4th gen tahoe . FLEX fuel sensor gone wacko

[mach]

so. disclaimer. i am the farthest thing from a tuner. but im checking one out to see why gas mileage is less than stellar. quick drive and scan of STFT shows fairly decent tune it seems. until i notice its commanding 12.93AFR,and in the 10's under PE. i then scan the alcohol content and its at 30% (which should be pump 87). so....as a check, i set full stoich table to 14.6 across the board and also disabled flex fuel. now the scan shows commanding 14.6 as intended and 0 % ethanol. is that normal for these virtual sensors to go bonkers like that? i then proceeded to use my WB and set up to dial the maf in since it was now approx 15% off. once i made a few logs, it is spot on everywhere now. just making sure this is the right thing to do here as ive not been around one that did this yet.

[5FDP]

Yes, the virtual flex fuel is garbage and I've seen it happen many times where it gets stuck at 30% or even higher for no reason even when the customer has run regular pump gas for thousands and thousands of miles and years of driving.

I always disable it or install a real sensor.

[mach]

Yes, the virtual flex fuel is garbage and I've seen it happen many times where it gets stuck at 30% or even higher for no reason even when the customer has run regular pump gas for thousands and thousands of miles and years of driving.

I always disable it or install a real sensor.

correct. this thing has been this way for a couple years i hear. lol. i reckon i will carry on . it was quite doggy on first drive before i found this

[Casey A]

i then scan the alcohol content and its at 30% (which should be pump 87).


Can you explain to a noob how to scan the alcohol content? I think I may be experiencing an issue from the same problem. My commanded is 11.63 on the scan tool, but 14.6 on the tune file....

Casey

[Casey A]

I always disable it or install a real sensor.

Could you explain what you mean by installing a real sensor?

Casey

[Innojazz]

Here's a link that discusses adding the flex fuel sensor into a Gen 4. Good YouTube video for explanation in the first post.

https://www.corvetteforum.com/forums...r-install.html

[blindsquirrel]

"Virtual" flex fuel:

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 operation.

Switching Between Gasoline and E85
No special precautions need to be taken when switching back and forth between gasoline and E85 other than re-fueling events must be 11 liters (3 gallons) or greater, and the vehicle must remain in closed loop long enough, usually by the time the engine has maintained full operating temperature, to calculate the composition of the new blend in the tank.

Real flex fuel that uses a real sensor to measure the actual alcohol content:

Fuel Composition Sensor Description
The fuel composition sensor (FCS), or flex fuel sensor (service parts term), measures the ethanol-gasoline ratio of the fuel being used in a flexible fuel vehicle. Flexible fuel vehicles can be operated with a blend of ethanol and gasoline, up to 85 percent ethanol. In order to adjust the ignition timing and the fuel quantity to be injected, the engine management system requires information about the percentage of ethanol in the fuel.

The FCS uses quick-connect style fuel connections, an incoming fuel connection, and an outgoing fuel connection. The two connections have different diameters, to prevent incorrect attachment of the fuel lines. All fuel passes through the fuel composition sensor before continuing on to the fuel rail. The fuel composition sensor measures two different fuel related parameters, and sends an electrical signal to the powertrain control module (PCM) to indicate ethanol percentage, and fuel temperature.

The fuel composition sensor has a three-wire electrical harness connector. The three wires provide a ground circuit, a power source, and a signal output to the PCM. The power source is vehicle system voltage, +12 volts), and the ground circuit connects to chassis ground. The signal circuit carries both the ethanol percentage and fuel temperature within the same signal, on the same wire.

The FCS uses a microprocessor inside the sensor to measure the ethanol percentage and fuel temp, and change an output signal accordingly. The electrical characteristic of the FCS signal is a square-wave digital signal. The signal is both variable frequency and variable pulse width. The frequency of the signal indicates the ethanol percentage, and the pulse width indicates the fuel temperature. The PCM provides an internal pull-up to five volts on the signal circuit, and the FCS pulls the 5 volts to ground in pulses. The output frequency is linear to the percentage of ethanol content in the fuel. The normal range of operating frequency is between 50 and 150 Hertz, with 50 Hertz representing 0 percent ethanol, and 150 Hertz representing 100 percent ethanol. The normal pulse width range of the digital pulses is between 1 and 5 milliseconds, with 1 millisecond representing -40?C (-40?F), and 5 milliseconds representing 125?C (257?F).

[blindsquirrel]

[I][COLOR="#FF0000"]Can you explain to a noob how to scan the alcohol content? I think I may be experiencing an issue from the same problem. My commanded is 11.63 on the scan tool, but 14.6 on the tune file....

Open VCM Scanner, connect to vehicle. Click 'Add Channel'. Search for 'alcohol', add Alcohol Percent to your channels list.

[2xLS1]

07-09 are the worst for doing virtual flex wrong.

[Casey A]

Thanks to all!!!

I will check it out later!

Casey

[mach]

so, couple things i found. i hooked WB up for the original tune ,and even though it shows commanding 12.9 and 30% alcohol content., it has 14.6 coming out the pipe. maf is pretty spot on with narrowbands. so the next part is confusing. if i disable flex, then my commanded goes to 14.6 as intended, and content goes to 0. now my maf is 15%lean across the board. im guessing the maf was tuned with the alcohol content at 30%? but what confuses me is that the narrowbands are right in line with my WB at 14.6. so, it seems it is ignoring the commanded part somehow. so , if the narrows only correct to stoich(14.67) ,then how does it richen anything when you actually do have e85 in the tank if the narrows are going to try to bring it back to 14.6? or is that not how they work? ive never found anybody that can tell me if an 02 will only correct back to 14.6, or if it will correct to whatever you set stoich to.

[mach]

so i went ahead and tuned it with flex disabled. got everything spot on and ve really close,as it was out of wack also. truck seems great now with everything back on and tuned. im just not sure how disabling flex made everything so far off,yet now its basically back to running like it did AFR wise before. i turned it off

[Casey A]

For starters, Alcohol (ethanol) is not as efficient as gasoline. You need more of it to reach that stoich need. On the other side, ethanol can give more horsepower, especially in higher compression engines. The flex system monitors the amount of alcohol in the gasoline, and increases fueling as the alcohol percentage goes up. When the flex system fails, it usually does so by misreading the alcohol content by more than what is in the actual gas. There for, if you tune with a incorrect flex reading, you are actually tuning an engine to run lean (The engine thinks it is using more fuel due to the higher ethanol blend). Then, when you turn the flex off, the engine goes back to the full gas measurements and leans out the motor.

If you dont plan to use blended gas, turn the flex off, and don't worry about it (most pump gas is 10% or less and this is fine). If you do plan to use a fuel mix, i seem to find a lot of people saying to bypass the virtual sensor, and install an actual sensor inline. Tune with the flex off, and turn it back on when you are finished. Just make sure you are not using a high alcohol content while you tune.

Casey

[mach]

For starters, Alcohol (ethanol) is not as efficient as gasoline. You need more of it to reach that stoich need. On the other side, ethanol can give more horsepower, especially in higher compression engines. The flex system monitors the amount of alcohol in the gasoline, and increases fueling as the alcohol percentage goes up. When the flex system fails, it usually does so by misreading the alcohol content by more than what is in the actual gas. There for, if you tune with a incorrect flex reading, you are actually tuning an engine to run lean (The engine thinks it is using more fuel due to the higher ethanol blend). Then, when you turn the flex off, the engine goes back to the full gas measurements and leans out the motor.

If you dont plan to use blended gas, turn the flex off, and don't worry about it (most pump gas is 10% or less and this is fine). If you do plan to use a fuel mix, i seem to find a lot of people saying to bypass the virtual sensor, and install an actual sensor inline. Tune with the flex off, and turn it back on when you are finished. Just make sure you are not using a high alcohol content while you tune.

Casey

i get that....but the thing is. the old tune ,although showing 30%E and commanding 12.9AFR, actually put out 14.6 the whole time out the pipe via wideband. when i turned the alcohol off, the maf and the VE table went lean. thats what im not seeing.

[Casey A]

Mine was and partially still is doing the same thing.

If you or someone tuned the truck with the bad flex sensor, than the engine was tuned to 14.6 output while commanding a 12.9. The 12.9 is the added fuel the engine is trying to compensate with to make everything happy, while the 14.6 is what the AFR reads the engine is burning.

When you tune, you are telling the computer that what is commanded is what is returned. So if the computer requests 12.9, and you see 14.6 and tell the computer this is right, it will put 14.6 thinking it is 12.9. Think of it this way, if you teach someone the color red is actually called green, they will say green when they see red. This is sort of what is happening. Once you turn off the Flex, the computer brings the fuel down from 12.9 to 14.6, but at this time, 14.6 is probably somewhere closer to 17-18 on the AFR.

On my truck, I added a larger cam, and when I went to tune, very little was needed. In fact, at some points I actually pulled fuel out. This was because my computer was commanding an 11.63 (53% alcohol on the broken sensor), which satisfied the air consumption for the new cam almost perfectly. The only reason I knew something was wrong is my warmup idle would vary in the 17-21 range. This is because the warmup is pulled from the normal fuel MAF and MAP tables before alcohol gains. As my engine warmed, it slowly moved to the alcohol tables and richened up. When I turned my flex off, my entire MAF curve was 20-25% off!

Hope this helps
Casey

[mach]

oh, i know somebody tuned it before for sure. and it had to be in the state it was in when i got it.they couldnt have tuned with afr error though, or they would have noticed something WAY off id think.
but riddle me this. IF i take the tune after its correct and then change the stoich from 14.6 to say, 13.5. shouldnt the 02's bring it back to 14.6? because in the state i got it where it was commanding 12.9,the 02's were happy within 5% trim as normal(while making 14.6 on the wideband). OR, do the 02's follow the stoich thats entered in the tune?

[5FDP]

The o2 sensors bring it to whatever the current stoich is.

If the wideband is on the gas scale, then it's always going to be 14.7ish. It doesn't matter if you have E60 or E0, the wideband is going to show 14.7ish.

Using Lambda is easier when dealing with this stuff.

[LSandlteditor]

One thing I have noticed though, is that the Virtual System goes haywire if you pump a different blend, and drive a short distance and shut the car off. That almost always causes it to get stuck in a limbo, from what I have experienced. That's why it's important to get the flex fuel sensor and even then it's wise to keep driving a few miles to give the system a chance to learn the new content. Just my 2 cents

[mach]

The o2 sensors bring it to whatever the current stoich is.

If the wideband is on the gas scale, then it's always going to be 14.7ish. It doesn't matter if you have E60 or E0, the wideband is going to show 14.7ish.

Using Lambda is easier when dealing with this stuff.

so, if i were to just go in and change stoich only from 14.7 to 12.0. ,then the 02's would suddenly try to richen the tune?

[5FDP]

I think you know that answer.