1991 BMW 325i "Check" light flashing

The Engine Computer detected a problem, set a diagnostic fault code, and turned the Check Engine light on to tell you. The place to start is by having the fault code(s) read and recorded. A flashing light indicates the most serious conditions and means you're supposed to stop the engine right away because too much raw fuel is going into the exhaust system and is going to overheat and damage the catalytic converter.