- Faulty Oxygen Sensor: An aging oxygen sensor can't accurately monitor oxygen levels in the exhaust. This hampers the ECM's ability to modify fuel trim.
- Incorrect Fuel Pressure: High fuel pressure can cause a rich mixture, but it's more common for vehicles to run lean with inadequate pressure.
- Clogged Fuel Injectors: If one or more fuel injectors gets stuck open, it keeps spraying fuel into the cylinders regardless of the engine's needs.
- Vacuum Leak: Leaking intake manifold gaskets or vacuum lines, can cause unmetered air to enter the engine, causing the computer to overcompensate for the air and send too much fuel.
- Faulty Mass Air Flow (MAF) Sensor: The MAF sensor measures the air entering the engine. If it's malfunctioning, it can skew the fuel mixture calculations.
- Leaking Fuel Pressure Regulator (FPR): A malfunctioning FPR can result in excessive pressure to supply more fuel than needed.
- Choke or Idle Air Control Valve Issues: Older cars may have a mechanical choke or idle air control valve that, when malfunctioning, can keep the mixture rich.
- Engine Coolant Temperature Sensor Issues: A faulty coolant sensor might prevent the vehicle's engine from warming up properly, leading to a rich fuel mixture that will correct itself during warm-up.
