* Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): These sensors tell the computer how much air is entering the engine. If they're faulty and reporting incorrect readings (e.g., less air than is actually present), the computer will inject too much fuel, leading to a rich mixture. This is a very common cause.
* Fuel Injectors: Faulty injectors can leak or stay open too long, constantly spraying fuel into the engine even when it's not needed.
* Fuel Pressure Regulator: This regulates the pressure of fuel delivered to the injectors. A failing regulator might allow excessive fuel pressure, leading to a rich condition.
* Oxygen Sensor: This sensor monitors the exhaust gases and tells the computer how to adjust the air/fuel mixture. A faulty oxygen sensor can prevent the computer from correctly adjusting the mixture.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the air/fuel ratio, causing a rich mixture. This is particularly important on older vehicles like the 1991 Explorer.
* Computer (PCM/ECM): While less common, a faulty computer can also cause incorrect fuel delivery.
* Fuel Filter: A clogged fuel filter can restrict fuel flow, but this usually results in a lean condition, not a rich one. However, if it's severely clogged and causing unusual pressure spikes, it *could* indirectly contribute.
Troubleshooting Steps (in order of likelyhood and ease):
1. Check for vacuum leaks: Visually inspect all vacuum lines for cracks, holes, or loose connections. Use a vacuum gauge if you have one to check for leaks.
2. Inspect the MAF sensor and TPS: Clean the MAF sensor with MAF sensor cleaner (following instructions carefully). Check the TPS for proper resistance using a multimeter (consult a repair manual for specifications).
3. Check the fuel pressure: You'll need a fuel pressure gauge to do this. This requires specialized tools and knowledge of the fuel system.
4. Check the oxygen sensor: This requires a multimeter and knowledge of the sensor's voltage output. A code reader might indicate a faulty oxygen sensor.
5. Inspect fuel injectors: This is a more advanced procedure and may require professional help. A leak test or injector flow test can diagnose problems.
Before you start:
* Safety First: Work in a well-ventilated area. Gasoline fumes are extremely dangerous.
* Consult a Repair Manual: A repair manual specific to your 1991 Ford Explorer will provide detailed diagrams, specifications, and troubleshooting guides.
Because several components could be responsible, professional diagnostics are often recommended. A mechanic can use a code reader to identify any trouble codes stored in the vehicle's computer, which can significantly narrow down the possibilities. Ignoring the problem can lead to more serious damage (like catalytic converter failure).
