Bones232

I’m curious about the relationship of the stoich value at LD232 ($61) and how it relates (if it relates) to the Rich O2, Lean O2 and Mean R/L thresholds? I noticed that the stock bin’s R/L values are set around the 450mv range (which for stock O2s is about 14.7 IIRC). I also noticed that the values used at idle (LD24c, LD24d & LD24e) are in the 600mv range. If that is the case wouldn’t the ecm continually pull fuel out at idle?

fun Pain

gas mileage, you don't need that fuel for idleing, a good tuner will even do that to a carb'd car on a dyno and wb.... I imagine GM thought about that too.

dimented24x7

iTrader: (2)

In general, the stored stoich O2 volts are used as setpoints to determine how far the O2 is away from 'stoich' and what direction its heading in (going richer, going leaner). From these, you get error terms, which are used by the closed loop control routine to determine how much fuel to add or remove to get toward the desired fueling.

Some of the values in the tables may vary because you may want to make the engine run slightly richer under certain conditions.

All of those closed loop O2 terms are part of the PID loop, which is basically a negative feedback loop. To get the error term, the ECM subtracts the current reading from the stored setpoints to get how far away the fueling is from whats desired. This is then multiplied by various gain terms to get the desired fueling correction.

The stoich value is used in the fueling, but in the calculations of how much pulsewidth is needed. Its the term that is used for closed loop fueling. Its not really used in the O2 stuff, its more just to make sure that the computer doesnt enter closed loop at some strange AFR.

Bones232

So if I read this correctly, the Rich O2 and Lean O2 terms create boundries where any readings between these two setpoints require no fueling corrections. Any voltage outside these limits would requiring fueling corrections with the Mean R/L term as the target.

dimented24x7

iTrader: (2)

Id have to look at it again to be sure, but I believe those are used to determine which way the fueling is going of the long term. If the fueling remains above or below those thresholds for some time, the integrator begins to step to correct the fueling and move the average fueling to stoich if the error is great enough. Its been quite some time since Ive looked at it so Id have to double check.

The proportional term is always active, and never stable at any time (for diagnostics of the O2 sensor), so in that regard the PID always doing something.

dimented24x7

iTrader: (2)

I was almost right. The mean rich/lean threshold is used basically to define 'stoich' for the O2 sensor. The rich and lean O2 thresholds are used along with this and compared to the O2 readings to determine the direction that the fueling is going in.

The thresholds themselves are subtracted from the filtered O2 readings to get the O2 error. The one used depends on whether the engine is going richer or leaner. And yes, you where correct in that if the O2 is between the thresholds, the O2 error is cleared. This means that the integrator wont/cant do anything. But, the proportional term will still push the O2 readings past the stoich point as the proportional term will never be zero, even when the O2 error is zero. This is done to make sure the sensor still works. If the readings stop (or never start) swinging around .450 volts and linger there, then the O2 is assumed to be not functional and a code 13 will be set.

You can make adjustments, but whatever is adjusted, you have to make sure that the O2 readings always manage to go outside the rich and lean window. This is what the ecm uses to determine if the O2 is working properly. Before making changes, though, be sure you understand how it works. There is a good paper by RBob on DIY-EFI.org thats worth reading.

Bones232

Thanks for following up That was very helpful.

Would you be able to confirm that the following terms are for "in idle" R/L thresholds?
LD24c R/L mean threshold for in idle (disassembly description: use when in idle)
LD24d Rich threshold for in idle (disassembly description: use when in idle2)
LD24e Lean threshold for in idle (disassembly description: use when in idle3)

Thanks again. I appreciate all your help.

dimented24x7

iTrader: (2)

I havnt looked at the $61, but Id imagine that there are seperate terms for when your idling vs driving.