I've only had the truck for about 5 years, but when I bought it the fuel economy was better than it is now. I have a tuner/code reader and I also know that the long and short term fuel trim numbers are higher than they should be - causing me to burn more fuel than necessary. I've read that a vacuum leak can cause these symptoms. Also, when towing I notice that when the truck shifts all the way out and I go up a hill or put more load on the truck, the air flow from the AC system will automatically shift to the defrost and back again which is also indicative of a vacuum leak. I've been trying to chase this down for a while and I'm fairly confident that the problem isn't under the hood, but I also have had the dash apart and cannot seem to find an issue there either. Anyone else been able to run this down? Please share your experience with me - I would greatly appreciate it!

 

Check the PCV elbow behind the intake. Also check the vacuum accumulator for the HVAC and lines to it behind the battery.

 

Roger. Will do. Thank you.

 

exactly what I’d suggest

 

What exactly are the fuel trim numbers and what are they when the engine is revved up? When you say higher than normal do you mean they are positive numbers?
A smoke test would be the best way to find a vacuum leak. In a Chis Fix video, he showed how to use a shop vacuum hooked to the exhaust pipe with the vacuum reversed blowing up the pipe and spraying with soapy water all the suspected leak points. Could even be the brake booster leaking as I saw in a south main auto repair video.