Likely Culprits:
* Mass Airflow Sensor (MAF) or Throttle Position Sensor (TPS): A faulty MAF sensor provides inaccurate air flow readings to the computer, leading to a rich fuel mixture. A bad TPS provides inaccurate throttle position, also causing improper fuel delivery. These are common culprits for poor fuel economy, black smoke (unburnt fuel), and rough running. Clean the MAF sensor with MAF sensor cleaner (use a sensor-safe cleaner, *not* brake cleaner). Check the TPS voltage output with a multimeter to see if it's within specifications (check a repair manual for the correct values). Often, cleaning the MAF sensor fixes the problem temporarily, a replacement might be needed.
* Fuel Injectors: Clogged or leaking fuel injectors can cause a rich condition. Leaking injectors constantly spray fuel, even when the engine doesn't need it. This leads to poor fuel economy, black smoke, and potentially a rough idle or stalling. Fuel injector cleaning (professional or DIY with a cleaner) is a good place to start. Replacement might be necessary if cleaning doesn't solve the issue.
* EGR Valve: The Exhaust Gas Recirculation (EGR) valve routes exhaust gases back into the intake manifold to reduce emissions. A stuck-open EGR valve can significantly enrich the mixture, causing the symptoms you describe. Inspect it for proper operation and clean it if necessary (often carbon buildup is the issue).
* Oxygen Sensor: The oxygen sensor monitors the exhaust gases and tells the computer how to adjust the fuel mixture. A faulty oxygen sensor can cause the computer to deliver the wrong amount of fuel, resulting in a rich condition. Replacing the oxygen sensor is often necessary; it's a relatively inexpensive component.
* Vacuum Leaks: Vacuum leaks can introduce unmetered air into the engine, causing the computer to overcompensate by adding extra fuel. Check all vacuum lines for cracks or disconnections.
* Fuel Pressure Regulator: This regulator maintains the correct fuel pressure in the system. A faulty regulator could allow excessive fuel pressure, leading to a rich condition.
* Computer (PCM): In rare cases, a problem with the Powertrain Control Module (PCM) itself can cause these issues. This is less likely than the other problems listed above, but it's something to consider if other repairs don't solve the problem.
Troubleshooting Steps:
1. Check for Vacuum Leaks: Visually inspect all vacuum lines and hoses. Listen for hissing sounds with the engine running.
2. Clean the MAF Sensor and TPS: This is the easiest and cheapest thing to try first.
3. Check Fuel Pressure: This requires a fuel pressure gauge.
4. Inspect the EGR Valve: Check for carbon buildup and ensure it's moving freely.
5. Check the Oxygen Sensor: While it might need replacement, you can use a scan tool to check for sensor faults.
6. Fuel Injector Cleaning/Testing: This can be done professionally or, with some DIY skills and tools, at home.
7. Diagnostic Scan: Use an OBD-II scanner (even a cheap one from an auto parts store) to check for any trouble codes stored in the computer. This can provide valuable clues about what's wrong. A 1993 Ranger may not have a fully OBD-II compliant system, but it might still give some relevant codes.
Important Note: A 1993 Ford Ranger is an older vehicle, and some components might be worn out or nearing the end of their lifespan. The difficulty in starting in the morning *could* be related to a weak fuel pump, but that's less directly linked to the running rich condition. Addressing the rich condition is crucial before tackling starting problems.
Without a diagnostic scan and some hands-on testing, it's impossible to pinpoint the exact cause. It's advisable to start with the simpler checks (vacuum leaks, cleaning sensors) and then move to more involved repairs if necessary. A mechanic specializing in older vehicles would be very helpful in diagnosing the problem.
