Service information: Smoke formation

 There have been isolated complaints of smoke formation in vehicles equipped with common rail diesel engines.

There are four main situations where this problem is seen. The following is designed to provide you with supporting information.

It should first be ensured that there is no malfunction in the common rail injection system, in the air ducting (inlet port shutoff, charge air ducting) or in the components of the exhaust gas recirculation system (EGR valve, Hot film MAF sensor).

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 Black smoke formation during full-load acceleration following extended partial-load driving:

During partial-load driving, low flow rates result in a build-up of soot particles in the exhaust system. These particles are deposited on the surfaces of the catalytic converter and on the walls of the exhaust system. During full-load acceleration, the increased exhaust flow velocities causes these soot particles to be blown out of the exhaust system. This effect is seen to a higher degree in direct injection diesel engines than in predecessor engines with indirect injection because the particles found in direct injection engines contain more moisture.

When compared with engine 602.982 and with rival make engines, the CDI diesel vehicles currently on the market display no especially unusual characteristics.

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 Black smoke burst during full-load acceleration:

Vehicles with CDI diesel engines are especially commended for their responsiveness. This increased responsiveness over earlier type engines was achieved in part by substantially increasing the slope of the accelerator pedal curve.

To reduce pollutant emissions under partial-load conditions, exhaust gases are returned to the system for afterburning. During acceleration, the amount of exhaust gas returned drops very rapidly.

The exhaust gases still present in the intake manifold combined with the rapid rise in the quantity of fuel injected cause the air/fuel mixture in the combustion chamber to become overrich for a brief period. This applies only to the start of the acceleration phase.

This configuration represents a compromise between smoke formation and responsiveness. Any change to this strategy would lead to a significant drop in responsiveness.

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 Blue smoke formation following engine starts under low outside temperature conditions:

This effect is seen only in uprated CDI engines with variable turbochargers (CDI2). The primary cause for this problem is the reduction of the compression ratio, which is done for emissions reasons and to decrease the load on the engine.

The problem occurs more often when the engine compression ratio is at the lower tolerance limit.

Changes introduced to a number of components in 01/01 have resulted in decreased compression ratio variation and should lead to a significant reduction in complaints.

Due to the complexity of the modifications made, however, no repair solutions are available for vehicles already on the market.

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 Blue smoke formation under full-load conditions following extended partial-load driving:

During extended travel under partial-load conditions, uncombusted hydrocarbons in the exhaust condense onto the surfaces of the catalytic converters. The process is exacerbated by the relatively low exhaust-gas temperatures in CDI diesel engines. As engine load increases, exhaust-gas temperatures rise resulting in the evaporation of these components, a certain amount of which are then blown out of the exhaust system. The cool ambient air behind the vehicle causes these components to condense once again in the form of visible blue smoke. This behavior is inherent to the system. Studies on ways to modify the configuration of the catalytic converter resulted in no improvement. Changing this behavior by increasing the exhaust-gas temperature would result in a significant rise in fuel consumption.

In vehicles running on diesel fuels with a significantly higher sulfur content, increased blue smoke formation may also be seen when driving at constant speeds. A modified exhaust system is being prepared for engines 611 and 612 for those markets where high sulfur fuels are sold and where permitted by emissions legislation. We will notify you as soon as the required parts are available.