Sales at Isuzu Motors Ltd. for 1998 will drop 11 percent to 631,000 units amid the economic slump in the domestic market and Southeast Asia, the company said Tuesday.Isuzu produced 705,029 vehicles last year and plans to produce 623,000 this year, down 12 percent. The economic crisis in Thailand will nearly halve Isuzu's sales there this year, down from 75,000 units to 38,000, Isuzu said.Isuzu, which temporarily stopped production in Thailand in November and December, said it has already cut 600 temporary jobs there. It resumed operations this month. "If we stopped our production in Thailand and withdrew from the country, it may deal a serious blow to the already worsening economic conditions," said Isuzu President Kazuhira Seki, explaining the reasoning behind resuming operations.Isuzu is currently building a new plant in Poland, which is due to begin operations in 1999. It has already agreed with Opal AG, a subsidiary of General Motors in the United States, to supply diesel engines produced there.Isuzu also announced that it has developed a new 3-liter, direct-injection diesel engine with 20 percent more fuel efficiency and power.