Stempel joined the engineering department of the Chevrolet Division as chief engineer in 1974 and was named Chevrolet's director of engineering in 1975. In 1978, Stempel was appointed general manager of the Pontiac Motor Division where he worked on the Fiero – which used a plastic-body/space frame technology that became integral to GM's subsequent minivans and the Saturn lineup. In 1980, he was moved into the Managing Director position at Adam Opel AG, the German subsidiary of GM. In 1982, he returned to Detroit as General Manager of Chevrolet.