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1981 Buick Century 3.8L failed smog test for high HC

1981 Buick Century 3.8L failed 2nd test High HC PPM reading. Plus engine diesels when shutting down. We have changed dist cap, rotor, oil & filter, air filter, added higher octane, plus some liquid heat. Should we change plug wires? Plugs are new. Plugs were chhanged prior to 1st test. 1st test @15mph hc was 106 @ 25mph all passed. made changes above. 2nd test 15mph all passed, 25mph hc was 81. Also engine diesels when turning off. Help? Any suggestions?
Answer:

We recommend looking into why your Buick Century is dieseling first. Dieseling can be caused due to carbon built-up in the combustion chambers which burn hot after the engine is off, providing ignition for any unspent fuel. Dieseling usually occurs when an engine is running very rich, depositing unburned fuel on piston heads and valves. Or when a carburetor doesn't completely cut off fuel supply once the engine has been turned off. Mike, ASE Advanced Emissions Specialist www.SmogTips.com posted by SmogTips Support

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