Q: Idle fluctuates

I have been driving my truck for about two or three years. I don't know the specific model or trim, it's a two door and two seater, with a little room behind the seats. My check engine light is on and I think it's the engire air filter (i think that's what it is called). The guy I bought it from did a lot of after market work on it. Under the hood, on the passenger side is a homemade box that is starting to fall apart. There's an aluminum/steel pipe that connects it to another pipe that I think connects to the engine. I have been connecting the two tubes together with duct tape because if they separate, I can't drive. I can push on the accelerator and the rpm's go up but its as if the truck is in neutral. I think it has to do with the homemade box. I think the box has an air filter in it. I just need to know what part(s) I need and how much it will cost to either have a mechanic fix it or to buy the part(s) and fix it myself. Thank you.

My car has 124000 miles.
My car has a manual transmission.