Some 2008-14 GM cars and light trucks with 3.0- and 3.6-liter direct-injected engines may produce black smoke, a rough idle and slight misfires on start-up following a prolonged cold soak.
While the symptoms may be disconcerting to car owners and technicians alike, they're actually quite normal and due to GM's strategy to light off the catalytic converter quicker in an effort to reduce cold start emissions.
GM achieves the quicker converter light off through a unique, dual-pulse injection strategy that runs the air/fuel mix very rich for up to a minute after start-up. Of course, the rich mixture causes the converter to heat up very quickly, and that's exactly what the dual-pulse injection strategy does—only in a very controlled manner. Once the converter lights off and the feedback system goes into closed loop, fuel injection pulse width returns to normal.
Vehicles that are equipped with the dual-pulse strategy and thus prone to producing the symptoms noted above include the 2008-11 Cadillac STS, 2008-14 Cadillac CTS and SRX models; 2009-14 Buick Enclaves and Lacrosses; Chevy Equinox and Traverses and GMC Acadias; 2010-14 Chevy Camaros and GMC Terrains; 2012-14 Chevy Impalas; and 2013-14 Cadillac ATS and XTS models.