'Cheat Mode' In VW Diesels: Tests Show It Boosts Performance, MPG

October 9, 2015

The so-called defeat device (otherwise called a 'cheat mode') that’s in up to 482,000 U.S. VW and Audi vehicles, and up to 11 million vehicles worldwide, helps circumvent emissions rules by running in a cleaner mode during an official emissions test, then emitting up to 40 times the legal limit of some pollutants—specifically oxides of nitrogen—during ordinary driving.

It's a mess that the automaker must now fix. But very little has been said by the automaker about how these vehicles might perform once fully legal. And a cleaner-burning engine—in terms of health-endangering, smog-forming pollutants like NOx—is not necessarily a more fuel-efficient one either.

Thanks to the auto-testing division at Consumer Reports, we now have an idea just how much performance and fuel economy could potentially be impacted.

CR placed a couple of the affected models on the dyno—a 2015 Volkswagen Jetta TDI sedan, and a 2011 Jetta SportWagen TDI, on and off the dynamometer to try to figure out how to reliably place their vehicle in its ‘cheat mode.’

Figuring out the tricks

To use its ‘safe’ emissions-test mode and prevent the vehicle from cycling over to its dirtier mode, the testers then disconnected some sensors that might tell the computer it was off a test machine and in normal driving conditions.

For the 2011 VW Jetta SportWagen TDI, with its older EA189 version of the 2.0-liter turbo-diesel four-cylinder engine, performance from 0-60 mph dropped from 9.9 seconds to 10.5 seconds, while fuel economy in a highway test cycle dropped from 50 mpg to 46 mpg.

2014 Volkswagen Jetta SportWagen

2014 Volkswagen Jetta SportWagen

Yet for the 2015 model, with its newer EA288 version of the 2.0-liter engine, fitted with a Selective Catalytic Reduction (SCR) exhaust-aftertreatment system, there was only a tenth of a second difference in acceleration, while fuel economy dropped from 53 mpg to 50 mpg.

Of course, this is an unusual case for fuel economy, as VW’s TDI four-cylinder engines—going by what we’ve seen in real-world conditions, or what you’ll see in observed figures from a wide range of sources—produce numbers that are rather consistently a bit higher than the official ones.

Likely still good for mpg, but not the standouts they've been

“While overall annual extra fuel costs may not be dramatic, these cars may no longer stand out among many very efficient competitors,” noted CR auto testing director Jake Fisher.

2014 Volkswagen Beetle TDI

2014 Volkswagen Beetle TDI

For the newer engine, it’s likely that the solution will be purely software-based, although models with the older engine—of which there are roughly 325,000 in the U.S. alone—will likely need a combination of software changes plus serious hardware alterations.

“Whatever they do, we will test the results, and if fuel economy or performance is hurt, Consumer Reports will urge VW to make restitution to its customers,” said Fisher.

Especially for those older TDI models of the group, that would be an essential part if performance is impacted to the degree seen here.

And VW needs to hold triage in a very tight time frame. Today, California regulators have stepped up the timeline, giving Volkswagen until November 20—45 days from an official compliance letter—to come up with a plan for fixing the cars.

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