2012 GMC Yukon Denali 6.2 worse gas mileage. No codes or check engine light.

[testdepth]

While sitting in the truck at idle I can watch the computer display "Average fuel mileage" it starts dropping while just sitting there. 1/4 tank of gas in it. The computer started at 8.2mpg which is low and in about 15 minutes the computer read 6.9 mpg. My wife is complaining that this truck is using too much gas. This truck has the 6.2 flex engine and normally around town its about 14mpg and thats what the display shows.

I own a very good active scan tool that allows me to check and activate most things on this truck.

No check engine light. No codes are shown. No misfires on any cylinder. I removed and cleaned the MAF sensor and the scan tool shows it functioning.

I do not smell any fuel while I'm under the truck and don't see any fuel leaking.

About 3 months ago I had to replace the Fuel pump, EVAP canister, EVAP Solenoid and purge valve.

Does something have to be reset so this system gets better gas mileage? ECU?

 

[Plumminfool]

Most vehicles do this. If you are sitting still, you are consuming gas but not traveling any distance.

 

[Ken Ford]

Mine drops like a rock sitting.

 

[haole396]

Does your scanner have the ability to read data? If so, check the short and long term fuel trim. You want to see it in the +or - single digits. If it isn’t, it’s running rich/lean with the computer trying to compensate.

 

[testdepth]

Well I found either the problem or part of the problem. We recently had the fuel pump changed out. I just filled the tank and brought it home. It was sitting in our carport and my wife starts smelling gas and looks under the truck to see a puddle. The car is parked and there was gas actively dripping off the side of the tank. Its coming from the drivers side just under where the pump is mounted in the tank. I'm guessing the guy that replaced the fuel pump screwed up and didn't get the lock ring tight or didn't put the o-ring in.

 

[testdepth]

In case anyone needs it here is how to drain your full gas tank on a 2012 Denali or similar vehicle. On the passenger side fuel rail there is a pressure test fitting. Unscrew the schrader valve and attach a 3/8 tubing to the valve and place the other end in another vehicle or a empty gas cans. Drop the spare tire located under the vehicle in order to get to the Fuel System Control Module or FSCM. Unplug the connector and get two jumper wires. Jump the connector as shown in this video at 8:20. Keys do not have to be on as this applies direct battery voltage to the fuel pump and now you are emptying your tank. Works like a champ.

 

[yardboss1]

Change the fuel filter and have a look at the spark plugs anyway.

 

[testdepth]

Change the fuel filter and have a look at the spark plugs anyway.

No external fuel filter on these vehicles. Fuel filter is part of the fuel pump assembly in the tank.

 

[CAMSDADDY]

Well I found either the problem or part of the problem. We recently had the fuel pump changed out. I just filled the tank and brought it home. It was sitting in our carport and my wife starts smelling gas and looks under the truck to see a puddle. The car is parked and there was gas actively dripping off the side of the tank. Its coming from the drivers side just under where the pump is mounted in the tank. I'm guessing the guy that replaced the fuel pump screwed up and didn't get the lock ring tight or didn't put the o-ring in.

Did you reach out to the guy that installed the pump?

 

[testdepth]

Did you reach out to the guy that installed the pump?

Yes I did and it was nothing but excuses and I'm sorry. I had already taken it back to him once for the tank leaking. He claimed to have fixed it but obviously that was BS. Like most times at my home, when I have a problem I end up having to fix it. Luckily or unluckily I know how and can fix it.

While gently moving over the heater hose at the firewall on the passenger side the ****ing hose just snapped off at the fitting. I've never seen a hose do that. Rubber hose just broke clean off. Then I start looking how to repair it, it's a known issue with GMC and Chevy SUVs and trucks. Inferior rubber and plastic parts (cracked dash) on $70000 vehicles.