Bad mileage on my 2004 Yukon

[TheRaptor]

I need some help here. My truck is a 04 Yukon with the 4.8 V8 in it and the mileage I get out of it sucks. I get about 11 MPG combined driving even driving on the highway I don't get what I should. The cats were bad and I had them both replaced. Now heres my question. There is a minor rattle in the exhaust (muffler). If my muffler is messed up would that hurt my mileage by this much? I know guys with the same truck as me and they get 14 city.

It also has 150k on it if that makes any difference.

 

[2Canoes]

Sounds like parts of the cat got into the muffler. I don't know if it would hurt mpg that much, but probably a good place to start. My guess is that it's getting too much gas and that's what messed up the cats. Was mpg down before cats had to be replaced?

 

[TheRaptor]

Sounds like parts of the cat got into the muffler. I don't know if it would hurt mpg that much, but probably a good place to start. My guess is that it's getting too much gas and that's what messed up the cats. Was mpg down before cats had to be replaced?

I bought it with bad cats and had them replaced so I don't know what the mpg was like before. If it is getting too much gas what does that mean? What causes that? The throttle response isn't as good as I think it should either too.

 

[chauncey0337]

Has it had a tune up lately? Plugs, wires, air filter, maf sensor and throttle body cleaning will all help the performance of your vehicle. As for the rattle, I've heard that something in the co2 sensors can come loose and make a rattling sound. Might be worth checking out.

 

[Sasquatch]

What 2canoes is saying is that if your old cats broke apart some of the pieces could be down in your exhaust pipes. I`ve heard it before and it only rattles under heavy acceleration or free revving the engine.

 

[2Canoes]

Is it throwing any codes?

 

[TheRaptor]

What 2canoes is saying is that if your old cats broke apart some of the pieces could be down in your exhaust pipes. I`ve heard it before and it only rattles under heavy acceleration or free revving the engine.

Thanks. I think I misinterpreted his comment.

The check engine light was on before the cats were replaced. I took it to a shop, they did the cats and inspected it and the light is off and has been off for a few weeks. No codes now.

chauncey0337, Thank you. I figured it could probably use a tune up. The previous owner said he was very religious about maintenance though.

 

[2Canoes]

The cat on my Camaro broke into pieces and plugged up the muffler. I don't recall gas mileage going down when it happened, but it ran like a new car when I installed a new muffler.

 

[TheRaptor]

Okay, reviving this thread because after talking to mechanics and friends I still haven't got to the bottom of the issue.

I'm just wondering if anyone had any additional thoughts. I checked the air box and the throttle body and did a general look around and everything looks nice and clean. So what I'm mainly wondering about is the sluggishness in the throttle response. It acts like its towing a heavy trailer all the time. I think the mileage issue and the performance issue is caused by the same thing. I just don't know what.

I've appreciated the responses so far. Thank you.

 

[zieglerj-pdm-specialist]

I have a 2004 Yukon XL Denali 6.0 litre and when I got it , it would get 17 highway and 13 City so I went to work. I put a 6 PSI turbo on it, Cold Air K&N filter system. Pulse Plugs $22 a piece, 10MM Wires and coils. Magnaflow Exhaust minus the cats.SHHH. I also put the highest PSI pressure I could in the tires. I also replaced two tough to spin wheel bearings. Carbon Fiber Skid plate removing all the weight that I could. Did an airflow test on it where you take it in and they put visual air passing over the Vehicle. So now I drive the with Side mirrors tucked in because they are automatic. I replaced the rims and tires. 23" rinch wrapped with KuhmoSTX 305/40/23 which is 20 pounds lighter than my stock wheels. I Drive Steady at 70MPH and I get 26MPH and when I had a 30 mile an hour back wind I actually seen 30MPH and my city mileage is from 18MPH to 20MPH. Take Care of your ride and it will take care of you. Oh and I also you Mobil One 0w-40 protecting the engine and giving it smoother and easier component movement. I am an Oil analyst so I constantly check if there is any defects from using the lighter oil and Mobil 1 is the only oil I trust. I put 40,000 miles and no problem at all with the engine oil analysis. If you choose to go my route I'm not liable for anything that may happen to your vehicle this is only what I do because I can monitor and discrepancies. Take Care.