"Our primary concern is people introducing these fuels and modifying these vehicles changes, the emissions from these very clean cars that have already been demonstrated to meet the emissions standards," so that the vehicles release emissions that haven't been tested by the EPA, Kimes says.

People who are involved in the "introduction of vegetable oil for use as a motor vehicle fuel would violate the Clean Air Act," says Catherine Milbourn, an EPA spokeswoman, based in Washington, D.C.

The maximum penalty could be $32,500 per day per violation for mechanics and companies that convert vehicles to run on vegetable oil. Individuals who make the fuel can be fined as much as $2,750 a day, according to the EPA. Milbourn says the EPA has investigated some alleged violations but can't comment because the cases are ongoing.

Locally, the Utah Division of Air Quality does not regulate fuel or individual vehicles, because the division's responsibility is to implement the state's clean-air plan. "There's no mechanism to test fuel," says Joe Thomas, a division manager.

Processing vegetable oil

The main difference between biodiesel and SVO fuel is that biodiesel is sold at many gas stations in Utah and throughout the United States, although biodiesel also can be produced in private garages.

Biodiesel sold by retailers is legal, but biodiesel from private garages is not legal unless it is certified by ASTM International, an organization of technical, consumer and government experts who work on standardizing products and services, in part to ensure safety, say makers and consumers of vegetable-oil fuels. SVO fuel is not sold by retailers in the United States.

Another difference between biodiesel and SVO fuel is that biodiesel can be filled into diesel tanks without much modification of the vehicle. Some experts recommend fuel lines or fuel-injection systems be changed when using biodiesel.

Drivers interested in using SVO fuel, however, must purchase conversion kits that typically cost $1,200 to $3,000 before installation. The cost depends on the vehicle.

In the back of the Heber City welding shop, Hartlieb demonstrates the different processes for making biodiesel and SVO. A chemical process is required for biodiesel, to strip glycerine from the oil. A mechanical process is required for SVO.