Fuel Injection and Rich Air Fuel Ratios
A rich fuel condition occurs when the air to fuel ratio is less than 14.7:1. This (14.7:1) is considered the perfect blend of air to fuel mixture or stoichiometric air fuel ratio. This is when there are just enough parts of air (14.7) to burn one part of fuel with no excess oxygen or fuel left over. Faulty components such as dripping fuel injectors, faulty fuel pressure regulators, ECT's and stuck open thermostats cause this problem. A rich air fuel ratio will result in high emissions (HC's) and black colored exhaust.
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 left over fuel dries in the injector’s hot tip as the engine cools. 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 fuel 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’s controlled by intake manifold vacuum. Electronic regulators do the same thing with more precise computer control.
Engine Coolant Temperature Sensor: The ECT is tasked with informing the PCM of the engine’s current operating temperature. This is important to the air fuel mixture because a cold engine requires a rich fuel mixture. If the ECT were faulty and the PCM were receiving a signal indicating a colder than actual engine, the mixture would be enriched.
Above are a few of inputs that affect a vehicle's air fuel ratio and fuel trim adjustments. 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 colored exhaust, fouled spark plugs, and poor engine performance.