I've seen a bunch of links that talk about how speed density works, but I haven't seen a lot of stuff that goes in-depth about how the newer OBDII cars perform EFI with input from the MAF and make adjustments to short term and long-term fuel trim. Does anyone have any links or reading material that they can recommend?

Thanks!

 

The difference in the PCM's does not come from the fact that they are either speed-density of mass-air. Both generally function the same, the only difference being the way the PCM gets the number for mass air flow.

In mass-air, it gets a mass flow value dierectly from the calibration table of the MAF sensor. In speed-density, the PCM calculates the mass air flow, by looking at RPM, Volumetric efficiency (the "speed" portion) and MAP and IAT (the "density" portion).

Once it gets the calculated mass air flow, it controls the A/F the same way..... (mass-air rate) / (A/F ratio) = fuel mass flow. Closed loop feedback alters a "multiplier" (consisting of both short term "integrators" and long term "block learn multipliers) that modifies the fuel mass flow based on the O2 sensor readings. There's no difference in the way the PCM works at this point, whether its speed-density or mass-air, or whether its OBD-I or OBD-II. The main difference in OBD-II is the addition of several additional functional checks on the emissions control equipment. They didn't really change the way the system operates.

There's a section in my scanner writeup on air/fuel management that provides a sort of simplified explanation:

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