The MAP sensor has the biggest say in how much fuel enters the engine. The acceptable range of signal voltage is 0.5 to 4.5 volts. A failing sensor, (or older ones with a leaking vacuum hose), can report an incorrect value, but as long as it is within that range it will not set a fault code.
The best way to attack too much fuel is by connecting a scanner that can read live sensor data and look at the fuel trim numbers. If the short-term numbers are high negative, that means the Engine Computer knows something is causing too much fuel to enter the engine and is trying, unsuccessfully, to reduce that amount. If the numbers are high positive, it means the computer is requesting more fuel in response to something it sees from one of the sensors.
A cylinder misfire can also cause a very rich condition. You will smell the unburned fuel at the tail pipe, but the oxygen sensor only detects oxygen in the exhaust, not fuel. It will tell the computer to command more fuel into the engine. No matter how much fuel enters, there will still be that unburned oxygen from the misfire.
For engines that have a fuel pressure regulator on the fuel rail on the engine, a kinked / blocked fuel return line to the tank can cause fuel pressure to go too high. That will cause extra fuel to enter the engine.
An exhaust leak ahead of the front oxygen sensor can result in a rich mixture. Between the pulses of exhaust pressure, the momentum creates pulses of vacuum that can draw in outside air. The oxygen sensor will pick that extra oxygen up and report a lean condition that the computer will try to correct by adding more fuel.
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Friday, April 8th, 2011 AT 12:32 AM