I have a 1998 Chevy 6.5. It had check engine light on but exhaust was screwy. Well I put new crossover and 4" exhaust on and cleared codes. It has popped P0236 and P0251 again. What causes these and how to fix. Injectors I have never replaced (had 30k miles), shows 205k. Thanks.

check your vacpump(faulty,no vacuum),cracked lines and faulty waste gate actuator.(code 236)

Code 251,Could be dirty OS (internal in IP) or air in the fuel.
Check for airbubbles in the system
run fuel conditioner in the tank to clean out the fuel system