Help! My car is miss firing bad

I have a 2000 ford focus. It starting having a lot of trouble. The car only has 66k miles on it. It was idleing hard shaking and running very rough. I friend said it might be the mass air flow sensor. So I Replaced that, still did it so I replaed the oxygen sensor and the coil pack. Now it idle 's a bit smoother but you hear a poping noise out of the tail pipe. And when I try to move I can push the pedal to the floor and I only go 5mph. I need some help. The check engine light is off so they said they cant pull a code