We have a 90′ Cavalier, we thought we blew a head gasket so we got that replaced and its definitely good. It sounds like a misfire of the spark plugs so we took them out and checked them all one by one and they all seem fine and its a good spark, no cracks or anything. The coil pack is good. The car doesnt have a distributor cap so it can’t be that, the vaccum lines are all good. Theres white smoke coming out the exhaust. The car will run but it jumps and is really rough. We put gas treatment in it. What else could it be?