Improper operation of the IAC system can have significant impact on idle quality and driveability. If idle speed is too low, the engine may stall or idle very rough. If idle speed is too high, harsh A/T gear engagement may result.
On some IAC systems, the IACV step count or ECM duty ratio may provide hints as to whether a major correction is being made to offset a idle speed problem. For instance, if false air entry causes idle speed to be much higher than normal, the IAC system may correct for this condition by decreasing bypass air volume in an effort to bring idle speed back to the "target" idle speed.
The IACV step count or duty ratio may also identify a restricted air passage, misadjusted throttle, or IAC valve problem. Observe IAC signal data at idle, while applying various "loads" to the engine. Look for a corresponding change to IACV step count or duty ratio, as loads are placed on the engine. Also, a signal comparison to other known good vehicles may be helpful.
IAC Check Using Active Test
Some late model OBD-II vehicles are equipped with an active test feature that allows you to command changes to IACV positioning.
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