Piston Slap: Decomposition of the Fuel Composition Sensor?

Sajeev Mehta
by Sajeev Mehta

Longtime TTAC commentor rrhyne56 writes:

Flex Fuel. I see it more and more. From what I’ve heard, this mainly means the vehicle has a fuel system that alcohol wouldn’t eat up. (Mainly, yes. — SM)

So here’s my question: do the more recent models of these vehicles have the ability to sense what level of alcohol is in the fuel lines and adjust the engine accordingly, to make best advantage of whatever current gasoline/alcohol or alcohol/gasoline mixture is entering the engine? I watched a build on Mighty Car Mods where the Haltech engineer was tweaking just such a system.

I know, I know, I ought to just Google it. But I thought it might make for some lively discussion.

Sajeev answers:

Yes indeed, flex-fuel vehicles have a flex-fuel sensor (b.k.a. fuel composition sensor) to give such information to the engine computer. Considering the following video (wiring diagram and the “blocky” output via the scope), this older Buick’s system meets the most basic need: it triggers a switch — E85 on/E85 off. Then the computer switches between a unique fuel/spark table for whatever’s in the tank.

The only proof I see (i.e. Googled) of a system that eschews the on/off switch for a rheostat is in the aftermarket. That said, who else but the hardcore ethanol racers worry about blowing up a tuned motor from a bad batch of ethanol? Factory sensors and tunes are good enough for most, but the Zeitronix sensor gives a varying signal that interfaces with standalone fuel injection systems.

You can bet it ain’t cheap, and most folks won’t pay for all of this. So let’s get back to the current factory setup.

Replacement sensors (on older models?) are pricey, causing drivability issues and engine codes when they fail. One enterprising company is offering a defeat sensor so that an E85-ready machine returns to being a normal gas guzzler. Considering a large number of Bush-era ethanol vehicles are fully depreciated, on their last legs, etc., this defeat sensor makes financial sense.

[Image: Shutterstock user PM8899]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

Sajeev Mehta
Sajeev Mehta

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  • Scoutdude Scoutdude on Jun 09, 2017

    Sorry Sajeev but you've got a lot wrong. When Ford, GM and The automaker formerly known as Chrysler started working on Flex Fuel Vehicles they all used a fuel composition sensor (FCS) to detect the percentage of Ethanol or Methanol in the fuel. It is not a switch as shown in the one video they send a signal that varies in direct proportion to the alcohol content. If they did a simple switch it could be way off when you fill a less than empty tank with winter blend E85 and in most areas it would always detect at least a little ethanol now a days. With GM and Ford not only is the injector pulse width adjusted so is the timing. However again it is not an A or B thing. It is closer to an FCS% x (B-A) + A, where the actual timing commanded is somewhere between the values in the two tables and proportional to the Alcohol Content. However even for the mfgs the FCS was an added cost that was not insignificant. So they switched to the VFCS or Virtual Fuel Composition Sensor as they figured out that they have the technology to infer the E% since in the mean time Methanol had lost the war and disappeared. So how do they do it now? By using the other sensors already required on the vehicle. As most people know the computer stores a ton of information for multiple reasons. For testing the EVAP system the computer has to know the level of fuel in the tank, how long since the engine last ran, intake and coolant temps. So it can figure out when it has been refueled and knows more or less how much was in the tank and added. Since it also knows and remembers the temp it can also make an Educated guess on what the actual E% is being added, whether it is more likely to have Full summer blend E85, winter blend which is really E70 or the transition blend. So here is what happens once you turn the key on a modern FFV. Before you can get to crank the computer takes readings from all of its sensors. If there is enough difference in the fuel level reading between the last shut down and the engine is up to temp it will start the fuel learn mode. Once it enters fuel learn mode it does a rough calculation based on current learned E% and the possibility of what could have been put in the tank. It then uses that best guess in fueling calculations. Monitoring the O2 sensor it then makes the corrections necessary to get closer to the true E%. Once it thinks it is in the ball park on fuel then it learns the timing. Again it uses a E0 and an E85 timing look up table and uses the current inferred E% to scale between the two extremes. Next it will play with that timing while monitoring the knock sensor to check how close it is to the optimum timing. If necessary it will adjust and infer its own idea about the E%. Then it will compare the calculation based on timing to the calculation based on fueling. Once that goes back and forth a few times the computer will declare the fuel "learned", use that in the fueling and timing algorithms, and wait for the next refueling event. If you look at the right scan tool you can see this happen in real time if you look at the "fuel learned" and inferred E% PIDs. Because Ford and GM use adaptive timing they tend to have a much lower drop in MPG when operated on E85 vs those other mfgs who have just allowed for a larger range of acceptable long term fuel trims. Now for those of you with a flex fuel vehicle, particularly if it is a GM you need to be careful with your refueling event. Never ever leave the ignition on while you are fueling, you'll mess up the computer's triggering of fuel learn mode. Also don't fuel on ground that is too far off level, ie keep all 4 wheels on the concrete pad if there is a ramp leading up to it. Fill your vehicle after it has reached operating temp, particularly in cold weather. Once you restart the vehicle drive it for at least 5 minutes. Don't fuel up at the pump, drive 20' to the parking in front of the store, shut the vehicle off and go in for snacks or to take a leak. Otherwise the vehicle can get its calcs way off causing things like off idle stumbles, general miss fires and reduced fuel economy. For GM there is a TSB noting that the first step in diagnosing driveablity concerns on FFVs that lack codes is to check the inferred E% and do a rationality check, including taking and testing a sample of the fuel in the tank. If the learned and tested E% are too far off then the solution is to manually reset the learned E% value to match what was tested in the tank. Then provide the customer with the above refueling cautions. Additionally stick to one grade as switching back and forth between various octanes can also mess up the computer's calculations.

  • Ronnie Schreiber Ronnie Schreiber on Jun 09, 2017

    Maybe it's Christian Koenigsegg, but someone invented a flexfuel system that works with a infrared spectrometer that tells the ECU the precise composition of the fuel, allowing engines to run on any combination of alcohols and napthas. I think GM originally used a capacitive sensor while Ford usedoptical sensors based on the refractive indices of the fuels.

    • Scoutdude Scoutdude on Jun 09, 2017

      Ford used capacitive sensors on the E85 vehicles. But they found out that they had enough other information to learn the fuel very accurately w/o a sensor.

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