Base Running Airflow vs MAF Calibration

[NPD]

I am wondering if the Base Running Airflow >Idle Airflow table interacts with the MAF Calibration>Airflow vs. Frequency table? More specifically, if there was a mismatch between the two tables at a given frequency or engine RPM at the same g/s value could that cause fuel trim problems in the MAF calibration? Application is a 2007 LQ4 with a P59 PCM.

[5FDP]

Yeah.

If the overall airflow model is way off it will make all your idle tuning off as well. It's not always the easiest but getting the low rpm MAF curve and VE table sorted first is the best route before tackling idle airflow corrections.

[mowton]

The MAF Airflow model, as 5FDP stated needs to be tuned down to and actually a bit lower than the idle rpm as a minimum to remove as much error as possible. This will establish the overall Air/Fuel settings by the PCM.

The Base Running Airflow is used to begin the closed throttle, TB blade/IAC opening requirement. There are numerous airflow "buckets" that are added/subtracted from the BRAF which include but are not limited to LT/STIT (idle error from PID), Fans, AC (in some cases) and Throttle Follower/Cracker... to name a few. The PCM combines the value of each of the "buckets" and adds to the BRAF to develop what is called Idle Desired Airflow. This is converted to an "opening " defined as millimeters squared and then converted to either a number of IAC steps (DBC) or TPS % (DBW) to control the the amount of air required to set the idle/cosastdown rpm desired.....it does get some help from the Under/over speed spark control otherwse know as Adaptive Idle spark control.

Ed M

[NPD]

Thank you both for the explanations. Does the MAF then override the Idle Airflow setting if the Idle Airflow is wrong? Say the Idle Airflow is set to 16 g/s but the MAF model calls for 14 g/s at 3250 Hz (in this example where idle occurs). Would this create a fuel trim problem only at idle or all RPMs?

[cobaltssoverbooster]

the MAF is a device that accurately calculates the density of air coming into the engine. it is used predominantly for calculations related directly to fueling.
Idle Airflow is a system that tells the ecu how much airflow is required to idle under specific conditions set out by its many sub tables. Think of BRAF as the main tables and trims, ac, fans, etc... (as ed mentioned) are the modifiers.

i am looking at my p59 file and i dont see any mentioning of MAF in the Idle section. This confirms my statements that the two are completely independent.


best way to look at this is the Idle Airflow tables dictate the required amount of air needed and compare it to what its actually receiving. any errors are typically tracked via stit and ltit and the corrections are made by controlling the IAC valve.
the maf sensor records the incoming airmass and is used for calculating fueling errors. you program the curve of the maf so that the ecu outputs the correct fuel for the airmass calculated by the maf sensor. if this is wrong its error is tracked via stft and ltft then applied changes are made through injector pulse modifications.

the reason the maf has a huge role in this is because the cylinder load changes when the fueling error is high. this causes swings in the Idle Airflow section to compensate for the incorrect cylinder loads. by fixing the maf you have the correct cylinder loads and can thus accurately model the required idle air to keep the idle trim errors low.

im not the best at explaining it which is why some of what i mentioned may sound like repeated information. The thing to understand is maf is directly related to injector control, and Idle Airflow is directly tied to IAC control.

[scoob8000]

This is something I've been curious about since I started messing with the idle tuning stuff.

When we're logging desired airflow, how is the PCM calculating that? I assume based on fueling, it knows how much air it wants to bring the AFR in line? ie, ve is too rich, so desired air will be a lot higher?

[cobaltssoverbooster]

I kind of miss represented the effect of the maf sensor I'm sorry.
The maf as an airflow device can be used for calculation of air density which is also used for idle trims. Having it wrong not only effects cylinder loading but also effects the measured airmass value.

The ECU should use the airmass value for the mode it's operating in SD, maf.

[cobaltssoverbooster]

This is something I've been curious about since I started messing with the idle tuning stuff.

When we're logging desired airflow, how is the PCM calculating that? I assume based on fueling, it knows how much air it wants to bring the AFR in line? ie, ve is too rich, so desired air will be a lot higher?

Fueling errors are corrected via fuel trims... If they are disabled then there is no correction.
Actual rpm vs desired rpm is a balancing game between idle airflow settings and idle advance settings.

[scoob8000]

I meant what determines the desired airflow. Wasn't sure if it took fueling into consideration, or it just knows how much air it needs to keep a specified RPM. (Just being curious)