An intake manifold vacuum leak can develop from a variety of sources, often causing illumination of the Check Engine Light with codes P0171 & P0174 stored. Thorough diagnoses will be necessary to pinpoint the cause in order to make the correct repairs. A "smoke" machine is often used to isolate the cause of difficult to find vacuum leaks.
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Repairpal verified 2009 Buick Lucerne problems
A Loose or worn gas cap may cause Check Engine Light to illuminate.
The fuel pump may fail causing the engine to stall and not restart.