1991 toyota corolla mileage 110,000. If I start the car up with all 4 spark plugs hooked up it will knock when it gets to about 3,000 rpms. If I unhook the first 2 spark plugs It will not knock any more. The first 2 spark plugs is not firing because it has a rough idle and the engine does not change when i unhook the first 2 sprak plugs.