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GMC Pickup Truck Failed High HC Smog Pump Not Working

I own a 1979 GMC 3/4 ton pickup with a 5.7 Liter engine. It failed the smog check because the HC was too high. On the first smog test it measured 116 HC at 25 mph.

I changed the spark plugs, adjusted the ignition timing to 8 BTDC, and checked all the vacuum lines. It failed the second smog test with even higher HC. This time HC measured 135 at 15 mph and 166 at 25 mph. Can a failed or broken smog pump (air injection) cause these HC to go up?

Answer:

High HC represents an excessive amount of unburnt fuel in truck's exhaust. HC is fuel. Your GMC trucks air injection system (air pump/smog pump) is designed to inject air (oxygen) to the exhaust (downstream) during cold starts, acceleration and sometimes under load. Introducing oxygen to the exhaust allows continued burning of the fuel mixture as it travels down the exhaust system and ultimately out the tailpipe, thus lowering HC. If your GMC truck has a defective smog pump or air injection system (i.e. diverter valve, smog pump, disconnected or broken hoses), we highly recommend repairing the air injection system prior to your next smog inspection. posted by SmogTips Support

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