some Volvo MD engines.

 

[FlyingDutchman]

Thanks Bilgediver,
The fuel pumps (lift pump and injection pump are mounted externally on the MD22, so I don't think this could lead to fuel in the oil. I will give it a sniff when I am on the boat over the weekend.
Interesting is your comment on the dirty injectors. I have always thought that these would cause black smoke! i will also have a look for traces of oil on the water surface.