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You've paid $2,000 to convert your car to run on compressed natural gas (CNG), but are you really polluting less than before?

"A lot of people run around thinking they're meeting emissions standards, and they're not," says Al Jessel, a Chevron Corporation CNG analyst.

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Cars designed to run on gasoline lose a little power when converted to CNG. Discovering this, owners may "tweak" the system to restore the power and worsen the emissions, Mr. Jessel says. Or the conversion kit might be poorly engineered. Or the men installing the kit might not do so properly.

The California Air Resources Board tested a number of cars that had been converted to run on propane and found "significant increases" in emissions, says Jack Kitowski, a manager in the CARB's toxics and fuels section. Although no CNG-converted vehicles were tested, the CARB suspected that such cars might have the same experience, since the problem with the propane-powered vehicles lay with the conversion kits rather than the fuel itself.

The CARB is now considering toughening the rules on conversions. Mr. Kitowski says manufacturers would have to make a kit specific to each model of car, and have the kit certified by the state. They would have to offer a three-year/50,000-mile warranty. Durability of the kit for 100,000 miles would have to be demonstrated. And if testing found problems in converted cars, the kits would be subject to recall.

"It's a transitional problem," says Rusty Bevilacqua, president of the Clean Air Vehicle Technology Center in Hayward, Calif. Conversion technology will improve, and longer term, more vehicles will be manufactured as CNG-fueled.

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