First let's take a quick look at what happens when you step on the gas. The computer reads the throttle position (TPS), checks the coolant temperature (ECT), checks the intake manifold air flow and pressure (MAF and MAP), air temperature (IAT) and engine RPM. With all that info the computer then performs a "look-up" function. Similar to finding a coordinate on a map, except this map has many more variables than horizontal and vertical! Once the computer finds the correct coordinate for all of those operating conditions, it finds the result it was looking for - exactly how much fuel to inject for that condition in order to get *almost* the perfect mixture, giving you the best performance, economy and lowest emissions. All of this is continuously recalculated, dozens of times per second.
Impressive! But why is it *almost* the perfect mixture? Because dozens of things could influence your engine to the point where that factory-programmed value isn't quite right. To "close the loop" of information flow, there is a program running on your car's computer that uses information from our friend the Oxygen Sensor to check if the mixture is right where it needs to be. If it's a little off, the computer uses that information to come up with a fudge factor. The name of that fudge factor is Long Term Fuel Trim (LTFT). That value is the amount of correction that the computer will automatically apply to the value that it looks-up. Measured in percent, we typically see stay well under + or - 10% when driving. If it gets over + or - 25%, the check engine light will turn on and a code is set telling you that your engine is running either abnormally rich or lean.
I've simplified this just a bit. Here are a couple of the complications (and there might be a few more that I haven't learned yet). There's another fuel trim called Short Term Fuel Trim. There are also a bunch of those lookup tables for different driving conditions, each one can have a separate fuel trim fudge factor.
So - what does this mean to you? A few things.
First, when you get a code that your car is running too rich or lean, don't rush out and buy new Oxygen Sensors. That's just shooting the messenger. Those codes are usually caused by bad information from one of the other sensors that the computer uses to do its lookup.
Using AutoTap to monitor your engine's Long Term Fuel Trims can give you some insight into its health. Nice low numbers mean that your engine is pretty content using factory-programmed values. Big numbers mean that, for some reason, a big fudge factor is being applied.
Finally, you'll get the best engine power with a nice low fuel trim number. Why? Because your engine quits paying attention to the Oxygen Sensor at full throttle. It still applies the last known fuel trim correction, but since maximum power requires a bit richer mixture than what is ideal for the perfect mix of economy and emissions, you typically get the best performance if a large correction doesn't need to be applied.[/b]