hey guys got a 96 gsr and i have a 4-1 header test pipe and catback exhaust and when I am going down a hill in 4th gear and the rpms build up to like 2500 i give it a little gas and it sometimes back fires but it always blows a puff of white smoke and sometimes its a cloud and i am wondering if this is my head gasket? Some people says its because i have the test pipe instead of the cat and thats what is cause it to do it and i have both o2 sensors on the car.