Poor Gas Mileage After Tune Up

[txranger]

I have a 2007 Yukon 5.3 Flex purchased new Oct 2007. 3 Weeks ago at 98K miles I changed the Plugs & Wires (AC Delco 41-110 & 748RR) and removed and cleaned the throttlebody and did a throttle position relearn with the GM Tech 2. I was previously getting around 16 MPG as a daily driver. After the tune up, I'm looking at a best of 14 MPG for the past 1K+ miles. Prior to the tune up, the only maintenance items have been regular oil changes and one transmission service at 50K miles. I use either Shell or Chevron regular fuel (87 octane). Has anyone else experienced this, or have any ideas?

 

[phileaglesfan]

Since you noticed it after the tune up then it is most likely something you did during the tune-up. Different plugs? Plug gap? Wires? Throttle position relearn?

 

[dsl100]

i would suspect you may have a lean condition (remove throttle body did not get it resealed) have your tech put a scanner on it and watch the o2 sensors ,when operating 2 will give a - value ,2 will give a number in the positive side .they should equal 0 when added together

 

[Zed 71]

Check the spark plug gaps, confirm connections are tight, and throttle position relearn was done correctly.