I read your problem does this truck use a k21, engine and if so you are dealing with a throttle position sensor problem not a petal problem check the wiring to the throttle it is located under the main fuel injector holder on lpg it has six wires coming out two go to the motor to open the butterfly one is ground one is power and two are signal wire to the two throttle position sensor. If the wiring checks out it is possible that the two switches in side do not agree on the position of the butterfly if so you need to replace the throttle body complete hope this helps