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  1. #1
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    Default odd sound when cold

    When I first take off while cold there is a noticeable phht phht sound almost like the old sound of a leaky exhaust manifold to pipe cracked gasket. After a few minutes it goes away, no noticeable performance issues or codes.
    Any and all ideas appreciated. Thanks

  2. #2
    Member compyelc4's Avatar
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    Quote Originally Posted by rogerda View Post
    When I first take off while cold there is a noticeable phht phht sound almost like the old sound of a leaky exhaust manifold to pipe cracked gasket. After a few minutes it goes away, no noticeable performance issues or codes.
    Any and all ideas appreciated. Thanks
    1. Start 'er up cold and listen and carefully feel top and bottom. You should be able to hear a leak if there is one.
    2. I'd start at exhaust manifolds, then work your way down to cat. connections, then rearwards to each and every connection. One give-a-way might be carbon black staining around the leaking joint.
    3. Why is it just on cold start? Because a leaky exhaust will happen with "shrunken" cold pipe connections. As the connections heat up the metal expands and seals up the leaks.
    4. I have the same thing with my '07 C6. The culprit was back where the tail pipe connects to the exhaust pipe coming from one converter. I can clearly see where black condensation as collected around that joint, and on a lift I can feel the leak on a cold start and see a bit of black condensation seep out. After a few minutes the condensation evaporates, the pipes swell, and the air leak stops.
    Ted

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    thanks for the tip, car is winterized till March or April so probably cant get to a lift till then.

  4. #4
    Member compyelc4's Avatar
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    Quote Originally Posted by rogerda View Post
    thanks for the tip, car is winterized till March or April so probably cant get to a lift till then.
    Yeah. Same here.
    Ted

  5. #5
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    The reason you hear it most often after cold starts is as soon as the engine is running, the ECM retards the spark, to heat the exhaust quickly so the cats light off. When the spark is that retarded, some combustion is going on downstream of the exhaust valve and that makes the engine more noisy. Once sufficient time has past, the timing advances and the noise is reduced or goes away.

    If the problem is a small exhaust leak, the leak will be louder after start up when spark is retarded.

    On some engines, GM surrounds the exhaust head pipes, just downstream of the joint between the pipe and the manifold, with a tight fitting, waffle looking shield. I have also found that if that shielding is not fully closed at its seam, the gap can allow cold start-up up noise to escape and the sound is very similar to an actual leak but...there is no leak.

    You have to get under the car while its running and use a good automotive stethoscope to fully diagnose this problem.

  6. #6
    Member compyelc4's Avatar
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    Quote Originally Posted by Hib Halverson View Post
    The reason you hear it most often after cold starts is as soon as the engine is running, the ECM retards the spark, to heat the exhaust quickly so the cats light off. When the spark is that retarded, some combustion is going on downstream of the exhaust valve and that makes the engine more noisy. Once sufficient time has past, the timing advances and the noise is reduced or goes away.

    If the problem is a small exhaust leak, the leak will be louder after start up when spark is retarded.

    On some engines, GM surrounds the exhaust head pipes, just downstream of the joint between the pipe and the manifold, with a tight fitting, waffle looking shield. I have also found that if that shielding is not fully closed at its seam, the gap can allow cold start-up up noise to escape and the sound is very similar to an actual leak but...there is no leak.

    You have to get under the car while its running and use a good automotive stethoscope to fully diagnose this problem.
    Sage advice. Good input.
    Ted

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