Cadillac's Rough Cold Idle Is 'Normal'
Owners of 2008-2019 Cadillac vehicles may comment on black smoke, rough idle and minimal misfires on cold start. The MIL may not be illuminated.
With the introduction of direct fuel injection, GM has revised the cold start control system to reduce cold start emissions. Quicker catalytic converter heating helps meet the ever-changing emission requirements and improve fuel economy.
A dual-pulse injection strategy it utilized during cold engine start to reduce the time required to bring the converter up to operating temperature. This duel-pulse strategy lasts for about 60 seconds on cold start. This process will cause the customer to see increased black smoke, soot, rough idle or minimal misfires during cold start and should be considered “normal.”
To verify that the dual-pulse injection is causing the roughness or misfires with no codes set, you should watch injector pulse width with the scan tool. Dual pulse injector pulse width will be nearly double that of normal idle. After 20 seconds, the pulse width will drop by about 50% and the engine idle will smooth out.
The high pressure fuel pump test will run either as soon as the dual pulse injection is completed or during the first stop after the engine is warmed up above about 155 Fahrenheit. This can feel like a slight idle roughness as the high pressure fuel pump is commanded to max pressure and then shut off to measure pressure decay.