I don't mean this to sound like a rhetorical question. I own Volkswagen Passat TDI (which is part of VW scandal) and currently awaiting approved modification for my car. What I'm trying to do is to understand why it is claimed that VW produces a higher level of NOx than other cars, when most diesel pick-up trucks (unsure on actual percentage) do not have a system for utilizing DEF to clean exhaust gasses to reduce NOx emissions. My car is equipped secondary tank for DEF fluid which resides in the trunk and I have to refill it with AdBlue (VW specific DEF fluid) every 10,000 miles or significantly less depending on driving condition.

My understanding on DEF system is very limited, same goes for my chemistry knowledge, but upon reading documentation about Cummins Filtration system in commercial vehicles which seems to be very similar to the technology employed by Volkswagen in later models, I'm confused why I'm buying and refilling DEF fluid that is supposed to do exactly what EPA says my car does NOT do.

Lastly, what is exactly causing diesel engines to produce black smoke which my car never produced?

1 Answer 1


The software in certain VW models was able to detect when the vehicle was being tested. It looked at inputs from the steering wheel, tire pressure etc. When in non test mode the software allowed the engine to make more power and pollution. It was not just a matter of turning the DEF system off. A simple analogy would be installing a performance tune for everyday driving or racing, then reverting to the factory settings for your annual emissions test. As far as anyone knows your vehicle pollutes no more than any other similar model. Diesel engines burn a fuel that is less refined than gasoline. When the fuel is not allowed to burn completely the result is soot and the black smoke at the tailpipe.

  • I'm still struggling to understand how a vehicle that utilizes DEF system can pollute more than same vehicle without DEF system. If it was simply a question of CPU programming I fail to understand how it would stop a company from reflashing CPU. I heard claims of "underground" CPU flash for my vehicle to extend gas mileage another 50% of what vehicle is able to achieve from the factory. I will have to wait until more information comes out to understand this entire issue. Plus who know what will happens with a change of administration in the USA. Dec 28, 2016 at 20:46

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