Piston miss fire.

Please help, I just bought a cavalier 96 and I'm not sure of what is happing. The cars seams to work fine but sometimes the engines starts shacking and the check engine light is on, I took to someone to scan it and it said piston miss fire. Almost every time I turn the car on a big smoke comes out of it, but it's only when I turn it on, if I accelerate not a bit of smoke comes out of it. I don't know if it has to do with the fuel cleaner injector that I poured on the gas tank I putted more gas in order to dilute the product. Also I changed the spark plugs but the wires.

Any ideas?

Thank you
jdff