Doug Decker's friends in high school knew how to cheat state emissions tests in their modified Mustangs and Firebirds: Just before the inspection, they'd adjust the carburetor to fill the engine with more air and less gasoline.

The car would generate less power, but the fuel would burn completely and the exhaust would come out clean.

"It was the way you played the game," said Decker, who is now in charge of motor-vehicle pollution for the Colorado Department of Public Health and Environment.

"It's still the same game," he added. Volkswagen recently admitted that it manipulated about 500,000 diesel engines in U.S. cars to bamboozle inspectors. Some automotive experts see vulnerability in government emissions-testing programs, despite their cost and inconvenience for drivers.

The Environmental Protection Agency on Friday said that it will launch sweeping changes to the way it tests for diesel emissions after getting duped by clandestine software in Volkswagen cars for seven years.

In a letter to car manufacturers, the EPA said it will add on-road testing to its regimen, "using driving cycles and conditions that may reasonably be expected to be encountered in normal operation and use, for the purposes of investigating a potential defeat device" similar to the one used by Volkswagen.

The testing would be in addition to the standard emissions test cycles already in place, the EPA said.

Regulators say that Volkswagen's so-called defeat device — it's in 11 million vehicles worldwide, VW says — used sensors for pressure, temperature, engine speed and more to register when the car was being put through the minutely prescribed federal testing procedure. If the onboard computer detected a test underway, it would reduce the car's emissions.

On the road, the car would run normally, likely with improved gas mileage and performance but emitting nitrogen oxide at as much as 40 times the federal limit, according to regulators.

Testing allows the EPA to enforce its fuel economy standards, but as the protocols are published in advance in detail, manufacturers are able to design cars that perform better on the tests than on the road, argued Gary Bishop, a research engineer at the University of Denver. Outright manipulation isn't always necessary.

Tests mandated by state and local governments didn't detect anything amiss because few states test diesel engines in light-duty vehicles. What's more, diesel inspections generally don't test for nitrogen monoxide, the odorless and colorless primary oxide of nitrogen produced by diesel engines.

For drivers with gasoline engines, these simpler tests are far easier to cheat than the federal inspection, a comprehensive shakedown that can take a few hours.

Gearheads might once have tinkered with an engine's timing using a screwdriver, but these days, they can buy aftermarket computer chips for better acceleration or towing capacity.

These chips sometimes mean more pollutants, too. An extreme example is "rolling coal" — disabling the pollution-control program in a diesel truck's computer so that the vehicle spouts a plume of thick black exhaust — which has no other purpose other than mocking environmentalists on the road.

Bishop's laboratory has developed a roadside sensor, which he and his colleagues have been using for more than a decade to see how cars actually do on the street in several major cities. The results allow Bishop to guess as to what modifications drivers have made to their cars.

Drivers can unplug these modifications before taking the car in for mandated testing.

Inspection "costs lots of money," Bishop said. "It does absolutely nothing to clean up the air."

This report includes material from the Associated Press.

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