AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 245)

Page 245 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
O2 Feedback
To control the engine's fuel delivery system, the EMS can be programmed to run open loop
and/or closed loop O2 feedback. Closed loop uses feedback from the oxygen sensor to make
temporary, but immediate, corrections to the injection to maintain a target AFR. The type of an
O2 sensor will determine how O2 feedback can be controlled. Due to the nature of standard
narrow band O2 sensors, 14.7:1 is the only air fuel ratio that can be accurately maintained in
closed loop. However, wideband O2 sensors can be used in almost every feedback condition.
A note on AFR data
14.7:1 is the ideal AFR right?
Not necessarily. 14.7:1 is what's called the stoichiometric AFR for gasoline. It is the ratio where
all available fuel is burned completely. For most performance applications, it is not the ideal
AFR to operate the engine at all the time nor is it the AFR that creates the most power or even
economy under all conditions.
When the fuel system is open loop, the O2 sensor is ignored and the injector pulse width relies
on the Fuel Map and fuel compensators to adjust injection duration.Open loop is necessary
during engine starts, when the O2 sensor has cooled below its operating temperature. It may
also be necessary when coolant temperatures arelow. In this state, the fuel vaporization is poor
and the engine will require a richer mixture to properly operate. When under heavy load, the
engine typically requires an air fuel ratio that is out of a narrow band oxygen sensor's standard
range and open loop will be necessary. When the accel fuel function is triggered during hard
accelerations, open loop may be necessary to help stabilize the O2 feedback. When the decel
function cuts fuel completely, O2 feedback will not be necessary.
User Definable Options for O2 Feedback
O2 FB Rich Limit
Units: Fuel % Description: The maximum amount of fuel to add when O2 feedback is enabled.
O2 FB Coolant Min
Units: Coolant Temp Temperature Description: O2 feedback will operate when the coolant
temperature has exceededthis value. This value is typically set near operating temperature.
O2 FB Global Gain
Units: Gain Multiplier Description: Overall O2 feedback gain. This value will always be one
unless the proportional and integral gains are not sufficient in controlling the air fuel ratio.
O2 FB Lean Limit
Units: Fuel % Description: The maximum amount of fuel to subtract when O2 feedback is
enabled.
O2 FB Max Load
Units: Engine Load Description: O2 feedback will operate when the engine load is below this
value. When using a narrow band O2 sensor, O2 feedback is typically not used at high engine