Flex Fuel

Flex fuel vehicles are designed to be compatible with any combination of ethanol and gasoline up to 85% ethanol (E85). The percentage of ethanol content in the fuel is inferred by the PCM flex fuel strategy.

The fuel level input (FLI) determines if a refueling event has occurred after an ignition ON or while the engine is running. If a refueling event is detected, the PCM saves the current inferred ethanol value. The PCM flex fuel strategy recognizes a refueling event as gasoline or E85, and enables the flex fuel learn procedure. The flex fuel strategy will infer the correct air to fuel ratio, based on the oxygen sensor input, to maintain stoichiometry after the vehicle refueling event occurs.

The new fuel is calculated to reach the engine after a calibrated amount of fuel has been consumed from the fuel lines and fuel rails. Normal long term fuel trim learning and EVAP purge control are temporarily disabled to allow the new ethanol content to gasoline percentage to be inferred. Ethanol content learning continues until the inference is stabilized within the engine operating conditions.

Typical flex fuel vehicle operation:

• The initial air to fuel ratio and flex fuel percentage is calculated for gasoline after a keep alive memory (KAM) reset. Vehicles that have E85 in the fuel tank after having a KAM reset may result in a hard start when cold, or a cold engine acceleration lack of power, until the PCM flex fuel strategy calculates the correct percentage of ethanol content in the fuel.
• A cold start with alcohol blended fuel may be more difficult than with gasoline, due to the lower volatility of alcohol blended fuel.
• Ethanol requires more fuel flow than gasoline, and flex fuel vehicles require a higher flow injector.
• Vehicles with flex fuel capability have the fuel type identified on the fuel filler pipe.