AFR question(how do they do it)

[myburb]

Dealing with 6395/7427 OE, they say that if open loop afr is selected the table "afr vs temp vs vac will be used for closed loop fueling instead of 14.7. If that is the case how does the ecm handle that. Does it redo the o2 crossing point and then do a calc? Always wanting to know more.

[lionelhutz]

If it's open loop then it's not doing any closed loop fueling and the O2 sensors are not being used. The PCM basically uses sensor data and the sensor calibration tables to calculate the air entering the engine. Once it has calculated the air it adds the correct fuel to produce the AFR.

[myburb]

When I first saw "open loop" I thought the same think but on reading the notes in the adx . Below is a copy of that.
Title: Open Loop AFR vs. Temperature vs. Vacuum

Location: 4BBC

Usage: Open Loop AFR Tables

Tips: Normally used for open loop but can be used in closed loop if L400D Bit 0 "Open Loop AFR Enabled" is enabled and temperature is greater than L48CE "Minimum Temperature for Closed Loop" and temperature is less than L48CF "Maximum Temperature for AFR Lookup in Closed Loop".

Conversion: AFR = 0.100 * X

[myburb]

In my post the reference was pasted there but for unknown to me it didn't show up in the post.
Title: Open Loop AFR vs. Temperature vs. Vacuum

Location: 4BBC

Usage: Open Loop AFR Tables

Tips: Normally used for open loop but can be used in closed loop if L400D Bit 0 "Open Loop AFR Enabled" is enabled and temperature is greater than L48CE "Minimum Temperature for Closed Loop" and temperature is less than L48CF "Maximum Temperature for AFR Lookup in Closed Loop".

Conversion: AFR = 0.100 * X

[lionelhutz]

Well, an EFI system can only do closed loop fuel control at stoichiometric when using a narrowband O2 sensor.

[myburb]

Those are the reasons I asked how do they do that. I had fooled with the table in open loop a few months back. I guess the only way will be to burn a few chips with it enabled and afr's in left field and record the results.

[lionelhutz]

The only way to what? Convince you that the PCM can not determine/measure any AFR besides stoichiometric when in closed loop?

[1project2many]

The ecm can be made to target a different AFR. The O2 quickly loses accuracy as the AFR moves from stoich but it is possible to change the rich and lean set points and / or the switching voltage. I used to try to lean bias closed loop in some applications by adjusting the O2 parameters.

I do not have a ready answer for the OP's question. This thread has some clues:
http://www.gearhead-efi.com/Fuel-Injection/showthread.php?1486-OE-7427-lean-cruise

[lionelhutz]

GM never did that because a narrowband O2 is only accurate when it is switching or transitioning between rich and lean. The PCM doesn't go and change the switching voltages from being around 0.45V to being around 0.3V to target 1.1 lambda instead of 1.0 lambda (in other words, lean cruise) as an example. The PCM either goes open loop or says it's still closed loop EXCEPT it locks down the integrator so the O2 sensor feedback is being ignored. In other words, it is only calculating the fuel the same as it does when in open loop.