Chrysler Flex Fuel Vehicles: How They Work

Posted 6/11/2009
By Arlen Mauland

Chrysler began building flex fuel vehicles in 1998. Flex fuel vehicles run on 85 percent ethanol/15 percent gasoline as well as 100 percent gasoline and any blend of the two fuels with less than 85 percent ethanol. A vehicle that runs on E85 as well as gasoline has a yellow sticker on the fuel door identifying it as a flex fuel vehicle.

"Flex fuel vehicles run on 85 percent ethanol/15 percent gasoline as well as 100 percent gasoline and any blend of the two fuels with less than 85 percent ethanol."

The fuel management system is basically the same for E85 vehicles and non-E85 vehicles, as is the hardware - although different part numbers exist for various components. The important thing to know is how the O2 sensor is used by the powertrain control module (PCM) to determine the percentage of alcohol and how the fuel trim reacts when E85 is used.

Ethanol does not have as much energy per gallon of fuel as gasoline; therefore, a mileage drop of around 30 percent should be expected when using E85 instead of gasoline. Its useful temperature range is much smaller than gasoline. Engines running on E85 may be harder to start in cold weather and have minor drivability problems in hot weather. E85 should not be used when the ambient temperature is below 0 F. Hard starting, rough idle and a decrease in driveability - such as hesitation and sag - are likely to occur from 0 F to 32 F until the engine is warm. Similar driveability problems can be expected when ambient temperatures are above 90 F. On the positive side, E85 is an extremely clean burning fuel, and it reduces dependence on foreign oil and boosts the rural economy.

Chrysler flex fuel vehicles do not have a sensor that directly detects ethanol/gasoline mix. When a minimum of three gallons of fuel is added to the tank (referred to as a refueling event) and then driven at least five miles, the PCM monitors the O2 sensor and modifies the value in memory for alcohol percent if the long-term fuel trim indicates too rich or too lean.

If the alcohol percentage is consistently lean, the PCM adds to the percentage of alcohol that is currently stored in memory, resulting in an increase in injector base pulse width. If the O2 sensor is consistently too rich, the PCM subtracts from the percentage of alcohol and decreases injector base pulse width. The only time that the alcohol percentage value is updated is after a refueling event and the vehicle is driven at least five miles. When the alcohol percentage is 30 percent or less, the scan tool will display 0 percent. Anything more than 30 percent will show the actual percentage, and the regular long-term fuel cells will be set to zero. A single adaptive cell called EAdp is used in place of the regular long-term fuel cells during this time.

When ethanol content is more than 30 percent, certain OBD-II monitors are disabled. These include O2 sensors, exponentially weighted moving average catalyst, EGR and misfire monitors. These monitors will remain disabled for up to five refills of gasoline. The number of refills required for a particular vehicle can be viewed as "Tanks til OBD Enabl" on the DRB III scan tool.

If the tank is less than 25 percent full, it is important not to switch from gasoline to E85 or vice versa. The fuel management system may not compensate properly, and the engine may run too rich or too lean. If switching from gasoline to E85, the engine will run lean until the alcohol percentage value is updated in the PCM. This can take up to five miles of driving immediately following a refueling event. The fuel management system can handle small variations in the alcohol/gasoline mix. If the PCM is replaced, the DRB III scan tool procedure should be used to copy E85 information from the failed PCM to the DRB III and then the information is copied from the DRB III to the new PCM. This must be done whenever the PCM is replaced on a flex fuel vehicle.

Since ethanol produces more water during combustion than gasoline does, a special oil containing an emulsifier is needed to keep the oil, water and alcohol mixed, increasing lubricity and reducing engine wear. Chrysler flex fuel vehicles must use oils with at least an SAE designation of AFV or FFV. All Chrysler oils are currently GF-4 rated with an API rating of SM. Oil with a GF-4 rating surpasses the flex fuel standards used previously. Currently, Chrysler recommends using SAE 5W-30 in all flex fuel vehicles. The driver should follow Schedule B in the maintenance schedule in the owner's manual.

Although flex fuel vehicles pose some unique problems, hopefully this will help you diagnose them more efficiently.

Arlen Mauland is an Identifix Chrysler specialist. He is a Chrysler master and ASE master technician with L1 certification.

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