My wife has a 2004 Denali XL AWD with a 6.0. All of a sudden gas mileage drops to 10.4 mpg from 12.1 mpg. One guy thinks my cats are plugged, another says that my fuel regulator is bad or maybe it needs new spark plugs.
We bought it used with 92k on it. Now it has 105k and only done the basic oil and air filter changes. The Denali does not have any misses but there is a hesitation as you try to give it gas from dead stop. No check engine lights. It just passed smog with the bad mileage but, all the info goes to Nevada's DMW so I do not have any spec's.
Is there away to test the items mentioned? Is there something else to look for?
Hate to start replacing parts on a guess! Any help would be appreciated.

 

Look and see if the pressure regulator is seeping fuel...usually when they go out they will start to seep. On the cats being plugged you could pull the front O2's and see if you pick up any lost power from them being plugged. I diagnosed my old 97 Dodge ram like that, they are notorious for the cats going out and getting plugged.

A bunch of other things you can check are the plugs and wires, Clean your MAF and throttle body. Check you Air filter also.

 

It may also be time for some front o2 sensors.

 

With 105k on the clock, its time to start replacing the spark plugs,maybe wires too, fuel filter, trans fluid and filter, belts, O2s and all the other small stuff. I would do one at a time to see if it helps.

Does it have a rotton egg smell when the truck is warmed up? If it does, that is usually the cats. Does it sound more muffled than usual?

You can try running a gallon or two of 110 octane gas to help clean out the top end. I have tried it a few times and it always seemed to help on the next tank.

 

Wow 12.4? My Denali only sees 11! lol Check your airfilter first, then my guess is front O2s and spark plugs.

 

Thanks for the advise from all who responded, I have changed the wires and plugs since the last post and the mileage is still going down, in fact it is down to 9.9.
I will try the sensors next and then the cats.

 

O2 sensors are typically the main culprit and the last thing everyone is willing to try. Not all the time though.