MORAINE — The DMAX diesel engine plant here is the area’s last facility with a substantial General Motors stake. And while the plant is not seeing the strong pre-recession business it once enjoyed, the plant is busy and workers are guardedly optimistic.

Late last year, the plant celebrated production of its 1.5 millionth Duramax truck engine. For the first time in several years the plant is scheduling overtime for workers. With the overtime, workers put in 10-hour shifts six days a week.

Earlier this month, Kurt McNeil, GM’s vice president of U.S. sales operations, said that a “recovery in new home construction” is benefiting truck sales. Often, Duramax engines are found under the hood of trucks at construction sites.

The 584,000-square-foot plant and its 517 workers make diesel-fueled heavy-duty truck engines for the Chevrolet Silverado HD (Heavy Duty) and GMC Sierra HD pickups. The engine is also available in the Chevrolet Express and GMC Savana vans.

The plant will produce an estimated 80,000 to 100,000 engines this year and last, compared with 250,000 units made in 2005 and 2006, a union official said.

GM has a 60 percent stake in the facility while Asian manufacturer Isuzu holds a 40 percent stake. Built in the late 1990s, the plant has seen layoffs and temporary shut-downs in recent years, but it has weathered the recession while bigger sites — including the nearby Moraine SUV assembly complex — have closed.

But the diesel engine market faces headwinds. Federal emission requirements have tightened, making diesel engines more expensive, and diesel fuel is more expensive than gasoline. On March 15, gas in the Dayton area ranged from $3.39 a gallon to $3.76, while diesel fuel was $3.79 to $4.19.

“What affects us most is the spread between diesel fuel and gas,” said Maho Mitsuya, DMAX chief executive.

Yet the Duramax remains a contender. Mark Williams, editor of PickupTrucks.com, said in 2011 head-to-head tests of heavy duty trucks, the Duramax “came out on top.” Testers pushed the trucks through 2,200 miles and four states, heading to altitudes of 11,000 feet, towing trailers that weighed almost 10 tons each.

The Duramax engine mated to a powerful Allison transmission was especially prized for its “responsiveness” and the “intelligence” of its on-board computer, Williams said.

James Winship, president of the International Union of Electronic Workers-Communication Workers of America Local 755, which represents about 400 DMAX workers, said diesels last longer, which means drivers aren’t buying new ones as often.

“We had an engine with a million miles on it,” Winship said. “We’re obviously building a quality product.”

Aaron Ford, a DMAX hourly worker since December 2000, said, “We’re really optimistic that (demand) is going to stay strong,”