What is cold start?? Is that the same thing as closed loop?

 

*technically* "cold start" is anything lower temperature than desired operating temperature.

since I don't know the oil temp in my cobalt (I don't have an oil temp gauge) I use water temp -- I don't put my car in gear until the water temp is 100 degrees (F)

but I don't boost until 170

to the poster that said "oil doesn't affect fuel mileage" ---it can. conventional oil has greater viscous friction than synthetic. Look at the API circle on various oils for the "resource conserving" mark, and read up.
OR, drain whatever is in your car out and put 10w40 in it, and report back with your fuel mileage

 

I've had the GMS1 installed since the week it was released in '09. I certainly didn't have it installed to help mpg but I did notice a slight improvement. I was averaging 28-29 running 93 octane exclusively until I parked the car a few weeks ago; mostly hwy and secondary roads. That said, whatever is ailing your car has probably been covered here. The only thing coming to mind is that if the dealership was incompetent enough to disregard whats on the filler cap they may have jacked up something else as well. Check for this-[URL="https://www.cobaltss.net/forums/2-0l-lnf-performance-tech-153/line-broken-intake-tube-294387/"]

Also if you've modded the intake/filter in any way revert to stock,reset ecm and try again.

 

GM dealers still have conventional oil for everything older than a 2011, unless it came with Mobil one.

 

did you try resetting the average fuel economy on the dic?