To start....I have and have had zero CEL codes or warning lights. Truck is 2006 Silverado with 5.3. Does have a SE cam, springs, push rods and has a 93 tune. Long tubes with Borla exhaust system.
Truck has been great for almost 20k miles since rebuild. Steady 12-14 mpg average, awesome power. Almost over night my fuel mileage dropped to 8-9 mpg and idle is randomly inconsistent and smelling more fuel in exhaust than usual. I've checked plugs, wires, MAF tested ok. It's not the mpg I'm concerned about, it's not my daily, it is however the change of mpg almost like a flip of a switch. Any ideas?


Plugs have 4500miles on them

 

How old are your O2 sensors? As they age/get high mileage on them (75,000) they get lazy.

Also can you monitor your long and short term fuel trims in real time?

 

The o2 sensors were changed when it was rebuilt so roughly 4500 miles on them. I would've thought faulty o2 sensors would give CEL codes. I'm gonna have it put on the scanner this weekend if weather permits. Unfortunately I do not own one yet. I drove it tonight for the second time since and ran fine except at idle and stayed at 8-9 mpg. Since rebuild and tune it's stayed at 12mpg average so larger difference all at once. I have owned 4 of this model trucks but first with any performance motor work done so still learning. Thanks for any advice

 

I could be wrong but the plugs also look a little rough to me for 4500 miles.

 

Lazy O2 sensors do not set a DTC nor the Check Engine Light.

Did you happen to use Bosch O2 sensors? If so those are known to be trouble for GM engines. Switch to either GM Genuine or Denso O2 sensors.

Another thing to check - make sure gasoline is not leaking from the fuel lines between the fuel tank and the engine.

 

Is it throwing any codes? Does it seem to have lost power?