This:
GM has developed a Virtual Flex Fuel Sensor (V-FFS) software program that calculates the ethanol content in the fuel, instead of using a sensor to measure it. When the fuel level in the tank increases as the vehicle is refueled, the computer recalculates the percentage of ethanol in the fuel and automatically changes the air/fuel ratio. To do this, the computer temporarily stops the operation of other emission systems and monitors the oxygen sensors to determine ethanol content. The test is done several times until calculations remain stable. This can take several minutes when the engine is idling, but much less time at higher fuel flow rates."
Quote:
In 2006 GM changed the way ethanol percentage of fuel was calculated by their trucks. This new system was deemed Virtual Flex Fuel System. Instead of using an additional sensor in order to measure ethanol content, the ECM had a software upgrade which allowed it to make this measurement by utilizing the fuel tank pressure sensor and level sensor signals. At a key on event the ECM looks to see if the fuel tank level sensor signals atleast a 2 gallon change from the last key off. If so, the ECM shuts the tank vent solenoid, monitors the fuel level and fuel tank pressure. These readings are applied to a formula which gives an ethanol percentage. The commanded air/fuel ratio is then changed according to ethanol percentage of the fuel.
^^ That obviously didn't work out too well in 2006!........ GM Brands use the Continental Ethanol sensor & it's mounted on the return line at the tank itself on current 2014/15 vehicles.
This part number suits 90% of the Current Chevrolet, GMC & Cadillac in the USA & the Holden Cruze and Commodores here in Aus, it was introduced in 2011.
Anyone who's dabbled with E85 tuning knows that you tune for the same AFR's as you do on 98 - the difference is that you need to add around 30% more fuel to achieve the same AFR's on E85 as you do on 98.