Sounds like an old/sick or sooted up catalytic converter. If your car runs well you probably need a new cat. This is one of those instances were a NOS part is not a good idea. The cats of today are much superior than they were originally.

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Darryl, Hill
2005 SL55 AMG Kleemanized
1984 500 SEC
1967 W113 California Coupe
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