E85 Flex Fuel Description

E85 compatible vehicles no longer use an alcohol sensor to determine and adjust for the alcohol content of the fuel in the tank. Instead, the vehicle calculates the alcohol content of the fuel through measured adjustments.

The ethanol calculation occurs with the engine running after a refueling event has been detected via a measured change in the fuel level sender output. The virtual flex fuel sensor (V-FFS) algorithm temporarily closes the canister purge valve for a few seconds and monitors information from the closed loop fuel trim system to calculate the ethanol content. This logic executes several times until the ethanol calculation is deemed to be stable. This may take several minutes under low fuel flow conditions such as idle, or a shorter time during higher fuel flow, off-idle conditions.

Air-fuel ratios and the corresponding ethanol percentage are updated following each purge-off sequence. The fuel alcohol content percentage value can be read on a scan tool.

When an E85 compatible vehicle is built, an ECM or PCM replaced, or if the learned alcohol content has been reset with a scan tool the fuel system will need to contain ASTM gasoline with 10 percent or less ethanol content.

A minimum of 11 liters (3 gallons) must be put in the tank in order for the vehicle to recognize a re-fueling event. It is not necessary to turn the ignition OFF in order to have the re-fueling event recognized, however local safety regulations should be followed.

After the re-fueling event, the system registers the amount of fuel that was added, relative to the amount that was in the tank. Reading fuel trim and O2 sensor activity, the system determines if the fuel added was either ASTM Gasoline or ASTM E85. Based on that determination, the system adjusts to the expected alcohol mix in the fuel tank, and then the fuel trim and O2 sensor activity fine tunes the adjustments. The system must remain in closed loop in order for this adjustment to occur. Numerous short trips after switching from gasoline to E85, or E85 to gasoline, can result in driveability symptoms due to the inability of the system to adjust for fuel composition by not attaining closed loop

Customers usually complain about driveability problems that are brought about by a vehicle fault or external conditions. Occasionally, the nut behind the wheel is responsible.
Fill It to the Brim
Earlier this year, a customer brought in his 2010 Chevy Malibu with a 2.4L engine and flex-fuel capability. His complaints were a Check Engine light, a DTC P0172 (System Too Rich - Bank 1) and an engine that’s hard to start when cold. After checking the vehicle with my Tech2 scan tool, I determined that the alcohol content data indicated over 30% and fuel trims also were around 30%. The customer told me he usually buys regular pump gas and hadn’t purchased any E85 fuel recently. I used the Tech2’s special test function to recalibrate the fuel alcohol content to 0% and all seemed well. It’s now a few months later, and the same customer has returned with the same set of problems. I’m wondering if I’m going to have to recalibrate the PCM a few times each year to deal with this issue. The GM TIS2WEB data does not indicate that there’s a PCM reflash or update available to address this problem. Suggestions?

Terry Gregory
Lincoln, NE


Thanks for your question, Terry. A flex-fuel vehicle must be capable of running, and running reasonably well, on a virtually limitless assortment of fuel mixtures. While most engines are designed to require a certain fuel octane level and added ethanol content limited to 15%, the engines in flex-fuel vehicles are designed to run on fuel that contains anywhere from no ethanol to fuel that’s 85% ethanol. This would not be possible if the PCM didn’t have a pretty good idea of what kind of fuel is in the tank at any given time.

Early flex-fuel vehicles used flex-fuel sensors that measured the ethanol content directly, either in the tank or in a fuel line, and could report their findings to the PCM in real time. Since the energy density of alcohol is much lower than gasoline and the burn rate is much faster than gasoline, the PCM must adjust the air/fuel mixture and spark advance timing to ensure power output is close to that of a normally fueled engine.

The flex-fuel sensor’s output frequency increases as the percentage of ethanol in the fuel mixture increases. For example, regular 87 octane gasoline may yield a 40- to 60Hz flex-fuel sensor output frequency. A fuel blend that’s 30% ethanol may have a flex-fuel sensor signal output frequency between 60 and 100Hz. A 60% ethanol blend may have a sensor frequency between 90 and 130Hz. The PCM uses this frequency input to calculate the correct air/ fuel ratio and spark advance for the vehicle.

Some of these direct-measure flex-fuel sensors weren’t terribly reliable and perhaps more importantly, they were also relatively expensive. Not surprisingly, vehicle manufacturers looked for a way to replace these sensors with something more efficient, reliable and less expensive. Enter the virtual flex-fuel sensor.

Rather than employing a physical sensor, the virtual flex-fuel sensor determines the amount of ethanol in the tank indirectly, using software and a controlled test. The test begins when the PCM determines that a “refueling event” has occurred. In less exotic terms, this means the driver has added fuel to the tank (at least three gallons, in most cases). Since it’s an E85 vehicle, the PCM needs to know exactly what’s been added so it can adjust the fuel and spark delivery to suit.

After the refueling event, the PCM waits until the engine is in closed loop before initiating the test. The purge valve is closed, as the introduction of any additional evaporative emissions could skew the test results. The PCM monitors the fuel control required to keep the system at lambda. If it trends one way or another, the PCM’s software logic will deduce that there has been a change in the ethanol content of the fuel.

If you were monitoring the exhaust with a five-gas analyzer both before and after a refueling event, you’d notice that CO2 readings will decrease and O2 readings will increase when fuel with a higher ethanol content is added to the system. Additionally, oxides of nitrogen (NOx) levels can be lower with E85, because combustion temperatures are lowered. These changes would be most pronounced if you went from a nearly empty tank containing conventional E15 gasoline to a full tank of E85 fuel.

Like most engine management tests, a virtual flex-fuel sensor test requires a precise set of parameters to function properly and to achieve reliable results. For this test, the most important parameter is time. The virtual sensor must have enough time to complete its test. So if your customer is in the habit of filling his fuel tank at the corner gas station, then driving a couple of blocks before parking in the garage, the virtual sensor may not have enough time to complete its test. The test may be aborted, then repeated at another time. Or the incomplete results may be reported to the PCM, resulting in a PCM that’s basing its decisions on incorrect information. As you have seen with your customer’s vehicle, the PCM’s ethanol percentage parameter did not square with what was in the tank.

Adding to this problem is a customer who habitually refuels when the tank is only partially depleted. Adding only a few gallons at each fuel stop makes it harder for the PCM to determine how much change in ethanol content has occurred with each fill-up. A customer who’s concerned about his fuel “icing up” during the winter and fills the tank each time he sees the fuel gauge drop below ¾ may be creating his own problem. And he may also have problems if he’s a short-trip driver.

It’s better to give the virtual sensor a larger dose of new fuel on which to run its test, which is why the software won’t allow the test to kick in unless at least three gallons have been added. Several additions of small amounts of fuel can skew the results, as can small fuel trim corrections due to vacuum leaks, incorrect airflow calculations or inadequate fuel pressure. The last three can cause enough variation during the virtual sensor test to corrupt the results.

If you suspect that the PCM is working with incorrect ethanol content calculations, take the time to confirm your suspicions by conducting a fuel sample test. A graduated cylinder, a fuel sample taken from the tank and some tap water will allow you to quickly and accurately determine how much ethanol is currently in the tank. If your results don’t match up with the PCM’s findings, don’t be too quick to blame the virtual sensor. It’s doing the best it can with the information available.

Before recalibrating the PCM (again), take the time to educate your customer on how to get the best driveability results from his E85 vehicle. It’s okay to run the tank down below ¼ from time to time. If he’s in the habit of switching between E85 and conventional fuels, the vehicle should be driven for several miles after a fill-up with either fuel to allow the virtual sensor to run its test. A quick trip to the station, then home again with a full tank of E85 can cause bigger problems than a Check Engine light. In winter, a vehicle filled with E85 just before parking for the night may not start the next morning. When changing from a tank of E85 to conventional gasoline or vice versa, the engine could start but run rough for a time.

It looks like E85 and vehicles capable of running on this fuel are going to be with us for the foreseeable future. Some of your customers are going to need your help to avoid preventable problems surrounding the use of this fuel.