Effects of Octane Rating on Engine Performance and Knocking

When diagnosing any customer concern related to poor engine performance or engine "knocking", always suspect fuel quality, or more specifically the octane rating of the fuel being used. The octane rating is a reflection of the fuel's ability to withstand engine knock, and is rated by its Antiknock Index (or pump octane rating). This number is displayed on a yellow sticker on the side of each gas pump.

Since octane requirements differ from vehicle to vehicle, always check in the Owner's Manual for vehicle's exact octane requirement and verify with the customer that their concern is not the result of low octane fuel. On vehicles with Knock Control systems, low octane may not cause the engine to knock, since the system has the ability to retarded spark advance; however, the engine may perform poorly as a result of a conservative spark advance strategy. If the engine knocking or performance concern is not the result of a sub-system problem, you may want to suggest to the customer a change in fuel grade or retailer.

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