Today I picked up my truck from the dealership because of a transmission issue. I bought the truck with 15K miles and it now has 55K. The truck has always had a rough idle and very poor MPG along with power issues. Well they agreed to hook up the Laptop and take it on a drive to show me all the different parameters. The fuel mileage would not go above 12mpg on the highway. Then he proceed to the test where it checks the fuel mixture. He said the truck was running about 15-17% lean under normal driving conditions and when I stomped the gas from a dead stop and raced the engine to 90MPH it was showing over 30% lean. I left the truck there for the weekend so they can figure it out.
My question is why wouldnt the check engine light come on in this situation?

What damages have occured from driving the truck for 25K miles running lean?

Is it posibbly a fuel Pump?

 

pump, filter, injectors, some intakes can produce lean conditions to varying extents, fuel rail pressure, etc.