Hey guys I have an '08 Sierra 1500, 4.3L 6 cylinder. I had a misfire a good while back, took it in and the dealer changed 1 plug, 1 wire. I noticed right away after my next fill up that my MPG had went way down. I check every fill and prior to the misfire I got 22mpg hwy and somewhere around 18 in town, sometimes a little worse, but overall great miles for a full size truck. After the misfire the best I get is 15, my last fillup was 13.3 mpg.

Ive been to 3 service centers, all say the truck is fine, there is no problem. The last one had the truck for 3 days and checked everything they could imagine and said it was fine. I did get a recall on the wires and thought that was the problem, but after changing the wires MPG wasn't affected.

I'm to the point Im about ready to just live with it, but Im wondering if there might be anything you guys could add that might help. Its hard to get used to going from 650 miles out of a 30gal tank to 400.

I have an 09 Chevy Silverado regualar cab 4.3 automatic. The best I get with mine is 15.5 to 17 on regular unleaded. That is on a 20 mile one way commute with few stops. The truck does what I want and expect other than the MPG. I have had zero service issues and I use the recommended 5W-30 oil and AC filters. I have almost 20K miles. Let me know if you figure out how to up MPG a little bit without breaking the bank. It pulls my 20ft Glastron F&S in and out of the lake OK.

I forgot to tell you that my wife's 09 Tahoe with 5.3L and full trailer tow package gets better Highway MPG than my 4.3L Silverado. Go figure.

I'm glad my v8 quad cab gets better gas mileage than a v6

I would suspect a bad injector

The truck runs fine, could it be a bad injector and the only symptom be mpg decrease?

probably just the winter blend of fuel, the ecm is very sufisticated on that truck im sure its runnin good i would love to get that mpg in my truck

Well, the problem has been going on for a long time, so its not the fuel. Also, one of the local dealers "reset" the ecm when I explained the problem to them. Another dealer told me that was a big no no, and said what you said that is very sophisticated. They did it before asking me. But, the fuel economy wasn't effected by that, the problem was there before and after. Wish they hadnt have messed with the cpu though.

Also, the MPG I get would be normal for a v8 4wd, but not what I have. I was thrilled with what I got before this deal.

Make sure the 4wd isn't engaged! U shouldn't have dropped that much!

Andrew, go here www.dieselplace.com its all GM trucks (gas too) Post the same question and I'll bet you get an answer..

Andrew disconnect the battery and count to ten,then rehook it up,and drive it like you have been and see if the computer will reset.sometimes the computer adjust the fuel and air mixture to get it to run its best while misfiring and it takes time for it to average back out,but this should reset it.

Could be a bad o2 sensor

How many miles on are the truck if one wire and spark plug where bad I would assume you must be getting close to due for a full tune up. If so there may be some other plugs that are bad enough to not fire the air fuel mix as well as they should but not yet create a miss.

If that isn't the case then the cylinder that had the miss may have built up some carbon while the miss was present which will kill fuel mileage. If thats the case running a carbon clean procedure would help but would require new plugs afterwards.

Clean the MAF sensor.