The Engine Computer is preprogrammed at the factory based on cam lobe profile, injector size, fuel pressure, and base timing. Non of those things are adjustable. What I mean is, those things are known and assumed and do not change in such a way the computer has to monitor them. What the computer DOES adjust to is readings from the oxygen sensors by way of short-term fuel trims (STFT) and long-term fuel trims). Those numbers represent how much fuel it needs to add to or subtract from those preprogrammed values.
Carburetors could only be adjusted perfectly at idle with the low speed jet and high speed with the high speed jet, and everything else was somewhat of a compromise. With electronic fuel injection, the computer makes adjustments based on incoming air temperature, coolant temperature, engine load, intake air volume, engine speed, then it looks at the oxygen sensor readings to see how it did. From there it makes tiny needed corrections, but that's when it learns those corrections and places them in the "lookup tables". The next time it runs into the same conditions, meaning the same engine load, same throttle position and speed and direction of change, same air temperature, etc, it uses the number in that "cell" that it learned previously. Those are the short-term fuel trim numbers. They change constantly while you're driving.
When the computer sees that it's always making the same corrections for the same conditions, and those short-term numbers are remaining fairly steady, it moves them into the long-term fuel trims. Those are the numbers it starts out on when you start the engine. Technically, the long-term numbers are modifications to the factory values that meet the needs of the specific engine for best performance and lowest emissions. If they work well in that respect, the short-term numbers will usually be pretty close to "0".
Other variables include the amount of fuel vapor in the engine oil that is being sucked out and burned, minor differences in fuel pressure regulators, blockage or leakage in the injectors, and how much fuel vapor is being burned from the charcoal canister. None of those things are monitored by the computer so it has to rely on long-term fuel trim numbers as a starting point, then adjust them in the short-term as conditions change. Many computers monitor the flow rate from the charcoal canister but it still doesn't know what percentage of that is gasoline vapor. You're just adding one more variable that is not monitored by the computer by changing the camshaft profile. The computer won't make adjustments due to increased power or torque but it will try to correct for increased emissions. Basically that means it will adjust the amount of fuel going in and it can tweak the ignition timing advance.
The starting point for how much fuel enters is also a product of how much air is flowing through the mass air flow sensor. The goal of an RV cam is more low-end torque but to get that, it opens the valves in such a way that more air enters the cylinders faster. Regardless of how much air goes in, the computer is going to match the amount of fuel to that air. If something should happen where it is unable to hold the injectors open long enough during each pulse, it will assume it has lost control of the fuel / air mixture, set a related fault code, turn on the Check Engine light, and default to factory preprogrammed values to make the engine run as well and as cleanly as possible.
If you disconnect the battery, the long and short-term fuel trim numbers are erased from memory. Those lookup tables will be rebuilt as soon as you start driving again. Most of the time the short-term trims do a fine job and you don't even notice the tables are being updated. Once in a while the needed modification for a set of conditions is so extreme that it takes a while to reach the fuel trim number. That's when you might notice a hesitation or stumble one time but not the next time. You have to have something rather unusual going on for the performance to be that bad.
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Tuesday, July 10th, 2012 AT 3:02 AM
