Common problem with the 7.3l IDI. There is a cold start switch which is activated during proper start up procedures that advances the timeing and pushes the accelerator a bit which stops this chugging. Turn the key to on. Push the pedal to the floor, and let go. Do not turn the key back off, but instead wait for the glow plug light, then start. If it still chuggs, then the solinoid is burned out, and needs replacing. They are cheap. You can also jumper a wire from the battery to the solinoid to see if it pops out. If not replace it. Alot of people buy these trucks not knowing about having to depress the pedal to activate the cold start solinoid. Note the solinoid won't activate when the engine is warm, as it is not needed at that point. If it does activate you will notice no chug, but also the RPM's will be higher, around 800-900. If you wait until the motor warms up, you will notice the RPM's drop off to about 600 after warm up of about 5 min. When it does this it is really noticable because the solinoid stops pushing the pedal, and it pops back up. Scared the daylights out of me the first time it happened to me. I didn't expect it, as I was usually driving already when it shuts off, and it poped up and scare the daylights out of me. LOL.