Ferrocene is used as an antiknock additive to replace lead alkyls. To clarify the influence of one metal additive, ferrocene, on engine, following experiments were carried out. The insulation resistance of spark plugs was measured, deposits in the engine were analyzed, and an exhaust emission and fuel economy tests were conducted using gasoline containing ferrocene. The deposit, which contained iron oxides, adhered to the combustion chamber, spark plugs, and exhaust pipe when the engine operated with gasoline containing ferrocene. When vehicles operated with gasoline containing ferrocene, fuel consumption increased and the exhaust temperature rose. In addition, an abnormal electrical discharge pattern was observed in spark plugs operating at high temperatures. Iron-oxide of Fe3O4 is changed into Fe2O3 under high temperatures. Discharge current flows in iron oxides including Fe2O3 because the conductivity of Fe2O3 increases at high temperatures. Therefore, a defective ignition causes the rise of exhaust temperature because the current flows to iron oxides, which adhere to the insulation of the spark plug. An engine bench test was also conducted using gasoline containing ferrocene and a detergent additive. However, the abnormal electrical discharge did not improve by detergent additive. These results indicate that ferrocene decreases the insulation resistance of spark plugs and causes increase in fuel consumption, exhaust emissions, exhaust gas temperature, and irregular electrical discharge.