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hoaga 11-02-2009 10:10 AM

Gray Smoke -- under load -- Volvo MD2040
I am noticing a lot of gray smoke in my exhaust when I push it to higher RPMs. What could be the cause? (I don't think it is white smoke, nor black smoke either... seems pretty solidly gray).

I have just changed to larger props (for completely independent reasons)--and the problem got worse. Note, the problem existed before the new props--just not as much. I am replacing the new props with new blades to reduce the pitch a bit, so I can get into my normal WOT range.

The engines have about 2800 hours on them. And have been continually maintained, with oil and filter changes, and fuel filter changes. Etc....


Faster 11-02-2009 10:29 AM

We had brownish-grey smoke when an injector partially failed (low breakover pressure) - but you could definitely hear the problem too - a nearly mechanical tapping sound at the same time from the un-atomized fuel.

sigmasailor 11-02-2009 10:33 AM

You're probably asking too much of your engine and it is going through its knees. Because of the larger props revs are probably too low in relation to the torgue curve (your engines characteristics) and you engines governor is trying to correct that by pushing more fuel in the engine than it has oxygen to burn. Hence the grey/black smoke; lots of unburned fuel and other nasty stuff.
If the problem was there already I would suspect the injectors; all this assuming it is a diesel of course. Yep, Volvo's are diesel....

klem 11-02-2009 11:07 AM

Gray/black smoke is a sign of overfueling. Generally, when it is gray, that means that the fuel is not burning efficiently for whatever reason. A very common cause of this is worn out injectors that are popping off at too low of a pressure and have greatly enlargened holes. If you can find someone local with a pop tester, it should not be expensive to have them test it.

Taking some pitch out of your propeller is definitely a good idea.

Capnblu 11-02-2009 11:13 AM

Blue, or black smoke, oil related. White smoke water / coolant related. First guess head gasket, second guess a crack in block. Course of action? Take an oil sample in to have analyzed, then a wet and dry compression test of all cylinders. Next action will depend on the outcome of the analysis and the wet/dry test.

hoaga 11-06-2009 09:29 AM

Thanks for the info All.
Will be doing some testing this weekend to see what can be figured out.

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