From:
http://www.thorssell.net/hbook/intro.html
Sometimes it helps if you try to think like a computer. A computer is just a set of circuits that can switch one way or the other. It makes "this" or "that" decisions based on data it is receiving.
Fuel system control is a good example of how this works.
Say the oxygen sensor (O2S) signal line was grounded. The PCM says "the O2S is lean (0.0V) , so I'll richen the mixture". The O2S line is still grounded so the PCM keeps on richening. After a bit of this the vehicle is blowing black smoke.
The Manifold Absolute Pressure (MAP) sensor which reads manifold vacuum is another example: When the driver accelerates, the vacuum to the MAP drops off and the frequency it outputs starts going higher. The PCM says "MAP frequency up = acceleration = richen mixture" and richens the mixture for acceleration.
Now suppose the MAP line develops a leak and the vacuum drops to the MAP. The PCM again says "MAP frequency up = acceleration = richen mixture" and richens the mixture. Now you have a vehicle with a vacuum leak blowing black smoke.
This is why, when the customer complains about fuel mileage and black smoke you may get a lean code, or no code. In the case of the lean code, the rich mixture is just the PCM trying to compensate. In the case of the MAP problem, the PCM didn't see a problem. It saw what it thought was acceleration and it acted correctly according to its rules.
Keep this in mind when you are trying to figure out a tough problem.
Try to put yourself in the place of the PCM and figure out what type of INPUT would give you the REACTION you are getting.