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  1. #11


    Just to try and narrow the problem, I'll ramble a little. As I understand the problem, the engine runs rough when hot, as well as cold.

    The O2 sensors are bypassed when the engine is cold, this is referred to as "open loop"; in this mode, the PCM provides air/fuel mixtures from a fuel map. When the engine is hot, the PCM switches to the data provided by the front O2 sensors (closed loop).

    Therefore, (IMO) the engine should only run rough when hot, if the front O2 sensors were bad.

    The rear O2 sensors monitor the condition of the cat (as some one said).

    If running rough, and no power, all of the time, the cat may be to blame. I only had one cat go bad (so my experience is far from anything but informative). In my case, the engine would idle fine (hot or cold), but would lose power as the gas was applied. The engine would also rattle (sounded like spark knock). The cat was plugged, a new one solved the problem.

    '09 Avalanche LTZ - Black
    '05 Envoy XL (sold)

  2. #12


    Ok, researched the price for the cat, for just one cat its $125-250, To replace the whole front of the exhaust is about $350+ but includes 2 cats gaskets bolts and such. Problem is there is no easy way to replace my cat it requires cutting it out and welding it back. Really I don't have the facilities to do that nor can afford the part and the labor at a shop.

    Tomorrow I will check it out and report back, but the symptoms are very similar to what @RayVoy is talking about and it sounds a lot like spark knock. I still need new plugs because the ones I have have who know how many miles and the wireset looks old as well. Gonna replace fuel filter as well because those things are usually done together.

    1996 Chevy Tahoe LT 5.7L V8 4X4 205,000+ miles. Built proudly at Janesville Assembly in Janesville, Wisconsin
    Basic mods: Lights all over, bunch of electrical work, and a couple cooling mods.

    Check out my other mods in My Garage:

  3. #13


    I haven't had any time to get under the truck I'm just so busy. Funny thing the truck is getting much better MPG with this issue not sure why but it is.

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