98 C3500 454/4L80 stock other than cats gone, and downstream O2 sensors are turned off. HP tune on the stock PCM.
Ran great for about 50 miles of a 60 mile trip, then started missing hard (like WHAM) backfiring and wouldn't take throttle input. Temp climbed to about 190° with no thermostat, ambient temp was about 85. Sat for about 5 hours, drove the same distance home, ambient temp was about 65°, coolant never got above 175° and it ran great the whole way.
Any thoughts?
Ran great for about 50 miles of a 60 mile trip, then started missing hard (like WHAM) backfiring and wouldn't take throttle input. Temp climbed to about 190° with no thermostat, ambient temp was about 85. Sat for about 5 hours, drove the same distance home, ambient temp was about 65°, coolant never got above 175° and it ran great the whole way.
Any thoughts?