1991 Chevy Cavalier - Runs rough til warm(hot)

I have a '91 Chevy Cavalier that wants to backfire and run rough until it warms up. It is hard to start and will either backfire thru the throttle body or the exhaust. As it warms up, it begins to run a little better. When it gets hot enough to trip the radiator fan it will run like there was never a problem! I'm mean this thing runs GREAT then!!!! Before this problem appeared I dumped some fuel system cleaner(9,999 mile fuel treatment) into the gas tank, and about 1/4 tank of gas later this problem appeared. It's almost like some sort of vacuum leak, and as things get hot they expand and no more leak??? Ideas anyone?

 

yeah here might be a solution does the car still have the Calatalic converter on it if it does have the converter replaced or cut off and replace with an ordinary piece of flex pipe I think that your converter is getting too much back compression and causing you to loose power and fuel consumption also check to make sure that you don't have a Head gasket leak mine did the came I have a 94 cavalier 5speed and I had to replace the Head gasket and now the thing has no power loss or back firing also use use some water out removal in the gastank it might be backfiring cause the fuel has water in it and is having a hard time to adjusting with the correct fuel pressure hope this helps you out if not take it to a professional mechanic let him hook it up on a diagnostic machine also autozone can do this for you at no charge good luck

 

Turned out to be the ignition module! For whatever reason this car is going thru them. This is the 3rd on in about 1.5 years!