* Mass Air Flow (MAF) Sensor (if equipped): A faulty MAF sensor can send incorrect readings to the engine computer, leading to excessive fuel injection. A dirty or failing MAF sensor is a common cause of this problem. The XR4Ti might have a hot-wire MAF, or even a speed density system (no MAF).
* Throttle Position Sensor (TPS): An inaccurate TPS reading can also cause the engine computer to inject too much fuel. A failing TPS can sometimes provide false signals at higher throttle openings.
* Fuel Injectors: Clogged or sticking fuel injectors can deliver more fuel than needed, resulting in a rich mixture and black smoke. This is less likely to cause black smoke *only* above idle, but it's a possibility.
* Oxygen Sensor: This sensor is crucial for feedback-controlled fuel mixtures. A malfunctioning oxygen sensor (O2 sensor) can prevent the engine control unit (ECU) from accurately correcting the air-fuel ratio, leading to a rich condition. A slow or faulty sensor is more likely to cause problems at higher loads (above idle).
* Fuel Pressure Regulator: A faulty fuel pressure regulator could maintain excessively high fuel pressure, flooding the engine with fuel.
* Engine Control Unit (ECU): While less common, a problem with the ECU itself (internal failure or bad programming) could cause incorrect fuel delivery. This is usually more complex than the others and harder to diagnose without specialized equipment.
Troubleshooting Steps:
1. Check for vacuum leaks: Vacuum leaks can affect the readings of many sensors and lead to a rich condition. Inspect all vacuum hoses for cracks or disconnections.
2. Inspect the MAF sensor (if applicable): Clean the sensor carefully with MAF sensor cleaner (following the instructions carefully). If it's damaged, replacement is necessary.
3. Check fuel pressure: Use a fuel pressure gauge to measure the fuel pressure. Compare it to the manufacturer's specifications.
4. Test the TPS: You can use a multimeter to check the voltage output of the TPS as the throttle is opened. Compare these readings to the specifications in a repair manual.
5. Check the O2 sensor: An O2 sensor test requires specialized tools and might involve observing its output signal with an oscilloscope.
6. Diagnose the ECU: This requires a diagnostic scan tool capable of reading the codes stored by the ECU. This is likely the last resort but could reveal a hidden problem.
Important Note: A 1988 Merkur XR4Ti is a somewhat specialized car, so access to a repair manual specific to that model year is highly recommended. The manual will provide detailed diagnostic procedures and specifications for all the sensors and components mentioned above. Improper diagnosis and repair could cause further damage.
