Tech Mail: The colour of smoke

By: WebmasterAdmin

I have a 2001 Audi 1,9-litre TDI model. I have recently replaced my turbo, clutch and a few other small mechanical parts. The car runs perfectly, however I have noticed that when I pull off or change up gears I see a cloud of white smoke coming out the exhaust (which only began to appear after the replacements). In addition to this, when I start the car I occasionally notice that it revs by itself with a white cloud again visible and the car tends to idle above its normal idle speed. Should I be concerned about this?

BRENDAN NELSON
Via email

Answer: Unfortunately you did not mention the mileage of the vehicle. The colour of excessive diesel smoke can point out possible problems with the engine:

Black smoke – poor or  incomplete combustion
This is the most common colour as diesel particulates form during poor combustion. A diesel particulate filter (DPF) in the exhaust will hide bad combustion to some extent as it traps the soot. Reasons for poor diesel combustion may include:
•    Worn injectors;
•    Over-fuelling;
•    Low compression (example worn rings);
•    Dirty air filter;
•    Poor fuel quality;
•    Turbocharger not supplying enough air (boost).

Blue smoke – oil being burned
This is true of diesel and petrol engine combustion when oil is burned. Oil can reach the combustion chamber by several means:
•    Worn valve guides/seals;
•    Worn piston rings;
•    Cylinder glazing affecting the ring-and-cylinder-wall seal;
•    Incorrect grade of oil – too thin for application causing oil to migrate past the piston rings and valve-stem seals;
•    Turbo seals worn allowing oil into the intake manifold.

White smoke – raw diesel in the exhaust
When raw diesel reaches the hot exhaust completely intact, it results in white smoke from the exhaust (this method is sometimes used by aerobatic aircraft to produce the white smoke trails behind the plane during a display). The following will result in raw fuel reaching the exhaust of a diesel engine:
•    Faulty or damaged injectors;
•    Incorrect injection timing;
•    Extremely low compression
(example damaged valves).

Water reaching the combustion chamber (example: faulty head gasket) can also result in white smoke. In your case we doubt if a serious mechanical issue is to blame as the engine still performs well according to your email. We would guess that you might have a leaky injector as this would also result in the high idle speed you see.

If there was a blue tint to the smoke then it might have been turbo seals allowing oil through to the intake manifold. If the leak is excessive, it can actually lead to a runaway engine as a diesel engine will happily run on its own oil in a worst-case scenario. Keep monitoring your oil and water levels just to be sure.

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