Long and short....Helping a guy in my church.....'93 Olds 88....his engine died....bought used 75k '91 3800....switched out the intake manifolds etc.... which were different. Ran great for about 18 months....then the H.B. went to pieces and took the crank sensor with it. I was out of town when it happened so I could not deal with it, the car was taken to a local dealership for repair (cost 2 arms and a leg and a half). Anyway, since the car was repaired it'* been running rough.....then it hit me.....I bet the dealership put a '93 H.B. on it and it'* a '91 engine. I checked the part numbers at the local Advance Auto and they are different. IS the difference enough to screw up the timing to make it run rough? What is the difference and what do I need to look for?