Nope - no need for a code reader - just google it. All the "OBD1" North American market BMWs have it (that's most Motronics from '88-95). Turn the key to position 2, don't start the engine, and stomp the gas pedal five times. If you do it right, the CEL will start blinking. Count the pattern & you'll get your codes.
You could also have a massive vac leak...even small cracks that cause poor running can expand and seal up when the car gets warmed up nicely.