Howdy I decided to reach out to the community in a last ditch effort to figure out this dang truck...I am getting CEL with p0455, p0443 codes idles rough after warmup so I replaced the evaporator vent valve canister and solenoidthen I also replaced the solenoid vent valve on top by throttle body as well problem persists things I noticed which make no sense is it only idles like crap when he air is off with air on it is pretty stable. I took to a GM tech that has good scan tool and we checked fuel trims they are 0 to -3 he says plus or minus 10 is within normal reading. So not too lean or rich. Nothing really jumped out from scanner but when the problem starts to occur it misfires on all 8 cylinders. Which is why I felt it idling like crap the rpms begin to drop then go back to normal constantly when this happens then we turn air conditioningback on and the misfires stop and it stabilizes...I am just hoping maybe just maybe somebody will see this post and have ran into the same prob or close to it. I did change plug wires and I took out a plug to inspect as well...I noticed that rather than the AC Delco iridium plugs that should be there it has autolite platinums so after playing around with the scan tool for a couple hrs and swapping plugs around on the even side he says to me I know it doesn't make sense but he believes I should change the plugs back to what it calls for AC Delco iridium plugs, which I agree with that observation but do y'all think it would actually have anything to do with misfires on all cylinders except when air is on? I would love some suggestions at this point as I'm pretty stumped will change plugs this weekend for sure but I just worry I'll waste more money although they do need to be swapped...I also sprayed several cans of carb cleaneraround intake manifold to check for gasket leaks and nothing changes as far as revving up. Thanks for taking time to read and comment I really appreciate any ideas y'all got even if they don't solve the issue I can't know til i try.

 

When you say missfire, does it log a missfire on the scan tool missfire monitor? Or does it just idle rough. A rough idle isn't necessarily outside the realm of possibilities. I say it's possible the limiting diode for the A/C compressor clutch might be sending a voltage spike into the PCM. Maybe check to see where the diode is and test it.