Check engine comes on only when pushing on accelerator

What is your question?

After driving for a while, my check engine light comes on, but I've narrowed it down to it only comes on when I'm pushing on the accelerator. As soon as i put it in neutral, or even leave it in gear, and take my foot off the gas pedal, the light will turn off after a few seconds. But as soon as i give the truck some gas, the check engine light comes back on. I've heard that it could be my fuel pump, but does anybody have any guesses?