Replace catalytic converters?
 

Hello all,
First post here. I have a 2008 suburban with 117k miles on it. I've had it almost a year now and overall runs looks very nice. I feel like the vehicle isn't getting necessary power at highway passing speed, like the exhaust is restrictive. Has anyone here found that replacing catalytic converters at 100k has made noticeable difference as far as restrictions? Planning on putting on sway bars, shocks and bigger rotors/calipers up front but exhaust would be my first start if needed.
Thanks!

I have the same year mine has 373 gears. has 121K miles. never has felt that way. Id suggest making sure injectors are clean, (run techron on a few tanks) inflate tire properly, I run 39lbs. remember heat and warm temps always makes a care feel sluggish. During the summer I run super and it always brings back a lil more power and pep. because the engine uses the higher octane to advance ignition timing and fuel curves.
Generallly give her a tune and filters. I also run Royal purple. oil which is good for another 1-2 hp lol.
Bad convertors will get scorching hot and you will throw codes.

The likelihood of bad cats is slim, unless you know the vehicle has been running poorly, with things like leaky injectors, bad plugs or wires , heavy oil burning, things like that will cause the cats to clog and become inefficient. If the vehicle has been over all well maintained and isn't burning oil or smelling like un-burnt gas then the cats are probably fine. Dragging brake calipers, worn wheel bearings, dirty air filter, and general tune up is where I would start. Use the OEM plugs they will perform the best

I forgot to mention all fluids, filters, plugs, wires had been changed at chevy dealer. Also both front hub assemblies have been replaced. Runs very good, and responsive at low speeds. However, just feels restricted at highway speed. I'm not looking to add air intake but am considering black bear tune? I'm not a big fan of this AFM system as I'm only averaging 15.8 mpg, or is this normal with 4x4?
and I'm not a lead foot either, this is primarily to drive kids around when I'm not using truck. I just want to make sure im addressing all areas of concern now as I will likely have this vehicle for some time, an heirloom so to speak. Thanks

Is there good way to "check" the cats? Or is that just a money sink provided that many places might just "say" they are bad to get the money out of you. For what its worth, my 2007 is averaging about 15 mpg, but it also has about 250k miles on it as well. I seriously doubt the cats or oxygen sensors have been replaced but maybe they need to be? I'd like to get more mpg, and while 1-2 more mpg wouldn't make a dent, it would at least make me feel better mentally lol.

A good shop will have the tools to check for back pressure.

Your mileage does sound low, my 03 5.3 gets about 19 hwy, depending on my foot. Bad cats would affect low speed or any kind of acceleration, not just hwy. Just a thought, but your issue sounds like it could be your ECM limiting you.

If all of the O2 sensors are in place and not by-passed they should be reporting back to the ECM on cat efficiency and would under normal circumstances report back with a code if there was a problem...
If you are not reading any codes then I think the cats are fine... a 4x4 is going to be heavy and harder to move at higher speeds, also keep in mind its throttle by wire so response is going to be different then a cabled system.
Check the air filter, make sure its the correct one, consider cleaning the MAF sensor with the proper cleaner, try running 100% or marine grade fuel in a tank or 2, or at least try a couple of bottles of fuel injector cleaner... If you are running E85 fuel, power and response will be decreased.

I will be needing to replace my catalytic converters soon. My old burb still runs strong but, I get an occasional P0420 code. I'm going to run some Cataclean through it when the gas gets low. I had to pay the IRS and put a new roof on my house so, no cats for now. Soon I will order the "Y" pipe from RockAuto and do the job myself. It's way cheaper than NAPA, Autozone, Advance, GM, etc.. They told me that an aftermarket cat wouldn't last very long on my Honda CRV....12 years and 100,000 miles ago.