Rough Idling

Diagnosing a rough idling condition

Have you ever experienced a rough idling condition on your vehicle? Maybe you have a rough idling condition and that is what has lead you to this site. A rough idling can occur for many different reasons. On this page we after going to be discussing some different things that can cause your rough idling condition.


Checking the Idle Air Control Motor

I have encountered a rough idle condition on just about every vehicle from a Ford F150 or a Jeep Cherokee all of the way to a Geo metro. There are many different types of vehicles out there and many different types of set ups an vehicle can have. It is important to understand the system you have in order to diagnose the root cause of a rough or unstable Idle.  Each certain set up go bad in their ways causing a rough idling condition. Most vehicles through the mid 90’s and up have an IAC motor that control the Idle. The IAC is short for the Idle Air Control motor. This motor sits on the throttle body and controls the idle. However, it can go bad causing rough idling or stalling condition.  OTC makes a really unique IAC Tester tool that can check the inputs to the Idle Air Control motor.  Keep in mind, this tool does not check the IAC motor itself.  Just the inputs such as powers and grounds.  So this will work great if you suspect a computer or wiring issue affecting the IAC’s performance.

However, starting in about 2005 or later (depending on the vehicle) drive by wire was introduced. This system replaces the need for any Idle air control motor. This system is very picky and the throttle plate needs to be in the right position when the vehicle is at idle or otherwise a rough or unstable idle or even stalling condition may result. These systems tend to affected more by dirt and debris on the throttle plate and throttle bore than the conventional systems with the idle air control motor.

My experience has been that a high idle or unstable idle is usually caused by an an air or a vacuum leak. Both of these two types of set ups can be affected by an air or a vacuum leak.  The idle may be higher or lower than normal if there is an air or a vacuum leak.  It is important to know the difference between the two when diagnosing a rough idling condition.  A typical air leak is before the throttle body.  For example A ford escort that has a mass air flow sensor that has a crack or ripped air intake tube or snorkel allowing air into the engine is just simply an air leak.  This can and will cause a rough idling condition as well as some other other symptoms.  However, a vehicle with a intake manifold gasket leak would be considered a vacuum leak.  This is because the air inside the intake manifold in negative pressure (with the throttle plate closed) or under a vacuum because it is behind the throttle plate.  Both conditions can cause a P0171 lean code, rough or unstable idle and sometimes even a stalling condition.

Rough idling can also be caused by bad motor mounts, an engine misfire, incorrect ignition or valve timing of even bad tune up parts.  I have even seen PCM updates fix a rough idling condition as well.  However below is a short video of a vacuum leak causing a rough idling condition.