They own a 1999 Chevy Venture with 142k miles, The engine knocks real hard at random times Vag com 16.8. They have replaced 3 spark plugs and wires (they cannot reach the back 3 without taking out the engine). When they use a little higher grade gasoline it seems to run better. Do you think a product like Zmax, or Sea Foam fix it? They think that we are only running on 5 cyl.