Chrysler Flex Fuel Vehicles–how they work
Flex fuel vehicles run on E-85, which is a blend of 85% ethanol and 15% gasoline. They can also run on straight gasoline. To determine if your vehicle can run on E-85, look for a yellow sticker on the fuel door that identifies it as a flex fuel vehicle.
The engine computer uses the same sensors for E-85 as for gasoline, but uses them differently for each fuel. Chrysler vehicles do NOT have a sensor that detects E85. It has to interpret its readings to deduce that it has E-85 in it.
First, the computer monitors the fuel level gauge to detect when you’ve added at least three gallons of fuel. That’s known as a “refueling event.” If the computer see fuel level rise, it wait until you’ve driven at least five miles before monitoring the oxygen sensors to detect a change in fuel. The computer is checking to see if the oxygen sensors are reaching rich or lean. If lean, the computer adds fuel by increasing the fuel injector “on” time (also known as injector pulse width). If the sensor shows rich, it reduces injector pulse width. Based on how much it had to add or subtract, it can calculate the percentage of ethanol in the mix. If you attach a scan tool, it will read out the percentage as long as it’s above 30%.
Gasoline vehicle computers always monitor oxygen levels and adjust fuel delivery. If an engine consistently needs more or less fuel, the computer makes either a long term or short term modification to its base fuel calculations. This is known as “fuel trim, or adaptive fuel trim.” For example, if a vehicle accumulates 100,000 miles and is no longer as fuel efficient as when it was new and the computer is consistently adding 25% more fuel in the short-term fuel trim calculations, it will move conclude that this is a permanent condition and move the fuel calculation over to the long-term fuel trim memory area.
Flex fuel vehicles are no different, except that the long term adaptive fuel trim is referred to as EAdp.
In gasoline vehicles, the computer also monitors for cylinder misfires, catalytic converter efficiency, and exhaust gas recirculation. However, in a flex fuel vehicle, if the computer determines that the tank holds more than 30% ethanol, it disables these monitors. It continues to disable the monitors for up to five refills of fuel. That’s why flex fuel owners are warned not to refuel with a different fuel if they have less than ¼ tank. It’s too difficult for the fuel management system to make those quick modifications.
If you switch from E-85 to gasoline, it’s not uncommon for the engine to run lean until the computer can recalculate the new ethanol percentage and update itself. This can take up to five miles of driving after refueling.
Another difference in flex fuel vehicles is the type of oil you must use. Ethanol produces more water during combustion and a special engine oil is required to keep the oil, water and alcohol mixed in the crankcase. If regular oils are used, the water and alcohol will separate and reduce lubrication, resulting in engine failure. Make sure your oil is rated as AFV or FFV. Also look for a GF-4 rating, as well as an SM rating. Chrysler recommends 5W30 in all flex fuel vehicles.
Be aware that Ethanol does not have as much energy per gallon of fuel as gasoline. Because of the reduced energy content, you can expect a mileage drop of around 30% when using E-85. In addition, a flex fuel vehicle running on E-85 will be harder to start in cold weather and may have driveability issues in hot weather, when the temps are above 90°F. From 0° to 32°, you may experience hesitation on acceleration.
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© 2007 Rick Muscoplat
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