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  1. #1
    FEP Power Member fgross2006's Avatar
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    Default Check engine light after upgrade engine pipe on 96 Yukon

    I have a 96 Yukon. Dropped a crate motor in September.

    Last month I had a Magnaflow Performance Exhaust 12588 Stainless Steel Muffler with 3 inch inlet/Outlets installed with new 3 inch tailpipe.

    Today I decided to replace the rest of the exhaust set up from manifold to the Muffler. I had 2 left over hi flow Magnaflow 59926 Metallic-Cat Universal Spun Metallic Catalytic Converters that I didn't use on my Mustang so I thought they would be useful. So I took the truck to Meineke and had them pull the old pipes and the old cats and redo everything from manifold flange to the muffler.

    Between muffler and cats I have 3 inch pipe. between the exhaust manifold to the cats inlets I have 2 1/2 inch pipe. They installed the O2 bungs in the exact places they are supposed to be and all 4 O2's are brand new. There was no CE condition prior to today.

    The truck runs awesome. It opened up a lot. Sounds throaty too. I drove on the parkway for about 5 miles, then while I was driving locally the check engine came on. I don't have a scanner to pull the code tonight but I'm wondering if the universal hi-flow cats messed with the reading the O2 needs or is it the larger pipes diluting the reading?

    Ill get my son to scan it tomorrow to see the exact code.

  2. #2
    FEP Member brianj's Avatar
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    Default

    It's possible that the far more free flowing exhaust is causing the O2 sensors to run too cold. I'm not sure if a '96 uses heated sensors or not, but if not, you may need them.
    1983 Mustang G.T. No-option stripper- I like strippers.
    5.0, GT40P heads, Comp Cams XE270HR-12 on 1.6 rockers, TFI spring kit, Weiand 174 blower, Holley 750 mechanical secondarys, Mishimoto radiator, Edelbrock street performer mechanical pump, BBK shortys, T-5 conversion, 8.8 rear, 3.73 gears, carbon fiber clutches, SS Machine lowers, Maximum Motorsport XL subframes, "B" springs.

  3. #3
    FEP Power Member fgross2006's Avatar
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    Default

    Quote Originally Posted by brianj View Post
    It's possible that the far more free flowing exhaust is causing the O2 sensors to run too cold. I'm not sure if a '96 uses heated sensors or not, but if not, you may need them.
    the downstream O2s are heated

  4. #4
    FEP Power Member fgross2006's Avatar
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    Default

    So Im relived to say the code was P0171, lean condition bank 1.

    That's before the cats so there no issue with the ECM and the downstream O2's not liking the hi flow cats.

    This lean condition bank 1 has come and gone a few times since I got this truck. We ran the smoke machine last time and the only place smoke came out was on the valve cover driver side. I have to ask if air escaping the valve cover could even be detected by the upstream O2 that's under the car before to cats?

    Apart from that, the Yukon runs great with the 3 inch pipe and hi flow cats and muffler. All in all Im glad I did it.

  5. #5
    FEP Super Member cb84capri's Avatar
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    The upstream o2 sensors are the ones that tell your pcm what the air/fuel ratio is, and then it adjusts fuel and ignition to optimize it. The downstream o2 sensors only tell the pcm if the cat is working or not, based on a comparison to the voltage spit out by the upstream o2. The pcm does not make any adjustments based on the downstream o2 sensors, it just sets a code like P0420 and turns the check engine light on if it doesn't like what it sees. So yes, the upstream o2 sensor is the one detecting the lean condition.

    The Vortec engines are known for intake manifold gasket leaks, though typically it is coolant getting into the oil. GM has revised the gaskets a few times, and Dexcool turning acidic plays a role, too. I don't know how you did the smoke test, but un-meterd air getting into the engine is hard for the pcm to adjust for.

    Cale

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