I used to have HPtuners, and I'm fairly well versed in OBDII stuff. THis is my first OBD I car though. It's a 94 LT1 camaro.

Does the PCM manage fuel the same way that the OBD II does?

I.E.

On startup/cold engine, the PCM uses MAF tables to meter fuel in "open loop" mode.

Once the engine has run for a preset amount of time(according to temp) the engine switches to "closed loop" mode and begins to meter fuel based on the oxygen sensor readings. While idling, cruising etc... the o2's are providing data for the fuel mixutre.

If you ever go WOT, the PCM disregards the o2's and goes back to MAF tables.

 

There does not appear to be a significant difference in the way the LT1 OBD-I and OBD-II LT1 PCM's function. The main difference in OBD-II is the addition of a few more sensors and diagnostic checks, solely for the purposes of monitoring emissions control equipment and systems. There is no change in the engine management strategy.

I beleive you may have a misunderstanding of the MAF sensor. Basically, the MAF sensor is used under all the conditions you described above. It measures the mass air flow. The PCM adds fuel based on the mass air flow, divided by the target A/F ratio. No difference, whether its in open loop, closed loop or in power enrichment (PE) mode (~WOT). The MAF "table" is a calibration chart that translates the MAF sensor's variable frequency output to mass air flow in grams per second.

In open loop, the PCM uses the MAF sensor to measure air flow, and uses the stored long term fuel corrections to correct the PCM's standard fueling calculation. Target A/F ratio is varied, based on things like engine temperature (e.g. - cold start enrichment). Learning is disabled, so the short term fuel corrections are not used, and the long term corrections are not adjusted, based on O2 sensor readings. O2 sensor readings are ignored, because they aren't accurate when they are cold.

In closed loop, the only thing that changes is the PCM uses feedback from the O2 sensors to adjust the short term fuel corrections, and learning is enabled, so that the long term corrections can be increased or decrease if required to keep the short terms at an average of 128 (0%). Target A/F ratio is generally 14.7:1. MAF sensor is used to measure mass air flow. The PCM intentionally swings the actual A/F ratio back and forth to either side of the 14.7:1 value, using the short term corrections. This swinging back and forth is required to allow the monolithic cat to operate, running lean to allow the cats to break down the oxides of nitrogen into free O2, then running rich to allow the stored O2 to combine with unburned HC's and CO, to reduce the levels of all three pollutants simultaneously.

When you exceed certain combinations of throttle position percent and RPM, the PCM will go into power enrichment mode. The target A/F ratio is generally richened significantly. The MAF sensor is still used to measure the mass air flow. The O2 sensors are ignored. The short term corrections are locked a 128 (0%) and the stored long term corrections may or may not be used. If the PCM was adding fuel in closed loop using the long term corrections, it will continue to use them in PE mode. If the PCM was subtracting fuel in closed loop using the long term corrections, it will lock the long terms as 128 (0%).

Sort of a basic explanation... there are fine points that may cause deviations from these general operating guidelines, but for the most part, they are accurate.

I have an online scanner guide that has an A/F management section that attempts to explain this in a bit more detail.

http://members.aol.com/InjuneerZZ/ScanMast.htm

 

Thanks for the clarification.