Это подсос воздуха на впуске. Возможно даже, что через уплотнительное резиновое кольцо самого моторчика. Моторчик снимал? Или дальше через гофру. В общем, надо внимательно смотреть.

Вот описание работы Моторчика
клапана ХХ и отчего зависят обороты ХХ:
2002 Dodge Neon L4-2.0L VIN C Copyright © 2009, ALLDATA 10.10 Page 1
Idle Air Control (IAC) Motor: Description and Operation
IDLE AIR CONTROL MOTOR
The Idle Air Control (IAC) solenoid is mounted on the throttle body. The PCM operates the idle air control solenoid. It is an electric stepper
solenoid.
The PCM adjusts engine idle speed through the idle air control solenoid to compensate for engine load, coolant temperature or barometric
pressure changes.
The throttle body has an air bypass passage that provides air for the engine during closed throttle idle. The idle air control solenoid pintle
protrudes into the air bypass passage and regulates air flow through it.
The PCM adjusts engine idle speed by moving the IAC solenoid pintle in and out of the bypass passage. The adjustments are based on inputs
the PCM receives. The inputs are from the throttle position sensor, crankshaft position sensor, coolant temperature sensor, MAP sensor,
vehicle speed sensor and various switch operations (brake, park/neutral, air conditioning and power steering).
When engine rpm is above idle speed, the IAC is used for the following functions:
- Off-idle dashpot
- Deceleration air flow control
- A/C compressor load control and power steering (also opens the passage slightly before the compressor is engaged so that the engine rpm
does not dip down when the compressor engages)
Target Idle
Target idle is determined by the following inputs:
- Gear position
- ECT Sensor
- Battery voltage
- Ambient/Battery Temperature Sensor
- VSS
- TPS
- MAP Sensor

Полярность естественно будет менятся при открытии/закрытии клапана ХХ , как уже указал Kroll.