Thank You for writing into AskPatty!! If your GM model is a flex fuel type, you may use E85. If NOT a flex fuel, you should use no more than an E10. This is ethanol blended with 85% ethanol and 15% gasoline. Using this blend in a non flex fuel vehicle can set the check engine light on, due to the burning process can seem like your vehicle is running too lean. Most non-flex fuel vehicle are not equipped with sensors to detect the ethanol content.
A. J. Valle
Here is a video clip for further reference: http://green.autoblog.com/2007/08/07/can-any-car-use-e85/
To be sure you should check your vehicle's manual or look on the label to the side of your gas cap. Also, GMC labeled the back of many ethanol friendly vehicles with "Flex Fuel." That said, even if your vehicle can run on Ethanol, you may not wish to use it. I found that when I used Ethanol in my Yukon XL, the fuel economy was reduced by about 40%, which didn't make up for the slight reduction in cost per gallon to fill up with E85.
〉 Answered on Jun 21st, 2012 by Kristin Brocoff, Director of Corporate Communications at CarMD.com
GM didn't come out with the Flex Fuel model until 2003, so unless you see a symbol on your vehicle that says "Flex Fuel", you can't run it. Check your owner's manual as well, it should tell you the type of gas your engine runs on.
Here's a website you can check out as well. It looks like only the 5.3L has it.
〉 Answered on Jun 21st, 2012 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines