The idle air clamp is used to place an upper limit on the airflow seen by the ECU when the throttle is closed and the idle switch is active. This is intended to limit the effects of over-reported airflow from disrupted airflow metering or from a BOV vented to atmosphere. The Idle Air Clamp table values are in units of the raw value AvgAirVolPerRevY.
Adjustment is made when the engine is fully warm (coolant temperature is at least 190°F or 88°C) by adjusting the table values low enough to prevent a rich condition as the engine rpm drops to idle with the throttle closed, but not so low that the mixture goes excessively lean under any closed-throttle conditions. Since the engine sees slightly more load when the air-conditioning compressor is engaged, two table columns are provided, one for comressor off and one for compressor on. Each will need to be adjusted separately.
At a given rpm, a cold engine requires a bit more airflow than a warm engine. The Coolant-based Adjustment table defines the adjustment made to the idle air clamp value when temperature is below 190°F (88°C). For example, if the closed-throttle mixture goes lean as the engine is warming through 70°F, the Coolant-based Adjustment table's 70° entry can be increased. However, changes to the Coolant-based Adjustment table are usually unnecessary and should only be made to alleviate over-clamping when the engine is idling at coolant temperatures below normal operating temperature.
The Enable idle air clamp checkbox must be checked for the idle air clamp to be active. Unchecking this checkbox is a quick way to test to see if a too-low idle air clamp value is causing a lean stumble.
Idle air clamp table values are in the same units as the raw value AirVolPerRevY. Idle air clamp operation can be monitored by logging AvgAirVolPerRevY (before clamping) and AvgAirVolPerRevW (after clamping). The airflow can also be displayed as MAFComp and MAFIdle, respectively, to show the difference in airflow/second. A difference between these two values indicates that the idle air clamp is actively limiting the airflow reading being used by the ECU.
Since MAF compensation is performed before applying the idle air clamp, proper idle air clamp operation requires that airflow has been calibrated by adjusting the MAF Comp settings. If you change MAF Compensation settings, you may also need to readjust the Idle Air Clamp table entries. Before making any MAF compensation adjustments, disable the idle air clamp by unchecking the Enable idle air clamp checkbox. After MAF compensation adjustments are completed, re-enable the idle air clamp and check its operation.
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