Quote Originally Posted by Scorp1us View Post
But even so how does the computer know how to achieve 12 or 13 or anything that isn't 14.7?
Can tell you how $8d does it. I have to believe other GM OBD I masks are similar. Here's the short answer:
  • If in CL and not in PE, the AFR is always reported in the ALDL Data Stream as that set in the calibration (at 0x41A in $8d = 0x01BD = 14.73% AFR).
  • If in CL and PE, the reported AFR is a calculated value based on 2 Table values: (1) PE AFR % Change .vs. RPM and (2) PE AFR % Change .vs. Coolant Temp, and stoich AFR. As an example, at 3600 RPM and 200*F, the calculated AFR is 11.42. Note that MAP is not a function of this calcualtion because MAP > a cretain value was used to determine if PE was entered. Also, this is merely a calculated AFR value and may or may not agree with that reported by a WBo2. Likely it would not agree because the calculation uses values that make a lot of assumptions for a given engine.
  • If in OL, AFR is a function of stoich AFR modified by a value from a table 'OL AFR % Change .vs MAP', and which can be further modified if in rolling idle (TPS < 2.3% and MPH < 15). But again, this is only a calculated value.


The only true AFR is that reported by a WBo2.