My truck is misfiring at 40 mph +, it's back firing. When it's idling it misfires and it hates hills. I've replaced the coil distributor cap and rotor new plugs and wires and had the fuel system cleaned by chemicals. My next step is have a diagnostic test on the truck. If anyone has an idea of what it might be I would be grateful. Thank you.

 

try cleaning the Throttle body by removing it, taking the sensors off and spraying it down with carb and choke cleaner. Pay special attention to the IAC motor port in the back. If it's gunked up, it won't meter air properly.

Other considerations: might have the catalytic converter checked to see if it's plugged, and replace the front O2 sensor.