Diagnosing poor mileage

[alvocado]

I’m trying to diagnose what’s causing a noticeable drop in gas mileage and have a question regarding some scan data related to assessing the fuel injectors. Mileage on two recent 500 miles trips was down 3-4 mpg. 
 

 2011 Yukon 5.3, 200k miles. Plugs and wires have 50k on them, air filter is clean, O2 sensors have been replaced. Fuel pump was replaced in September. 
 

LT FTRM1 is reading -5.5% and LT FTRM2 is -6.3%, both indicating rich running condition. ST FTRM bounces as from approximately-15 to +15%. 
 

Fuel pressure reads 44 psi on both sides. 
 

Any advice on interpreting the data and confirming if the injectors are an issue would be appreciated. 

 

Edited November 25, 2023 by alvocado
Added details on replacement parts

[ember1205]

Do you have any pending codes or have you gotten a CEL? When did you last change the oil?

 

Are you pulling fuel trim readings at idle or while driving?

[alvocado]

Oil and all drivetrain fluids were changed last week/1000 miles ago. No codes. I took the fuel trim readings while driving around the block so low speed only. 

[ember1205]

Those trim levels don't look all that bad to me. It isn't until you get to 10% or more that I would be concerned. What readings are you seeing at idle? If the readings at idle are high then drop back down when RPM's go up, there might be a vacuum leak. Other things to check would be the MAF sensor, or you might have an O2 sensor (or two) that are getting a bit flaky.

[Jsdirt]

Yeah,  those fuel trims aren't bad at all. My Flex 5.3 in a '07 Silverado 1500 had from -12% to -18% fuel trims any time I threw the scan tool on it. Mileage wasn't great, but wasn't nearly as bad as 3 MPG.

 

I'd be checking brake rotors  / drums for excessive heat. You could have a hung up caliper or collapsed brake hose causing the brakes to not retract. Parking brake cables could be rusted and both brakes applied fully. Tire pressure could also be low.

 

With mileage that bad this should be super easy to track down. If it's not dumping black smoke out the exhaust, then it's a mechanical problem elsewhere. Or, you've got no oil in the engine or other parts of the driveline.

 

Or you've got a bad fuel injector (or 8 )but the chinese o2 sensors aren't reading properly ...

Edited November 26, 2023 by Jsdirt