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Old 02-03-2006, 09:26 PM   #1 (permalink)
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smoke color

does anyone know why smoke from a cold diesel engine is white/blue and smoke from working the engine is black. both are caused by unburnt fuel right? does the engine temp make the difference?
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Old 02-03-2006, 10:31 PM   #2 (permalink)
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Re: smoke color

Black smoke is when the diesel is running out of excess engine air. Diesel run lean and power is controlled by the amount of fuel injected.

Every piston stroke, whether at full power or at idle, receives the same amount of air.

When the diesel is working hard, this excess amount of air is used up, and unburned black smoke is the result. If the engine is turbocharged, this amount of black smoke can be reduced or even eliminated as it is providing pressurized air to the engine at partial and open throttle settings.

White smoke is sure fire indicator of a bad injector, or that the engine is still very cold. A tuned, well maintained diesel engine should not emit white smoke at idle.

If there is a coolant leak, white smoke can be a result. Of course some diesel engines are smokier than others at idle. My 6.9 is in great running order but it still emits a faint trail of smoke at idle.

Good thing I run biodiesel now.
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Old 02-04-2006, 12:32 PM   #3 (permalink)
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Re: smoke color

White smoke is atomized but unburned fuel. It's the result of failure to ignite.

Black smoke is fuel which has ignited but only partially burned.
When there's too little air for complete combustion, the hydrogen fraction of the fuel burns first, leaving excess carbon. The excess carbon is the black smoke.
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Old 02-04-2006, 06:26 PM   #4 (permalink)
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Re: smoke color

[ QUOTE ]
White smoke is atomized but unburned fuel. It's the result of failure to ignite.

Black smoke is fuel which has ignited but only partially burned.
When there's too little air for complete combustion, the hydrogen fraction of the fuel burns first, leaving excess carbon. The excess carbon is the black smoke.


[/ QUOTE ]

Blue smoke, of course is associated with motor oil. it is common on older engines for oil to run down the valve guides when the engine is parked or even when it idles for a long time. this explains the old trucks that chase off the mosquitos when they are cold started.

If you are dealing with a diesel engine that won't start, the white smoke or lack thereof is an important clue. If the engine spins up and there is no sign of white smoke and it doesn't start you should get focused very quickly on various reasons why fuel isn't getting into the cylinder.
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Old 02-04-2006, 07:35 PM   #5 (permalink)
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Re: smoke color

[ QUOTE ]
White smoke is atomized but unburned fuel. It's the result of failure to ignite.

Black smoke is fuel which has ignited but only partially burned.
When there's too little air for complete combustion, the hydrogen fraction of the fuel burns first, leaving excess carbon. The excess carbon is the black smoke.


[/ QUOTE ]

^Exactly.

And as for blue smoke, that usually means that the engine is in bad mechanical condition. I once saw a '90-'93 cummins in Modesto, California, and when it started up, from the other side of the truck I saw a big ol' cloud of blue smoke come out at startup. That engine was probably in horrible condition. Also, like I said once before on this forum, I saw an F-350 (It was between 1999-2004) diesel that smoked blue at idle. I was skeptical about why a newer truck could do that. My friend (another truck person) was nearby, so I asked him. He said the turbo probably wasn't getting enough lubrication to not work hard. Also, the engine sounded very dry and not good. But either way, blue smoke isn't good.
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Old 02-04-2006, 08:32 PM   #6 (permalink)
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Re: smoke color

drcamble is the only one who got it right.
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Old 02-05-2006, 12:22 PM   #7 (permalink)
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Re: smoke color

Thanks for the replies.
drcambell what you said makes sense and was what I was looking for.

Al
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Old 02-26-2011, 10:42 PM   #8 (permalink)
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Only Under Load

I have an '86 6.9L n/a & my diesel experience is limited. It doesn't smoke on startup or idle, not even thoughout the rpm range, but it does smoke under load such as hard acceleration. This smoke is not coal-black as I have seen in pics & videos and it isn't pure white or like my old gasser when it was spitting out fuel. It is more of a grey color. Is this a sign I should have my injectors checked or do I really need to worry.
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