If you use a MAF translator or change the MAF tables in the PCM will it adjust your AFR or will the O2's compensate? I have read threads both ways. I also read that the O2's can only compensate so much. So I think I keep confusing myself the more I think about it.
I want to dyno tune the car. In my mail order tune one of the few things changed is the %Change to Fuel/Air Ratio at WOT table, basically making it leaner at different RPM's at WOT.
Is this what we change to adjust the AFR at WOT when we dyno tune?
My car still smells rich so, at other than WOT how does it know?
If the O2's are looking for 14.7 how can we program the curve for 12.5?
I don't see how the computer can be tuned for anything other 14.7 AFR unless you had wide band O2's.
On any BLM cell if I am hitting between 131 and 137 on the BLM reading is this a rich or lean condition? Does this have to do with the AFR when it is other than 14.7 or is it more to do with engine load?
Other than playing all day on a dyno just to see what happens were can you go to get a good sense of how the PCM uses the O2’s to make the car run other than 14.7 AFR? I have done a search and I understand that I want to be at 128 no matter what cell I’m in but what do the cells mean.
I’m not trying to learn to be a tuner on this board. But I would like to understand what you can change in the PCM to affect the AFR.
All I ever hear is you can’t do this or that because the PCM will compensate with the O2’s. For ex. changeing fuel pressure. But then we go and get a tune so something is going on.
Didn' mean to get too long.

 

The PCM only uses the O2 sensor feedback when the PCM is in "closed loop" = part load, part throttle. That's when it tried to maintain 14.7:1 A/F ratio, to maximize the performance of the catalytic convertor.

When you step on the throttle, the PCM ignores the feedback from the O2 sensors, and sets a "target" A/F ratio that it calculates from RPM and coolant temp. Typically, this value is around 11.7:1 on stock PCM tuning. This is called the "power enrichment" (PE) mode.

A "tune" will generally alter the PE vs RPM and PE vs coolant temps, to adjust the target A/F ratio at WOT.

The PCM sometimes uses what it learns in "closed loop" to calculate the PE mode fuel. A simplified explanation... if the PCM was using the long term fuel corrections (BLM's) to add fuel (BLM's above 128) in closed loop, it will continue to use the Cell 15 BLM in PE mode. If the PCM was using the BLM' sto cut fuel (BLM's under 128) it will default to Cell 18 and use 128 in the fuel calcualtion in PE Mode.

A BLM above 128 indicates that when the PCM used its "standard" fuel calculation, the O2 sensors told the PCM the A/F ratio was too lean (referenced to 14.7:1), and the PCM first increased the short term corrections (INT's), and then increased the long terms (BLM's) when the short terms were averaging above 128. Your 131-137 BLM's indicate that if the PCM was not adding the extra fuel, the engine would be running leaner than 14.7:1 in closed loop.

The "cells" are used to cover the entire range of RPM and engine load (MAP)..... Cell 01 is low RPM/low MAP. Cell 15 is high RPM/high MAP. Cell 16 is used for idle. Cell 17 is used for decel, and Cell 18 is used for open loop (TPS >0%)

You might want to read through the air/fuel portion of this:

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

 

yeah....what injuneer said.....i think.

 

Thanks Injuneer that is STARTING to make sense.

So the PCM DOES try to maintain 14.7 at other than WOT. Then that all makes sense.

A "tune" will generally alter the PE vs. RPM and PE vs. coolant temps, to adjust the target A/F ratio at WOT.

I can't seem to find the PE vs. RPM or the PE vs. Coolant Temp tables using Tuner Cat. Are they there under another name or do I have to buy LT1-Edit?

And what should you do if your long terms are not 128? If the PCM is trying to hit 14.7 AFR then eventually it should compensate. Why does it not adjust until it hits 128? Or the fact that it is not 128 saying that it IS adjusting so it maintains an AFR of 14.7?

Thanks!

 

The BLM is a "permanent" (= stored) multiplier. The PCM looks at the mass air flow from the MAF sensor. It then adds enough fuel to achieve the desired 14.7:1 A/F ratio (14.7 pouunds of air for every 1 pound of fuel). After the PCM calculates the injector pulse width required to deliver that 1 pound of fuel for every 14.7 pounds of air, it multiplies the pulse width by BLM / 128. If your BLM is the desired 128, the multiplier is 128 / 128 = 1.000000. The PCM starts the fueling this way, and as long as the O2 sensor is sending back an average signal of 450mV, it assumes it is getting the calculation right. The PCM uses a similar "short term" fuel correction (integrator = INT) in the same way, to "temporarily" (not stored) correct the mixture.

If the PCM sees that it always has to use an "INT" that is always above 128 to correct its standard fuel calculation, it ratchets up the BLM to a number above 128, until the INT returns to an average value of 128. This means that, using the standard calculation, the PCM can not achieve the desired 14.7:1. Lets say the BLM has been adjusted up to 135, and has been stored in one of the PCM "cells". That means after the PCM makes it standard calculation and gets the required pulse width, it multiplies the pulse width by 135 / 128 = 1.0547. In other words, the PCM needs to add about 5.47% extra fuel to its standard calculation, in order to achieve 14.7:1. If it didn't add this fuel, the A/F ratio would be too lean. The BLM IS the compensation that it applies to achieve the correct A/F ratio.

That means that there is something in the control system that is preventing the PCM from achieving the correct A/F ratio. Maybe There is air coming into the system that hasn't gone through the MAF sensor, perhaps someone has screwed with the MAF sensor, and the output signal frequency for a given mass air flow no longer matches the calibration table that is programmed into the PCM. Or maybe the fuel pressure is not exactly the 43.5psi that the injectors are calibrated for.

There are numerous things that can throw the PCM's fuel calcualtion "off"... and it compensates to bring the A/F ratio back to 14.7:1.... it does this by adjusting the BLM's. It does not try to bring the BLM's back to 128... it tries to bring the A/F ratio back to 14.7:1.

 

Thanks Injuneer for taking the time to write that.
That is what I'm looking for. The BLM / 128 turned on a light bulb.

Thanks!