Tech Bulletin - DIT Subaru - Fuel "Lean Limit" tables causing unexpected fueling behavior

Date: 9/12/18
By: Mike McGinnis

Vehicles affected:

• 15+ Subaru WRX
• 14-18 Subaru FXT

Software affected:

• 2.0.0.0-16735 or higher

Firmware affected:

• not applicable

COBB has identified the lean limit tables as causing behavior Protuners do not expect, and we'd like to help you better work with them and understand how they work.

In order to do so, the table names and look of the tables has been updated.

"Closed Loop Fueling Target Base (Main) Lean Limit A-D (Coolant Temp)" has become "Closed Loop Fueling Target Base (Main) Lean Limit and CFF Transfer Modify A-D (Coolant Temp)".

Rather than an air/fuel target, the Z data is being presented as an air/fuel target adder. As with many adders, you can zero the Z data out if you want to "disable" the adder, preventing these tables from impacting fueling. If that is your goal, you can skip the rest of this bulletin which explains how to tune these tables using non zero values.

The lean limit tables manipulate how the final closed loop fuel target is applied to commanded fuel final. As a reminder, commanded fuel final, or CFF for short, is the final fuel target for injector pulse width (IPW) calculation. 

Here are some examples of how the lean limit tables can impact fueling:

• If the closed loop fuel target, from the "Closed Loop Fuel Target Base (Main)" tables and other compensations, is 12:1 AFR and the lean limit table value is 2.000 (AFR adder), then the commanded fuel final (CFF) in closed loop will be (12 + 2.000 = 14:1 AFR, assuming no other CFF compensations). The final closed loop target would still be 12:1 AFR in this case.
• The lean limit tables can also limit the final closed loop target (if lean limit result is richer than the base CL target). For instance, if the lean limit table value is 3.000 (AFR adder), the lean limit will be (14.7 - 3.000 = 11.7:1 AFR). If the closed loop fuel target (from "Base (Main)" table and other compensations) is 12:1 AFR, the final closed loop fuel target would be 11.7:1 AFR (because the lean limit is in effect). The commanded fuel final (CFF) in closed loop would be (11.7 + 3.000 = 14.7:1 AFR, assuming no other CFF compensations). This would cause very high fuel trims and/or extreme lean condition.

WARNING: The manipulation of the transfer of the final closed loop fuel target to the commanded fuel final target by this table can cause more positive fuel trims (i.e. lean condition) in closed loop when this table is greater than zero (or more negative fuel trims when the table is less than zero). Care should be taken when tuning this table to non zero values, especially where it would be active at higher loads in closed loop.