ORIGINAL: z28pete

The fuel trim is variable depending on condition and how the car is being used. There is a specific range allowed before the PCM will light the MIL. ST stands for short term trim which isconstantly varying depending on the input from the various sensors, especially the upstream O2 sensors, and the MAF ( Because the ST is a variable, the value is not defined in the FSM). LT stands for long term trim. If the ST reaches the end of range, The PCM automatically adjuststhe LTto bring theST back to mid range. There is a limit to the amount of adjustment available for the LT,and if it is exceeded the PCM will light the MIL. The LT is defined in Counts, usually somewhere around 110 to 156, and is the same for both banks. You can check the FSM for your car (driveability and emission section)for actual values.

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ORIGINAL: z28pete

If you are really interested, the first thing you should get is a copy of the Factory Service Manual from HelmInc.com. Part #GMP98FF ($135), and the GM OBDII 1996-2000 Emission Diagnostic Manual Part #GMPT00IM240 ($38), both available on the web from WWW.ElmInc.com. These manuals explain how the system works and how to make it work to factory spec. More informant ion is available if you buy tuner software or ahand held programmer for your particular car. You need to learn the basics of engine operation and tuning before jumping on a programmer as you can scramble the PCM so that it is totally confused to the point where the engine won't run, or you could make changes that actually damage the engine. You can go to a good bookstore such as Borders, Barns & Noble, or even a good speed shopwhere you can find many books on the subject. If your engine is stock, it is pretty well tuned by the factory and you won't gain much by messing with the computer. Good luck & keep reading.