Fuel System Issues:
* Clogged Fuel Injectors: These injectors may be spraying too much fuel, leading to a rich mixture. This is a common cause of rough idling and black smoke.
* Faulty Fuel Pressure Regulator: A malfunctioning regulator could keep fuel pressure too high, resulting in excess fuel delivery.
* Fuel Pump Issues: A failing fuel pump might intermittently deliver too much fuel, especially at idle.
* Vacuum Leaks: Leaks in the vacuum lines affecting the fuel system can disrupt the air/fuel ratio, leading to a rich mixture. Check all vacuum lines for cracks or disconnections.
* Dirty Fuel Filter: A clogged filter restricts fuel flow, potentially causing erratic fuel delivery.
Ignition System Issues:
* Worn Spark Plugs or Wires: While less likely to directly cause *black* smoke (more likely to cause misfires and *white/grey* smoke if they're fouled by the excess fuel), severely worn plugs or wires can contribute to a rough idle and indirectly exacerbate the rich condition.
Engine Management System Issues:
* Oxygen Sensor (O2 Sensor): This sensor monitors exhaust oxygen levels and controls the fuel/air mixture. A faulty O2 sensor can prevent the system from correctly adjusting the fuel delivery, leading to a constantly rich mixture. This is a very common culprit.
* Mass Airflow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A malfunctioning MAF sensor can give incorrect readings, resulting in too much fuel being injected.
* Throttle Position Sensor (TPS): A faulty TPS can provide inaccurate data about throttle position, leading to improper fuel delivery.
* Computer/ECU Issues (Engine Control Unit): Less likely, but a faulty ECU can cause a variety of problems, including a rich fuel mixture.
Other Potential Causes:
* EGR Valve Issues: A stuck-open EGR (Exhaust Gas Recirculation) valve can increase the amount of exhaust gas returning to the intake, creating a lean condition. While usually associated with a *rough idle*, excessive black smoke is less common with a stuck-open EGR valve (though possible). A stuck-closed EGR could contribute, depending on other factors.
* Choke (if applicable): On older vehicles a stuck open choke could contribute to a rich condition, although a 1991 Tercel likely has an automatic choke that's less prone to this, though malfunctions are possible.
Troubleshooting Steps:
1. Check for vacuum leaks: Inspect all vacuum hoses visually.
2. Inspect spark plugs: Look for excessive fouling (black and sooty).
3. Check the fuel filter: Replace if necessary.
4. Test the O2 sensor: This requires specialized tools or a professional mechanic.
5. Test the fuel pressure: This also requires specialized tools.
Given the symptoms, the O2 sensor and fuel injectors are prime suspects. It's best to start with the simpler checks (vacuum lines, fuel filter) before tackling potentially more expensive components like the injectors, O2 sensor, or ECU. A mechanic with experience working on older vehicles could diagnose the problem more quickly and efficiently.
