OK, all you gearheads out there, question number one that's puzzling me today involves filling the tank. Can anybody tell me how my Flex-Fuel vehicle knows how much alcohol I'm feeding it? It's interesting that I'm supposed to put no less than three gallons of fuel into the tank when I switch between E85 and normal gas, and also that I'm supposed to drive at least seven miles after filling up with something different than what's been in the tank.

So ... what's it doing for those seven miles? Is there a hamster in the tank and a video camera to monitor how drunk he gets? Is it different conductivity or magnetic properties ... or what?

The inquiring mind wants to know!

 

There is a sensor that can detect the ethanol content of the fuel. I believe it works by sending light through the fuel stream. I'm guessing if you're switching that it needs time to accurately gauge the content of the fuel if it is obviously mixed. Maybe it needs time to see if the ethanol content is high enough to add timing?

 

Im curious about this too. All Ive heard is that theres a sensor in the tank but I have no idea how it works

 

atleast on the older trucks GMT800's there is a fuel composition sensor inline on the main fuel line and it bolts up to the transmission crossmember.

 

fuel trims.

x amount of ethanol in the fuel will trigger the fuel trims to change, a lot more will cause the base commanded afr {stoich} to change to compensate for the swing in fuel trims. above a certain amount of fuel being put in will trigger this change. every time you fill up, regardless of fuel it goes into a relearn mode for 10 or so miles where you do not have wot fueling. flashing a tune will lock the afr into the last stoich ratio, but will cause a slight loss in power. this is right after the fill up, or durning fill up.

 

Does it do this with tunes as well? Or when you get it tune, it kind of locks your vehicle into the tuned fuel (93 octane tune for me)?

 

I'm getting tuned by Wheatley a week from today and he says with his tunes you continue to have the flex fuel capability. When E85 is used the ECM will still add 3 degrees of timing.

 

I wonder if wheatly tunes for that, or is this a universal equation within the pcm tables?

 

Flex fuel vehicles have a spark correction table to be used in conjunction with E85. Maybe some tuners cheap out and leave it alone/disable it, but not Wheatley. This is a huge plus IMO because I live very close to several E85 stations and have run it exclusively in my trucks. If I go out of town and have to fill up then I will use 93, but that is the only exception.

 

Like area47 mentioned it uses the trims and a/f to estimate the ethanol content then adjusts everything accordingly. Ive heard it termed as a "set algorithm" in how it finds the gas/ethanol content, but im not sure if that is completely correct or not.
I believe they quit using the inline sensor because they are really darn expensive from what i remember hearing..