Fuel Injection: Rich Air/Fuel
A rich fuel condition occurs when the air to fuel ratio is less than 14.7:1. This is considered the perfect blend of air to fuel mixture or stoichiometric air fuel ratio. Where there is just enough parts of air (14.7) to burn one part of fuel with no excess oxygen or fuel left over. Faulty devices such as dripping fuel injectors, faulty fuel pressure regulators, ECT's, and stuck open thermostats cause a rich air fuel ratio.
Fuel Injectors: Fuel injectors are electromechanical devices that atomize pressurized fuel from the fuel rail. They can become clogged with deposits that form as the fuel dries in the injectors hot tip as the engine cools. This results in lean misfire causing unburned fuel to flow into the exhaust system. They can also leak or drip pressurized fuel. This can be diagnosed with a fuel pressure kit and a leak down test.
Fuel Pressure Regulator: A pressure regulator is designed to provide constant fuel pressure to the fuel injectors under different loads and operating conditions. A traditional regulator contains a vacuum operated diaphragm. It is controlled by manifold vacuum that would vary pressure to compensate for increased or decreased demand. The newer electronic regulators do the same thing with more precise computer control.
Engine Coolant Temperature Sensor: The ECT is tasked with informing the engine control unit with the engines current operating temperature. This is important to fuel trim because a cold engine requires a rich fuel mixture. If the ECT were faulty and the PCM were receiving a signal informing it of a colder than actual engine, the fuel trim would be enriched. This signal will also effect the automatic transmissions torque converter clutch.
Thermostat: The thermostat alters the flow of coolant as it opens and closes. It opens and closes based on the temperature of the coolant. As the engine reaches operating temperature a temperature sensitive spring opens the thermostat to allow more flow. A stuck closed thermostat would result in an overheated engine. A stuck open thermostat will result in a richer than normal fuel mixture because the engine would not reach proper operating temperature.
Above are a few of inputs that effect a vehicles fuel trim. A faulty oxygen sensor should be mentioned. An oxygen sensor sending the wrong signal to the engine control module can result in a rich fuel condition. Symptoms of a rich fuel condition include black exhaust smoke, fouled spark plugs, and poor engine performance.
