Need to replace exhaust system

[yukonxl1500]

I have been getting really poor gas milage in my 2000 Yukon Xl. I had the enging replaced 2 weeks prior to shipping out to Iraq 3 years ago due to major coolant loss. The mechanic could not guarantee it was the head gasket or internal leak and I did not want my wife to deal with it when I was gone. Prior to the engine swap I was getting 16 mpg and when I got back it was down to 12mpg. I installed a CAI and throttle body spacer and the mpg went up to 14mpg. I took to dealer to see if something was off, too rich/lean...anything. He said that the engine was not pushing any codes but that since my the SUV (not engine) had 170K the exhaust, mainly cat was getting ready to go and was probably really restricted.

I would really like to just get back to 16mpg or better, I was thinking about just replaceing the cats with a straight pipe. Does anyone know where you can buy one for "off road" use ofcourse and what you can do or go to get the computer flashed not to read rear o2 sensors. I was also planning on installing headers to and leaving stock exhaust on for time being.

Appreciate any help and if any one has any other idea on my poor mpg causes.

Thanks

 

[MadMedwyn]

Yeah after 170k miles a new cat may help your mpg. There are high-flow cats out there that can free up some restriction and keep emissions legal. I know nothing about them though. As for exhaust, any cat-back system should do nicely for power and mpg. Magnaflow and Flowmaster are really popular here, with maybe the occasional Corsa. I wouldn't advise straight piping at all. Straight piping is LOUD and it may not help your power output and mpg at all actually.

You can order entire cat-back systems online, so you wouldn't have make somone at the shop do any pipe bending ect. I ordered my entire system from autoanything.com and it's been good dealing with them.

 

[STREETutilityVEHICLE]

also i would not install the straight pipe in place of the cats. i would go for a couple of high flow cats welded in their place. any muffler shop will do this for you if you order the cats. i have seen a direct replacement piece for the cats. it's got the cats installed and it's basically a y-pipe that goes from both exaust manifolds to the single pipe leading to the muffler. they run in the mid to high $500 range and you could probably install yourself. if you do the high flow cats, i would say it's almost mandatory to install a high-perf muffler, like flowmaster. i like the flowmaster because it will likely last you until you no longer have the truck. but with the cats and high-perf muffler, you should deffinatly pop your MPG's back to where they were. if you are still not getting the mpgs you should have, then look into your spark plugs, wires, and oxygen sensors. thats a good start, good luck

 

[MadMedwyn]

You can always clean your MAF and throttle body. The MAF is really easy to do. Your throttle body might be already clean since you mentioned a new engine, but I'd look into it while you're down there.

 

[thdrill]

best way to clean MAF?

 

[dbdolan]

A local muffler shop cut out my 2 cats and replaced them with 2 high flow cats for $250. My 2002 is running much better, she has about 98,000 miles on her. I was getting a code about catalyst below threshold, or something. The Tahoe would rev but not accelerate when gunning it, it was almost like the transmission was slipping.

 

[boostaholic]

Knock those bitches out and get a tune
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