Ford rigged at least 500,000 heavy-duty trucks to beat emissions tests, drivers claim in a lawsuit, adding to the tally of automakers linked to diesel-cheating allegations worldwide, which started in 2015 with Volkswagen.

Ford’s F-250 and F-350 Super Duty diesel pickups, a slice of the top-selling F-Series, are spewing emissions as much as 50 times the legal limit of nitrogen oxide pollutants, according to the complaint. The trucks sold from 2011 to 2017 cost $8,400 more than their gasoline-fueled counterparts, the filing shows.

Ford marketed the trucks as “the cleanest super diesel ever,” while the lawyer behind the suit said they should have been called “Super Dirty.”

“The vehicle’s own on-board diagnostic software indicates emission control system to be operating as Ford intended, even though its real world performance grossly exceeds the standard,” attorney Steve Berman, a managing partner at Hagens Berman, said in the complaint.

Ford worked with German automotive supplier Robert Bosch to mask the vehicles’ inefficiencies in order to maintain overall performance, according to the complaint filed Wednesday in Detroit federal court. Bosch is named as a defendant in the proposed class action.

Ford is at least the fifth automaker accused of diesel cheating in the U.S., once again raising questions about the effectiveness of diesel technology. The dispute also casts a shadow on Ford’s F-Series, the top selling line of vehicles in the U.S. every year since 1981.

The lawsuit could pose a risk to Ford’s plans to introduce a diesel engine in its smaller F-150 pickup for the first time. The automaker has said it expects the powertrain will boost fuel economy to 30 mpg on the highway. That would confer significant bragging rights for Ford and potentially help its trucks outperform new models from General Motors and Fiat Chrysler Automobiles coming out this year.

Diesel engines, while more fuel efficient, produce greater volumes of nitrogen oxide pollutants, or NOx. During on-road testing the diesel trucks polluted at levels beyond legal limits and higher than their gasoline counterparts, according to the complaint.

Bosch is accused of developing software that enabled Ford to adjust fuel levels, exhaust gas re-circulation, air pressure and urea injection rates while being tested for emissions by regulators including the Environmental Protection Agency and California Air Resources Board.

The formula built by Ford and Bosch allowed the Super Duty trucks to reverse the traditional order of exhaust treatment, putting catalytic reduction before the diesel particle filter. This allowed Ford to market both fuel efficiency and power in its vehicles without compromise. The reordering should have forced Ford to burn off the collected particles, sapping the vehicles’ efficiency. Instead the particles were released through the vehicles’ exhaust, according to the complaint.

Bosch faces similar accusations in cases against VW, Fiat Chrysler and GM. The Stuttgart, Germany-based supplier has denied any wrongdoing in those suits.

In 2011, Ford embarked on a marketing campaign that promised “class leading fuel economy and towing capacity” tested more rigorously than its competitors. The campaign evolved over the years, and as recently as 2017, Ford’s vehicle brochures boasted of reduced NOx levels “in the real world” and “unsurpassed diesel fuel economy.”

“It is not plausible, given this ‘groundbreaking’ testing, that Ford and Bosch did not know that emissions controls do not work when the vehicle is operating in normal stop-and-go conditions, running under heavier loads, and going up modest to steep grades,” said Berman.