Black smoke from the exhaust

Symptoms

• Black or dark gray smoke from the tailpipe, especially under acceleration
• Reduced fuel economy noticed alongside the smoke
• Possible rough idle, hesitation, or check engine light
• Strong fuel-like smell in the exhaust

Likely causes

• CommonClogged or dirty air filter restricting airflowToo little air relative to fuel causes an overly rich mixture that burns black; a simple, cheap first check.
• CommonFaulty oxygen sensor or mass airflow sensorBad sensor readings can cause the engine computer to add too much fuel to the mixture.
• PossibleFailing fuel injector stuck open or leakingDelivers excess fuel to one or more cylinders regardless of what the sensors report.
• PossibleFaulty fuel pressure regulatorCan cause consistently too-high fuel pressure, over-fueling the engine.

What to do

1. Check and replace the engine air filter if it's dirty or clogged, since this is the cheapest and most common cause.
2. Have the check engine light's codes read with a scan tool if it's illuminated, since a rich-condition code will point toward the specific faulty sensor or component.
3. Note if the smoke is worse under hard acceleration specifically, which supports a fuel delivery issue reacting to higher demand.
4. Track fuel economy over a tank or two, since a notable drop supports an ongoing rich-running condition rather than a one-time event.
5. Have the oxygen sensors, mass airflow sensor, and fuel injectors tested by a mechanic if an air filter change doesn't resolve it.

Tools and parts

• OBD-II scan tool

When to call a pro

See a mechanic with scan tool access if a fresh air filter doesn't stop the black smoke, since sensor or injector diagnosis usually requires proper equipment.

Safety