They might not care for Mitsu engines, but the G6 in a 1991 Mazda was a Mazda design engine. 1988 was the last year of the G5x 2600 which was a Mitsu engine. The ECU was still mfd by Mitsu, but the engine was all Mazda. In 1992, they altered the ECU to eliminate a ping problem when using regular gas in warm climates. That ECU is interchangeable with the one from the 1991 model.