[Eisenhardt]

I have a 92 gmc with a standard transmission that is backfiring and sputtering after it has been running for a while. I have noticed that on longer trips, 30+ miles, the truck backfires when i go to accelerate on the highway. Also, after it has been running a while, I will stop at a light, When I go to accelerate, It backfires and tries to stall but wont. I have to turn the key off and wait for the truck to shut off. Once it shuts off, I will restart it and then it runs normal till the next light.. I have replaced the plugs recently, thinking that it was a misfire problem. All the plugs looked normal. None were dirtier than the rest. It only does this after it has been running for a long distance. If it is cold, it does not have a problem. I have also tried seafoam in the tank and direct into the carb. I am at a loss? the temp gauge never gets hot?