Open Loop

In this mode, the MCU determines the air/fuel mixture based upon engine operation rather than oxygen sensor input signals. There are 5 open loop modes of operation and each has a specific metering pin position. However, because each condition may be present at the same time, the MCU is programmed with a priority ranking for each operation. The MCU complies with the highest priority. The open loop priorities (listed from highest to lowest) are as follows:

• Cold Weather Start-Up & Operation  - If air cleaner air temperature is below calibrated value of TES, the stepper motor positions the metering pins a pre-determined number of steps richer than that at initialization. Air injection is diverted "upstream". Lean air/fuel mixtures are not permitted for a preset period, following a cold weather start.
• Open Loop 2 (Wide Open Throttle)  - Open Loop 2 (OL2) is selected whenever air cleaner air temperature is above calibrated value of the TES and the WOT switch has been engaged. In OL2 mode, the stepper motor drives the metering pins to a calibrated number of steps rich of initialization and the air control valve diverts air "downstream". The timer is activated in this mode.
  NOTE: If a "lean limit" condition (altitude jumper wire installed) is selected, the air is diverted "upstream". The timer is inoperative if "lean limit" is selected.
• Open Loop 4  - This mode is selected whenever manifold vacuum falls below a preset value. During OL4 operation, the stepper motor is positioned at the initialization position. Air injection is switched "upstream"; however, air is diverted "downstream" if the extended OL4 timer is activated or "lean limit" (altitude jumper wire) is not installed. Air is also diverted "downstream" if the WOT timer is activated.
• Open Loop 3  - This mode is selected when spark advance vacuum level falls below a preset level. In OL3 mode, engine RPM is also determined. If the RPM voltage is greater than the calibrated value, an engine deceleration condition is assumed to exist. If the RPM voltage is less than calibrated value, an engine idle condition is assumed to exist.
  NOTE: Both deceleration and idle conditions are independently selectable to be either an open loop or closed loop condition. If selected as an open loop operation, air is diverted "upstream".
• Open Loop 1  - This mode will be selected if air cleaner temperature is above calibrated value, OL2, OL3 or OL4 is not selected and if engine coolant temperature is below calibrated value. The OL1 mode operates instead of normal closed loop operation during cold engine operating condition. In this mode, 1 of 2 pre-determined metering pin positions are chosen, dependent if the "lean limit" (altitude circuit) jumper wire is installed.
  NOTE: With each engine start-up, a start-up timer is activated. During this interval, if engine operating condition would otherwise trigger normal closed loop operation, OL1 mode is selected.
• Open Loop Position Variation  - An additional function of the MCU is to correct for a change in ambient conditions (altitude). During closed loop operation, the MCU stores the number of steps and direction that the metering pins are driven to correct oxygen content of exhaust gases. If the movements are consistently to the same position, the MCU will vary all open loop operation preset metering pin positions a corresponding amount. This function allows the open loop air/fuel mixture ratios to be adjusted to the existing ambient condition during each uninterrupted use of the system. This is to optimize emission control and engine performance.