Ive gone through every RPO code I could find. My 1986 IROC-Z has a strange code that I can't find anything about anywhere. Ive been researching since I bought the car in 2015 and can't seem to find anything about it. The code is C4B and the only thing I've found was "performance engine upgrade" from only a few sources. Ive even talked to GM dealers and have been running out of ideas. Anyone know anything about it? Any help would be greatly appreciated!