Difference in Flex fuel and non flex fuel

[Bubba66]

Hello .. I have an '07 GMC Sierra Crew Cab with the 5.3L w/ flex fuel w/towing package 3.73 gear ratio. I bought it new and now it only has 20,000 miles on it. My only problem is crappy mpg. I live in Corpus Christi, TX so all flat land. I have tried every brand of gas and I use synthetic oil and the best I get is 16.5 mpg. I have took it in to several dealerships and was told nothing is wrong with it. When I bought this truck it had a big decal saying 20 - 22 mpg and I am nowhere near that. Last month I drove to Houston kept it on cruise at 65 mph, really babied it the whole way and thats when I was able to get 16.5 mpg. I have done my own research and found this exact same truck w/o the flex fuel option gets 18 - 20 mpg. So I want to know what I have to change to do away with the flex fuel option. I know I have to regap spark plug and change ECM but is there anything else.

[stumpuller454]

Just get the ECM re-tuned (probly much cheaper than a factory ecm and factory tuning), contact westers, wheatleys, nelson, blackbear etc, should be able to gain you 1-2 mpg easy. have you considered changing the exhaust?

Some trucks fall short of what the EPA says they will do, and some excell, its a funny thing.

[Bubba66]

OK thanksfor the info .. I was hoping not to buy a whole new ECM ..so you can re program them ... and I bet the dealership want a fortune for that.
I have beed thinking about changing the exhaust and cold air kit ... haven't decided on Flowmasters or Magnaflows .. any suggestions

[leolkfrm]

you can change the ecm and it will still be flex fuel, all it means is it was built with parts that can withstand alternative fuels like ethanol.
the dealer should have a metering guage to confirm mpg, are you sure the OD and lockup are working?

[Bubba66]

Hmm good thing to know ... I just know I have driven other trucks with exactly the same options but without the flex fuel option and they get 1 - 2 mpg more than I get. I watched the tach and it shifts 5 times.. I assume the 5th shift is lockup.

[hotrodpc]

Don't be disappointed Bubba on the Stikcer MPG issure. EPA FINALLY realised a vehicle inside a builing with 4 concrete walls around it, driving on 2 rollers does not give accurate measurements for mpg. The manufactures are not the ones who provide this information. It comes from the EPA. So it is rare to hit those marks on the sticker. If you read real litty bit fine print, they also give the worst and the best, then the big # you actually see is the average. But, as of 2008, the EPA standard for estimating mpg are much more accurate on the Maroney sticker. BTW did anyone know that Mfr sticker on a vehicle is called Maroney sticker and do you know why? Great piece of history. I may tell that story someday. So where the 2007 mpg may be 20-22, and if not changes were made in 2008 on the like model vehicle, it is very common to see that it is now 3, 4, 5 and even 6 mpg differant, usually lower. So find someone that has an equaly equiped 2008 and ask then what their maroney sticker said for mpg. That will get you closer in the ball park for expectations.

[hotrodpc]

BTW Leo, great tip. I know a guy had a late 90's Bonneville. Never got good mpg like others at work who had the same car. Dealers never found a thing wrong. Now comes, the guy has like 50,000 miles on it. He had one of the other guys that owned one at work go to lunch with him, and they started talking about their cars being alike and the mpg issue came up. The guy looks at his tach and said you must have lower gears, but that does not make sense in a transaxle, and the guy made the suggestion he was not in OD. The owner said yes he is. They stopped and counted shifts as they picked up speed. Here this vehicle is with 50,000 and NEVER had OD right from the day he bought it. It was a defective solenoid in the trans. The owner happy he got it fixed and improved mpg, but he was PO'ed that the few dealers he took it to did not find it much sooner and said his engine is now wore more than it should be, the mpg he wasted for 50,000. Do you think GM compensated the guy??? They sure did, but not willingly. It was settled out of court.

[scavanger]

If you reprogram the ECM be sure to change it back before it goes into a dealer. They now check for alterations. ANY alterations can void the warrenty.
Try unhooking the bat ground cable for a few hrs. This will give the ECM a chance to relearn. Just might help.

Scav.

[Bubba66]

Thank you guys for all the info. And I know the window sticker is a joke and don't believe everything you read ... It is just where I work there are 7 of the trucks. These 7 trucks have exactly the same options 3.73 gear Texas editions w/20 inch tires and all crew cabs ..the only thing difference is color and 4 w/o flex fuel. We have had many lunch break chats comparing notes. And we have found the trucks w/o flex fuel option get 2 - 3 mpg on the highway and driving normal

And the worst part is my wife has an '07 yukon with same options 5.3L, 3.73 gear and texas edition 20" tires and w/o flex fuel and on the highway and keeping it under 70 mph and we get 20 mpg ....just makes you scratch your head and wander.

[hotrodpc]

hmmm, I think I'd be frustrated too.