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Neal
Neal, Chevy Mechanic
Category: Chevy
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Experience:  Shop Owner and Mechanic on multiple makes.
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on 2001 chevy blazer 4.3ltr runs rough on idle . give it a

Resolved Question:

on 2001 chevy blazer 4.3ltr runs rough on idle . give it a gas and engine smooths out.i have changed plugs and wires dist cap and rotor.could this be a sensor?
Submitted: 5 years ago.
Category: Chevy
Expert:  Neal replied 5 years ago.
It sounds like it is your idle air control valve. It is pretty common and if the rough idle is your only symptom it is most likely your problem. I attached some info below.

IDLE AIR CONTROL (IAC) SYSTEM DIAGNOSIS

CIRCUIT DESCRIPTION
The Idle Air Control (IAC) valve is located in the throttle body of both the Throttle Body Fuel Injection (TBI) and the Multiport Fuel Injection (MFI) systems. The valve consists of a movable pintle, driven by a gear attached to an electric motor called a stepper motor. The IAC valve motor is a 2-phase, bi-polar permanent magnet stepper motor that is capable of highly accurate rotation, or movement, every time the polarity of a winding is changed. This change in polarity can be seen when observing a test lamp connected between ground or B+ and an IAC valve circuit while the PCM is attempting to change engine RPM. The test lamp will flash ON or OFF each time the polarity is changed. The PCM does not use a physical sensor in order to determine the IAC pintle position, but uses a predicted number of counts. One count represents one change in polarity, which equals one step of the stepper motor. The PCM counts the steps the PCM has commanded in order to determine the IAC pintle position. The PCM uses the IAC valve in order to control the engine idle speed. The IAC does this by changing the pintle position in the idle air passage of the throttle body. This process varies the air flow around the throttle plate when the throttle is closed. In order to determine the desired position of the IAC pintle at idle or during deceleration, the PCM refers to the following inputs:


The engine RPM
The battery voltage
The air temperature
The Engine Coolant Temperature (ECT)
The Throttle Position (TP) sensor angle
The engine load
The vehicle speed
After an ignition cycle, when the ignition key is turned OFF, the PCM will first seat the AC pintle in the air bypass bore, and will then retract the pintle a predetermined number of counts in order to allow the proper amount of air to bypass the throttle plate for engine start-up. This procedure is known as an IAC reset.

DIAGNOSTIC AIDS
Inspect for the following conditions:


A skewed TP sensor signal
A restricted air intake system
Objects blocking the IAC passage or the throttle bore
The correct, properly installed, and proper operation of the Positive Crankcase Ventilation (PCV) valve
Proper operation and installation of all air intake components
Proper installation and operation of the Mass Air Flow (MAF) sensor, if equipped
A tampered with or damaged throttle stop screw
A tampered with or damaged throttle plate, throttle shaft, throttle linkage, or cruise control linkage, if equipped
Excessive deposits in the IAC passage or on the IAC pintle
Excessive deposits in the throttle bore or on the throttle plate
Vacuum leaks
A high or unstable idle condition can be caused by a non-IAC system condition that can not be overcome by the IAC valve.
Neal, Chevy Mechanic
Category: Chevy
Satisfied Customers: 8334
Experience: Shop Owner and Mechanic on multiple makes.
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