I installed a cold air intake on my 01 4.3 Sonoma. Ever since the installation my mileage has dropped about 1 mile per gallon. I do not drive fast or floor it to hear the roar. Can I expect mileage to improve when the outside temperature starts getting warmer? My goal was to increase mileage, not decrease it!!!!!!!!!
Intake and exhaust modifications usually don't help MPG unless you're towing. A cold-air intake or low-restriction exhaust system will allow the engine to flow more air at wide-open throttle, which increases maximum power. But when cruising, you're using the throttle (a restrictor plate) to limit the amount of air that the engine gets. So you decrease the restriction of the intake or exhaust, but when cruising you increase the restriction of the throttle to compensate. The end result is that it doesn't matter to part-throttle cruising MPG at all.
When towing it can make a small difference since you spend a lot of your time at wide-open throttle.
You can expect better mileage when the switch from winter to summer gas takes place over the next month or so, depending on your location.
I have another explanation. I believe that your truck uses a MAF sensor. If it does and you reinstall into the cold air kit chances are its not sending the right information to your trucks ECM. If the sensor is in a larger diameter housing it actually thinks its getting less air. I found a bunch of Toyota nuts that have installed cold air kits. Their trucks would drop in mileage just like in your case. Someone suggested calibrating the MAF and it worked. They noticed on a dyno that after the cold air kits were installed the trucks where running richer. After calibrating the MAF to the proper fuel/air ratios they had more power yet and an increase in MPG.
Also If your using a filter that you can clean and re-oil. That oil can collect on the sensor and make it read improperly. So you may need to clean the sensor. But my guess is your MAF is out of range.