Toyota engines equipped with a knock detection system are very sensitive to fuel octane levels. Motor fuels with low octane ratings will cause the engine to detonate, which will in turn, cause the detonation retard system to retard timing. On some vehicles with advanced ECM operating strategies, an adaptive memory factor is used to track signals from the knock sensor. When detonation occurs frequently, the ECM relearns the basic spark advance curve, retarding spark throughout the entire engine operating range. This retarded spark curve will negatively effect engine performance and fuel economy under all driving conditions, even after a tank of higher octane fuel is purchased. The retarded spark curve will remain stored in the ECM keep alive memory until the engine is operated for a substantial amount of time on the higher octane fuel, or until the "keep alive memory" is cleared by removing power from the BATT terminal.
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