Researchers from the University of the Ruhr in Bochum, Germany, and at the University of California, San Diego, said this week that they had found evidence of a defeat device in a diesel Fiat 500X, a compact S.U.V. sold in Europe.
Fiat Chrysler declined to comment on accusations of cheating in the 500X. The company told the European Parliament last year that the car was not programmed to detect emissions tests or to deactivate the pollution control equipment after a certain period of time.
Mr. Carder’s team in West Virginia tested four models: 2014 and 2015 Jeep Grand Cherokees and 2014 and 2015 Ram 1500 pickups. All the models had diesel engines, which are more fuel efficient than gasoline motors but produce more nitrogen oxides, a family of gases linked to lung ailments, premature deaths and smog.
The group drove the vehicles on roads around Morgantown, W.Va., the home of West Virginia University, using equipment that measures emissions while the vehicle is on the road. Those results were compared with tests at the university’s labs, where cars run on rollers while emissions are checked. The testing included more than 100 road trips and several hundred lab tests, Mr. Carder said.
All four of the models have 3-liter diesel engines with a pollution control system that uses a spray of a chemical solution to neutralize nitrogen oxides in the exhaust.
That kind of emissions control system can be effective, but only if the car is programmed to spray adequate amounts of the solution. Volkswagen was found to have rationed doses of the solution because the company did not want to inconvenience customers with frequent refills.
Mr. Carder said he did not know why the Grand Cherokee appeared to pollute less than the Ram pickup, but he said it would be normal for different vehicles to have different software configurations.
A resumption of sales of the diesel Ram and Grand Cherokees in the United States is unlikely for several weeks or even months. Fiat Chrysler’s proposal must first be accepted by the Justice Department, and then its fix must be certified as compliant with E.P.A. emissions regulations.
Once the software is certified, Fiat Chrysler could alert owners with a recall — or possibly a less formal customer service campaign — to take their vehicles to a dealer to have the software updated at no charge.