Mechanics

Idling High or Low

AD
Step by step automotive repair guide on how to troubleshoot and fix an engine that is idling high (fast) or low (slow). (Note: The engine idle speed is controlled by the computer and is NOT adjustable.) The troubleshooting procedure for high and low idle conditions are similar, which are included in the following:

Difficulty Scale: 4 of 10

Step 1 - If the check engine or service engine light soon has illuminated retrieving the diagnostic trouble codes could lead you in the right direction of what the problem is, or is related too.


Check Engine Light

Step 2 - The idle air control motor (valve) is responsible for controlling the engine idle speed, if this part fails or becomes weak the engine idle speed could become erratic, additionally, this condition may or may not trigger the MIL (check engine, service engine soon light.) Because of the nature of this valve (large air volume processed) it should be serviced when a tune up is performed to remove "coking" which will hinder the valve's performance. To perform a rough check of the idle air control, remove the valve while reconnecting the wiring harness. While observing the valve with a flashlight, turn the key to the "ON" position, (DO NOT START ENGINE.) the valve should move slightly.


Idle Air Control Valve (Off Engine)

Step 3 - Engine vacuum is a sealed system and must hold a specific amount to maintain proper idle conditions, inspect the engine vacuum hoses and intake gaskets.


Engine Intake Vacuum Leak

Step 4 - The air intake boot or tube is used to deliver air to the intake system of the engine. If this tube becomes loose or broken it can leak unmetered air into the engine effectively causing an unbalance (via the mass air flow MAF sensor) in the computer system which can produce fast or slow idle.


Removing Air Intake Boot

Step 5 - A throttle bore is used to regulate the volume of air flow the engine consumes. Controlled either by computer or by manual foot pedal this device must be clean and free from coking build up (light tar substance.)


Throttle Bore Service

AD

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


COMMENTS TO THIS ARTICLE


Please use our question form if you have a specific question about your car as we are not able to give you a full answer on this page.



Article first published (Updated 2014-07-06)