Things that can be wrong as long as the car rools fine are the timing chain and exhaust restrictions or leaks. Dirty or leaking fuel injectors can too if the car has good power and doesn't lag at off idel ( like a timing chain loose will do ) oil pressure or too heav oil or oil that hasn't been changed. low tire pressure is worth about 3 MPG Timing chain can be 10 easy as stopped up exhaust. the car runns stronger with the pipes unbolted from the engine then theres a converter or muffler problem. If the pipes are good it'll not make any different except it'll sound like a race car. ( might seem faster but it will be faster if the pipes are stopped up. Race cars go faster with open pipes as the engine has been modifed to run better with open pipes but an ordianry car will have little difference until it's revved up real high.
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