I have a 2009 GMC Sierra w/ a 5.3L V8, stock as a rock. Can I disable the flex fuel? When I hit the gas suddenly it lags. My previous GMC 5.3L (99 model) had the same engine, but had much better throttle response even though it was basically the same engine. I have a feeling its the flex fuel. Any one experience this? Can I disable the flex fuel?