The U.S. Environmental Protection Agency is warning truck-engine makers not to persuade customers to buy vehicles equipped with diesel engines before an October deadline when new emissions rules take effect.

DaimlerChrysler AG’s Detroit Diesel unit, Cummins Inc., Caterpillar Inc., Navistar International Corp. and AB Volvo were sent letters by the EPA early last month after the agency received reports some companies were encouraging customers to buy before the new emissions rules kick in.

In 1998, the truck-engine makers agreed to lower nitrogen-oxide emissions from truck diesel engines by October 2002. The companies deny they are encouraging pre-buying, knowing the EPA is keeping close watch on their marketing activities.

This year, sales of heavy-duty trucks in North America are up 88% through April to 92,444 units. Customers buying in advance of the October deadline fear price hikes of as much as $5,000 on vehicles with engines that meet the new rules.