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Why Wont My 2000 Gmc Sierra Run Better Mileage?

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I have a 2000 GMC Sierra 4X4 with the 5.3 with 145,000 miles on it. This truck will just not run any mileage at all. I'm adveraging around 12 in town. I have replaced the plugs and wires with no change. I do not drive with a heavy foot either. The truck is all stock, no oversize tires etc.


I do however notice that the truck seems to be running alittle rich while ideling but it has not thrown any codes. Could this be something going on causing it?




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have you ever changed your oxygen sensors? over time oxygen sensors can get "lazy" - they perform poorly, yet don't set any diagnostic codes. at that high of mileage if they haven't been changed it would most likely be beneficial to do so. This mainly refers to both oxygen sensors before the cats, don't worry about the two after the cats unless a code comes on.

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