Diesel versions of the Toyota Hiace van, Land Cruiser Prado, and Nissan X-Trail produced up to 10 times more NOx than allowed. The Mitsubishi Delica D:5 was up to five times over the limit, The Wall Street Journal reports. There was no evidence of defeat devices in the vehicles.
Mazda performed well in the experiment, though. The CX-5 passed with nearly the same results on the road and in the lab. The Demio, better known as the Mazda2, did nearly as well with only slighter higher figures in the real world than in the controlled setting.
The experimenters theorized the reason for the excessive emissions was that cold weather caused the engines' software to shut off the exhaust gas recirculation to prevent damage, according to the WSJ. However, this behavior also increased NOx production.
Toyota, Nissan, and Mitsubishi don't have to worry about punishment from the transport ministry because this check was just an experiment. Their models already passed the mandated lab tests, which was the only requirement, according to The Japan Times.
As governments begin greater real-world emissions tests, the results suggests diesels aren't very clean. A recent check in France found models from Ford, Renault, and Mercedes-Benz that didn't perform up to the standards. Regulators in India conducted similar evaluations and ordered VW to recall over 300,000 vehicles.