Black Smoke from Exhaust: Causes and Solutions

Learn what causes black smoke from the exhaust after a cold start on a late-model vehicle

Most carmakers have shifted to gasoline direct injection (GDI) technology to improve engine performance and improve gas mileage. GDI works by injecting gasoline directly into the combustion chamber, as opposed to port fuel injection, where gasoline is injected into the backside of the intake valves. The downside to GDI is soot formation during a cold engine start due to the rich mixture. That can result in black smoke from exhaust after a cold start.

Understanding gasoline direct injection

Gasoline must be atomized and then vaporized in order to burn properly in an engine. Early fuel systems used a carburetor to converter liquid gasoline into atomized droplets that were delivered into the intake manifold. Once the engine heated up, the hot manifold would convert the atomized droplets into gasoline vapor.

Carmakers then switched to fuel injection, which sprayed gas droplets onto the back of the intake valves. Once the engine heated up, the hot intake valves would complete the vaporization process.

Now, carmakers rely on a far more efficient method: gasoline direct injection. By injecting fuel directly into the combustion chamber, the system can precisely control the amount of fuel delivered, reducing waste and optimizing combustion. This precision allows for a higher compression ratio, which translates into better thermal efficiency and, consequently, improved fuel economy.

Cold start GDI injection strategy causes black smoke from exhaust

Some carmakers employ a dual-pulse injection strategy on cold engine starts. The dual-pulse reduces the time required to bring the catalytic converter up to operating temperature. This dual-pulse injection strategy lasts for about 60 seconds on cold start.

The dual-pulse injector on time is nearly double that of normal idle. After 20 seconds, the pulse width will drop by about 50%, and the engine idle will smooth out. This is considered normal operation and no repairs should be attempted.

However, the downside to a dual-pulse strategy is the formation of extra soot due to a rich mixture and incomplete burn. This results in black smoke from the exhaust right after a cold start. The incomplete burn may also cause a brief period of rough idle and some misfires. This is considered normal.

While the dual-pulse temporarily increased emissions, the faster catalytic converter warm up due to the extra fuel, more than compensates for the short increased emissions.