Switching between E85 and gasoline

Z-Michigan · Dec 6, 2007

I have a 2007 Chevy Suburban with the 5.3L flex fuel engine. I have a few local stations that sell E85, but I can't be assured of having one convenient for every fill-up I need. My owner's manual says I should not regularly switch back and forth between E85 and gasoline. Can anyone tell me why? Will it actually cause wear or mechanical failure?

My guess is this: the 2007 model year 5.3L engine does not have a fuel type sensor and therefore presumably determines what fuel it's burning based only on the oxygen sensors. I can guess that after each E85 fill after using gasoline it runs lean at first (metering fuel at gasoline rates), and after a gasoline fill after prior E85 in the tank, it would run rich at first (metering fuel at E85 rates, which are higher). I don't abuse the vehicle and am not running over 1/3 throttle for the first several miles after a typical fill-up. If I'm guessing right on the fuel metering issue, will this cause me any problem?

BTW, I'm not looking to debate the merits of E85 - either cost or environmental. I have read the following thread:
http://www.tractorbynet.com/forums/...104535-chevy-hybrid-flexfuel-e85-ethanol.html

and I am not looking to re-hash it, just looking for a narrow answer to a narrow question about switching between the two fuels.

Thanks.

Wesdor · Dec 7, 2007

Good definition of what you want. Although I do not know for sure what is the reason, I believe you have the basis for the problem.

I have a 2000 Chrysler mini van with Flex Fuel capabilities. We often use E85, but recently it has been inconvenient to detour to fill up there. I am under the impression that the sensors in the engine have to make adjustments and switching back and forth causes problems (just as you so correctly described).

In my situation, I have gone from an average of 21 mpg to 19 mpg using regular gasoline. About time to take the van into the shop and see what is happening. I do wonder if the switch between E85 and regular gas has anything to do with it.

Diamondpilot · Dec 7, 2007

I have a 2007 F-150 Supercrew 4x4 with 3.73 gears that I have been running E85 through. It runs great on the stuff but with 87 octane I get 15.5 mpg mixed driving. With E85 the best I have seen is 11.5 mpg. This week 87 cost $2.80 in this area in round numbers and the E85 is around $2.15. Each way it cost $.19 per mile to drive my truck. It is great for the stations because you stop more and get a pack of gum and a coke, that is where they make the real money.

Chris

shvl73 · Dec 7, 2007

I drive an '07 Impala and fill up wherever I can. I never even look for E85. If its there fine, if not, fine. Its got 55,000+ miles on it and 25.9mpg average since new, based on the dash readout.

Travelover · Dec 7, 2007

I think you have got it basically right. The new engine controls have a learning curve to optimize performance and if you keep changing back and forth between straight gasoline and E85 it is always in the learning mode.

Z-Michigan · Dec 8, 2007

shvl73 said:
I drive an '07 Impala and fill up wherever I can. I never even look for E85. If its there fine, if not, fine. Its got 55,000+ miles on it and 25.9mpg average since new, based on the dash readout.

Just to clarify, are you switching between E85 and gasoline depending on whether E85 is convenient, or are you just running gasoline 100% of the time? It isn't clear to me.

shvl73 · Dec 8, 2007

Z-Michigan said:
Just to clarify, are you switching between E85 and gasoline depending on whether E85 is convenient, or are you just running gasoline 100% of the time? It isn't clear to me.

Mostly just gas. I stop only when low and always where convenient. I have used E85, just not regularly.

KAB · Dec 13, 2007

From GM service document # 2000114 (these a excerpts)

For 2006, GM has changed the method of "sensing" the alcohol content of fuel in E85 compatible vehicles.

Important: Only vehicles designated as E85 compatible are capable of operating on fuel with greater than 10% ethanol content. See Service Bulletin Number 05-06-04-035.

For 2006 and 2007 models, E85 compatible vehicles marketed in North America no longer use an alcohol sensor to determine and adjust for the alcohol content of the fuel in the tank. Instead, the vehicle "learns" (calculates) the alcohol content of the fuel, (and subsequent mixture in the fuel tank), through a series of measured adjustments.

A minimum of 11 Liters (3 gallons) must be put in the tank in order for the vehicle to identify/recognize a "re-fueling event." It is necessary to turn the ignition "off" in order to have the re-fueling event recognized; however local safety regulations should be followed.

---- SNIP------

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 vehicle has been driven for 11 km (7 mi) at full operating temperature) to comprehend and calculate the composition of the new blend in the tank.

For best driveability, the vehicle should be driven for at least 11 km (7 mi) following a refueling event when switching between gasoline and E85.

----SNIP----

shvl73 · Dec 14, 2007

Thanks KAB.
That makes sense, as I've never noticed a difference.

montanaman · Dec 14, 2007

Diamondpilot said:
I have a 2007 F-150 Supercrew 4x4 with 3.73 gears that I have been running E85 through. It runs great on the stuff but with 87 octane I get 15.5 mpg mixed driving. With E85 the best I have seen is 11.5 mpg. This week 87 cost $2.80 in this area in round numbers and the E85 is around $2.15. Each way it cost $.19 per mile to drive my truck. It is great for the stations because you stop more and get a pack of gum and a coke, that is where they make the real money.

Chris

Thanks for the hard numbers! Though I don't have a flex-fuel vehicle, mine just uses overpriced diesel, my brother-in-law just bought a flex fuel Chevy truck and I can relay your results to him.

Ken