Fuel Injection and Rich Air Fuel Ratios
A (14.7:1) air-furl ratio 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, fuel pressure regulators, ECT's and stuck open thermostats cause this problem. This results in high emissions (HC's) and black colored exhaust.
Fuel Injectors: Fuel injectors atomize pressurized fuel from the fuel rail. They can become clogged with deposits that form as the extra fuel dries in the injector's hot tip as the engine cools. They can also leak or drip pressurized fuel. Check for dripping fuel injectors by performing a leak down test with a fuel pressure gauge.
Fuel Pressure Regulator: A fuel pressure regulator provides steady fuel pressure to the fuel injectors under different loads and operating conditions. A traditional regulator contains a vacuum-operated diaphragm controlled by intake manifold vacuum. Electronic regulators do the same thing with more precise computer control.
Engine Coolant Temperature Sensor: Engine Coolant Temperature Sensor: The ECT informs the PCM of the engine's current operating temperature. This temperature is essential to the air-fuel mixture during cold starts because a cold engine requires a richer fuel mixture. If the ECT were faulty and the PCM was receiving a signal indicating a colder than an actual engine, the mixture would be enriched at the wrong time.
Above are a few inputs that affect a vehicle's air-fuel ratio and fuel trim adjustments. A faulty 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.