Do you have a scan tool to plug into the OBD port?
Open loop - computer is delivering fuel according to fuel map without regard to o2 sensor feedback...it thinks the engine isn't at the correct operating temp and will keep pumping fuel to stoke the fires up good and hot...which can actually quench the metal in the engine, preventing it from getting hotter (and killing your mileage in the process. That's why I asked if you had recently serviced your cooling system/checked tstat/water level)
Closed loop - ECT (Engine Coolant Temperature) sensor sends signal to the computer so it can monitor for what engineers have determined as correct operating temperature for that engine. when it hits a pre-set temperature, usually just below where your tstat opens, the computer starts to trim fuel injector pulse duration (fuel delivery) according to what the oxygen sensors tell it as far as air/fuel ratio.
If I were to guess, you've either got a tstat that opens at the wrong temperature, or a bad ECT sensor, or not enough coolant in the system.
your scan tool should tell you fuel system status...open is not good if it has been running for a while, and I'd look at the sensor first - see that it's sending correct temp info to the ECU. If it's closed, look at the oxygen sensor info - it should be sending a varying waveform representing the voltage it generates with values between 01. and 0.9Vdc to the ECU. (how the ECU handles the information -whether relying on the analog signal or digitizing it and sending frequencies to the computer, I couldn't tell you for certain on your truck)