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CW

Accidentally Purchased an E85 Vehicle

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One thing that is important on current FFVs is the refueling procedure. There should be instructions in your owners manual. The condensed version however is: not to use a pump that makes where you park not be level, purchase at least 5 gal, and drive it for a few minutes/miles before shutting it off. Otherwise the computer can get confused as to what the E% in your tank is and your MPG, performance and drivablity can suffer. It shouldn't be much of a problem if you always use E85, but if you are forced to switch back and forth it is important.

 

When the forum gets the karma feature, I'll come back and award this post.  Thank you!

 

My service adviser called and "warned" me about using the E85.  He said his wife bought some in Colorado and "barely made it home."  I now presume she didn't know about these usage factors.

 

I am more of a fill up on the half tank guy anyway, so I'll always be buying more than five gallons and have a long drive home.  I'm relieved to know the computer keeps track of it.  EMAS, do I need to research the General Motors thread for more goodness like these?

 

 

Good stuff!

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CW, also the computer does better if you tend to stick with the same fuel and gets confused if you alternate fuels a lot since it takes several hours to optimize for a new blend. This would be more important if you tend to run your tank closer to empty, which it sounds like you don't do.

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CW, also the computer does better if you tend to stick with the same fuel and gets confused if you alternate fuels a lot since it takes several hours to optimize for a new blend. This would be more important if you tend to run your tank closer to empty, which it sounds like you don't do.

 

According to the service bulletin I saw a few years ago, shortly after they switched to the "virtual fuel composition sensor", it only runs the fuel learn program for a few minutes or miles, not hours.

 

When it senses a refueling event it freezes the long term fuel trim and the short term fuel trim instead feeds the learned E% correction factor. If the after refueling event isn't done right it feeds the LT FT and therefore won't adjust the timing and warm up enrichment curves properly.

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According to the service bulletin I saw a few years ago, shortly after they switched to the "virtual fuel composition sensor", it only runs the fuel learn program for a few minutes or miles, not hours.

 

When it senses a refueling event it freezes the long term fuel trim and the short term fuel trim instead feeds the learned E% correction factor. If the after refueling event isn't done right it feeds the LT FT and therefore won't adjust the timing and warm up enrichment curves properly.

 

It? You mean "2010 Chevy Avalanche"?

 

I'm speaking in generalities based on my experience, which has never been a Chevrolet with E85 (and hasn't been Chevrolet/gas since 1977).

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According to the service bulletin I saw a few years ago, shortly after they switched to the "virtual fuel composition sensor", it only runs the fuel learn program for a few minutes or miles, not hours.

 

When it senses a refueling event it freezes the long term fuel trim and the short term fuel trim instead feeds the learned E% correction factor. If the after refueling event isn't done right it feeds the LT FT and therefore won't adjust the timing and warm up enrichment curves properly.

 

It? You mean "2010 Chevy Avalanche"?

 

I'm speaking in generalities based on my experience, which has never been a Chevrolet with E85 (and hasn't been Chevrolet/gas since 1977).

 

I'm talking about FFVs that have "virtual fuel composition sensors" in general and all GM's so equipped specifically.

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One other important fact about the vehicles that have virtual FCS. You can run into problems if the battery goes dead or is disconnected. The computer "learns" what the E% is and the default after a reboot is E0. So you need to either run a couple of tanks of straight gas or E10 if that's all that is available, have a bi-drectional scan tool to read learned E% value before disconnect and then manually enter the value, or run straight gas for a few tanks to get as close to E0 as you can and then reset again. Otherwise when it fires up on the gas program it will adjust the LTFT and you won't get optimum performance, MPG and driveablity on E85.

 

Hopefully you won't need this info for at least 5 years or so. But I bet many places that sell batteries and install them don't have a clue about this little secret.

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One other important fact about the vehicles that have virtual FCS. You can run into problems if the battery goes dead or is disconnected. The computer "learns" what the E% is and the default after a reboot is E0. So you need to either run a couple of tanks of straight gas or E10 if that's all that is available, have a bi-drectional scan tool to read learned E% value before disconnect and then manually enter the value, or run straight gas for a few tanks to get as close to E0 as you can and then reset again. Otherwise when it fires up on the gas program it will adjust the LTFT and you won't get optimum performance, MPG and driveablity on E85.

 

Hopefully you won't need this info for at least 5 years or so. But I bet many places that sell batteries and install them don't have a clue about this little secret.

 

EMAS- same is true today in many of the customization settings and security systems for some autos. I believe that on my Saab one is supposed to use a small battery (but I am not sure if it needs to be 12v) across the terminals while replacing a battery.

 

I would also add that if you are changing fuels (gas to E85 or the reverse), one should always have key off during filling AND should do this fill when you are going to drive at least 5 minutes before shutting off the vehicle to allow the ECU to adapt. While these things will likely not cause a problem if you forget- no reason to try to avoid potentially problematic situations.

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I have never seen more than a minor stumble for a few secs when removing my conversion so the ecu saw a drastic change in e%. Short term trim can quickly adapt. I think this was a bigger problem for older, narrow band engines.

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I have never seen more than a minor stumble for a few secs when removing my conversion so the ecu saw a drastic change in e%. Short term trim can quickly adapt. I think this was a bigger problem for older, narrow band engines.

 

In this case we are talking about a Factory built FFV that uses a virtual fuel composition sensor. Much different than a conversion or a factory FFV that uses a "Pyhsical" FCS. Both styles of factory FFV do much more than just change the basic fueling. They also vary the cranking pulse width, warm up enrichment %, and timing curves base upon what the computer reads the E% to be. With the virtual FCS algorithm it learns the E% based on refueling events it "sees". When the car is shut down the computer stores the reading from the fuel level sensor so it knows with in a fairly narrow range how gallons of fuel is in the tank, when it is turned on it compares the first reading it sees from the fuel level sensor. If it is higher the computer to it's VFCS algorithm. It assumes that on 1 of 2 fuels could have been added gas E0-E10 or E70-E85, and I wouldn't be surprised if they didn't store a little ambient temp data via the IAT sensor values to guestimate the E70-E85. It then does the rough math to find the 2 extremes of the new blend in the tank based on the currently stored value for learned E%. It also freezes LT FT values. This all occurs before you can get the key from the on to the crank position. Then once it fires it looks at the O2 sensors and uses the ST FT to find which of the 2 fuel curves is the closest. It stays in this mode for a while until it has seen a range of operation and has assured itself of the correct E%. It then returns to it's regularly scheduled program where the ST FT is used to adjust the LT FT not E%.

 

Which brings up another point you are better off not stopping for fuel until the vehicle as reached normal operating temp.

 

Why is this all so important?

 

Well lets assume that you have a full tank of E85 the battery is disconnected and the computer's learned E% is reset to 0%. Now when the vehicle fires up it may be hard to start and suffer from a lean surge until it warms up and enters closed loop. Once it enters closed loop the ST FT will max out driving the LT FT way up as well.  Which will get the fuel close enough, but the cranking pulse width, warm up enrichment, and timing curves will be optimised for gas not E85 and driveablity and MPG can suffer. Refueling with more E85 wont help since it will be the same as what is in the tank it won't drive a change in learned E%.

 

Which is why the service bulletins I've seen instruct to read and record the learned E%, (and other learned values) before disconnecting the battery. Then once the battery is reconnected use a bi-directional scan tool to put those values back in. Things like the exact Idle speed control motor position for different desired idle speeds can be relearned simply, but not so for E%.

 

Since the new learned E% is base don calculations of the old learned E%, continued refueling events that do not meet the criteria can cause the E% error to slowly drift higher and higher again leading to less than optimum driveability and MPG.

 

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I have never seen more than a minor stumble for a few secs when removing my conversion so the ecu saw a drastic change in e%. Short term trim can quickly adapt. I think this was a bigger problem for older, narrow band engines.

 

 

In this case we are talking about a Factory built FFV that uses a virtual fuel composition sensor. Much different than a conversion or a factory FFV that uses a "Pyhsical" FCS.

 

Read it again... not a FFV.  ;)

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