In the latest step in its crackdown on dirty diesel vehicles, the U.S. EPA yesterday issued a final rule that will require new diesel truck and bus engines to emit 40 percent less pollution by 2004. Later this year, the EPA intends to issue a rule that will mandate even cleaner engines by 2007 and require diesel fuel to be almost free of sulfur, a contaminant that hampers pollution-control equipment such as catalytic converters. When both rules take effect, diesel trucks and buses should run almost as cleanly as those powered by compressed natural gas. Diesel engines are currently a major source of smog and soot in the air, which cause 15,000 premature deaths, 400,000 asthma attacks, and 1 million respiratory problems each year in the U.S., according to the EPA.