Hi what, The Mass Air Flow sensor has the biggest say in how much fuel is commanded by the computer. If it is dirty, it will not accurately measure all of the incoming air so too little fuel will be sprayed from the injectors.
If your engine uses a barometric pressure sensor, the computer will believe its reading as long as it is between.5 and 4.5 volts. If the value is wrong, the computer will think you're up in the mountains or below sea level and will be asking for the wrong amount of fuel. Chrysler engines use only this exact same sensor to measure barometric pressure before the engine starts, then to determine fuel metering based on engine load once it is running. GM will sometimes use this too as a MAP sensor as a backup in case of failure of the MAF sensor.
If you have a single cylinder misfire due to a loss of spark, the oxygen sensor will detect the unburned oxygen in the exhaust. The computer will react by commanding an increase in fuel delivery from the other three cylinders on that side of the engine. No matter how much fuel it adds, there will still be that unburned oxygen from the misfiring cylinder. (O2 sensors don't detect unburned fuel). If you connect a hand-held computer, called a scanner, that can read live sensor data, you will see that O2 sensor reporting a lean condition while you see black smoke or smell raw fuel from the tail pipe.
A less common problem causes the same symptoms if there is a leak in the exhaust pipe before the first O2 sensor. Between the pulses of exhaust flow, the momentum creates little pulses of vacuum. Air can be drawn in through that leak and be detected by the O2 sensor. Again, no matter how much fuel the computer adds, the O2 sensor will continue to see unburned oxygen. An important clue here is your observation that the problem doesn't start until the engine is warmed up. The oxygen sensor doesn't start to do his thing until it reaches 600 degrees. They have electric heaters built in to hurry them along, but it can still take a few minutes for them to get hot enough to go into "closed loop". Until that happens, the Engine Computer bases all of its fuel metering decisions on all of the sensors except the O2 sensor.
Sunday, April 4th, 2010 AT 5:04 PM