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Originally Posted by tmoss
The EEC does know how much fuel should be being injected as it monitors the O2 sensors in closed loop operation. It will adjust injector pulse widths if needed to richen or lean depending on whether there is too much or too little fuel. In open loop (wide open throttle - WOT) the EEC ignores the O2 sensors and goes straight to the table look ups for fuel and spark. So, you can tune your fuel needs at WOT with the fuel pressure regulator and the EEC will tune A/F ratios back into spec when running part throttle normal driving conditions. This keeps fuel consumption at or close to stock.
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Well you have the theory right, but your deduction isn't really accurate.. It has absolutely no idea how much fuel it's injecting. It knows how much fuel it thinks it's injecting. Most of the time it also has an idea about how much more/less fuel it needs to inject. In addition, it knows how much air it thinks it's taking in which is where the problem arises. It's really clueless on both so if you modify only one variable it quickly throws a temper tantrum because nothing adds up.
The 02 sensors tell it how lean/rich the engine is running in closed loop, but since closed loop goes out the window about 3 seconds (unless you're an unlucky person with a Cobra) after you put the pedal down 75% or more, it doesn't even care what the O2 sensors are reading. Also, the O2 sensors don't know how much fuel or air is being mixed in the engine so it can only read lean/rich fuel conditions, not actual air or fuel volumes.
The SD EEC has a finite table (which is much more flexible than MAF car tables) of injection data to use. If you go outside that table, the EEC assumes a sensor is malfunctioning so it either discards data or goes into limp mode, and this is when the drivability issues surface.
The idea is to keep the ratio of air and fuel coming into the engine close enough to stock so the EEC can compensate.
An analogy is the EEC is balancing its checkbook, but it only cares to get within $200. It thinks it knows how much money there is, but its balance doesn't match the balance on the bank statement (oxygen sensor). It looks at it's tables and counts deposits for $1000 (air intake) and it counts its withdrawls for $1000 (fuel injected). The problem is mom (the E303 camshaft) made a birthday present of $500 (air) into the account that it didn't know about.