Home FAQ

Why is my Subaru Forester not idling properly?

If your Subaru Forester is not idling properly, it could be due to a variety of reasons, such as issues with the engine sensors, fuel system, or ignition system. In this article, we'll explore the common causes of an improper idle in a Subaru Forester and provide some troubleshooting steps to help you identify and fix the problem.

Common Causes of Improper Idling in a Subaru Forester

There are several potential reasons why your Subaru Forester may not be idling properly. Some of the most common causes include:

Faulty Idle Air Control (IAC) Valve: The IAC valve regulates the amount of air entering the engine during idle. If the IAC valve is stuck, dirty, or malfunctioning, it can cause the engine to idle too high or too low.

Vacuum Leaks: Vacuum leaks in the engine's intake system can disrupt the air-fuel mixture, leading to an unstable idle.

Fuel System Issues: Problems with the fuel injectors, fuel pump, or fuel pressure regulator can cause the engine to run lean or rich, resulting in an improper idle.

Ignition System Problems: Faulty spark plugs, ignition coils, or the engine control module (ECM) can also contribute to an unstable idle.

Sensor Malfunctions: Issues with the oxygen sensors, mass airflow (MAF) sensor, or throttle position sensor can affect the engine's ability to maintain a proper idle.

If your Subaru Forester is experiencing an improper idle, it's essential to diagnose and address the underlying issue to ensure the vehicle is running smoothly and efficiently.

Troubleshooting Steps for Improper Idling in a Subaru Forester

To troubleshoot the cause of the improper idling in your Subaru Forester, follow these steps:

Check for Diagnostic Trouble Codes: Use an OBD-II scanner to check for any diagnostic trouble codes (DTCs) that may be stored in the vehicle's ECM. These codes can provide valuable information about the specific issue causing the improper idle.

Inspect the Idle Air Control (IAC) Valve: Clean or replace the IAC valve if it appears to be stuck or malfunctioning.

Check for Vacuum Leaks: Inspect the intake system, hoses, and connections for any signs of leaks. Use a vacuum gauge or smoke test to identify and address any vacuum leaks.

Examine the Fuel System: Check the fuel pressure, fuel injectors, and fuel pump for proper operation. Replace any faulty components as needed.

Test the Ignition System: Replace the spark plugs and inspect the ignition coils and wiring for any issues.

Verify Sensor Functionality: Test the oxygen sensors, MAF sensor, and throttle position sensor to ensure they are working correctly.

By following these troubleshooting steps, you should be able to identify and address the root cause of the improper idling in your Subaru Forester. If you're unsure about any of the steps or need further assistance, it's recommended to consult a professional mechanic for a more thorough diagnosis and repair.

Why is my Subaru having idle problems?

Your engine could be idling roughly due to a number of different reasons. One reason for a rough idle is it could be time to clean out the fuel injectors or it could be time to change out the spark plugs. Other reasons for the rough idling include: Exhaust problems.

Why is my car idling rough but drives fine?

A rough idle when a car drives fine can be caused by issues with the air-fuel mixture, the ignition system, or engine mounts. Common culprits include vacuum leaks, dirty air or fuel filters, worn spark plugs, clogged fuel injectors, or faulty engine mounts. Addressing these problems is important for engine performance and longevity.

This video explains some common causes of rough idle: 55sHonest Mechanic ColoradoYouTube · Mar 13, 2024
Air and fuel mixture problems

Vacuum leak: A leak in a hose or gasket can let in unmetered air, throwing off the air-fuel ratio.
Dirty fuel filter or injectors: A clogged fuel filter or dirty fuel injectors can restrict fuel flow.
Dirty air filter: A very dirty air filter can restrict the amount of air reaching the engine.

Ignition system problems

Worn or faulty spark plugs: Old spark plugs can cause misfires, leading to a rough idle.
Bad ignition coils or wires: Problems with these components can lead to a weak or inconsistent spark.

Other potential causes

Faulty engine mounts: Broken or weak motor mounts can cause excessive vibration at idle, even if the engine is running smoothly.
Dirty throttle body: Carbon buildup on the throttle plate can impede airflow at idle.
Faulty sensors: Issues with sensors like the mass air flow (MAF) or oxygen sensor can cause the engine's computer to use an incorrect air-fuel mixture.
Stuck EGR valve: A stuck or clogged exhaust gas recirculation (EGR) valve can also cause rough idling.

This video demonstrates how to fix a rough idle: 38sChrisFixYouTube · Sep 3, 2020

How to reset idle on Subaru Forester?

To perform a Subaru Forester idle relearn, disconnect the negative battery terminal for at least 15-30 minutes to fully reset the Engine Control Module (ECM). After reconnecting the battery, turn the key to the "on" position (without starting the engine) for about 10-15 seconds, then start the car and let it idle for 10 minutes with all accessories off. After the 10 minutes, turn the car off, wait 20 seconds, and start it again for a final 5-minute idle period before test driving.

This video demonstrates the Subaru idle relearn procedure: 1mMrSubaru1387YouTube · Jul 12, 2019
Step 1: Reset the ECM

Disconnect the negative battery terminal.
Wait for at least 15 to 30 minutes to ensure the ECM is fully reset.
Reconnect the negative battery terminal.

You can watch this video to learn how to reset the throttle position: 59sBbernhardsrYouTube · Feb 3, 2016
Step 2: First idle phase

With the key in the ignition, turn it to the "on" position (but do not start the engine).
Wait 10 to 15 seconds.
Start the engine and let it idle for 10 minutes.
During this time, keep all accessories (like the A/C, radio, and defogger) turned off and do not touch the accelerator pedal.

Step 3: Second idle phase

After the 10 minutes of idling, turn the engine off.
Wait at least 20 seconds.
Turn the key back to the "on" position (without starting the engine) for another 10 to 15 seconds.
Start the engine and let it idle for an additional 5 minutes, again without accessories or touching the accelerator.

Step 4: Finalize the relearn

After the final 5-minute idle, turn the engine off and remove the key.
You can now take the vehicle for a test drive to ensure the idle is stable.

This video shows the final steps of the idle relearn procedure: 59sAuto ResourceYouTube · Jun 17, 2020

What is the most common problem with the Subaru Forester?

The most common problem with the Subaru Forester is the continuously variable transmission (CVT), which can suffer from issues like shuddering, delayed acceleration, and whining noises, and may require expensive repairs. Other frequent issues include problems with the engine (especially oil consumption), the electrical system, and, in certain model years, issues with windshields cracking and batteries dying quickly.

You can watch this video to learn more about Subaru Forester common problems: 51scar PROblemsYouTube · Feb 19, 2024
Transmission (CVT)

Shuddering and jerking: This often occurs when accelerating from a stop.
Delayed acceleration and slipping: The engine revs up but the car doesn't speed up as expected, which can be dangerous when merging.
High-pitched whining: This can be a sign of worn pulley bearings inside the transmission.
Repair cost: CVTs are often not rebuildable and require a complete, expensive replacement.

Engine

Excessive oil consumption: Many model years, particularly from the 2014-2018 generation, are known for burning through oil rapidly.
Oil leaks: Leaks, often requiring expensive engine replacements, are another reported problem.

Electrical and other issues

Electrical malfunctions: Issues can range from infotainment system glitches and GPS errors to more serious problems like a battery dying prematurely or the EyeSight safety system turning off unexpectedly.
Air conditioning (AC): Some models experience AC failure that can be costly to fix.
Windshields: In the 2019-2024 generation, windshields can crack from small chips.
Airbag malfunctions: Older models, such as the 2009-2011, have been associated with malfunctioning airbags.

This video explains some of the most common issues with the Subaru Forester, including the transmission and engine: 57sGear GazetteYouTube · Oct 23, 2025

Kevin Bennett

Company Owner

Kevin Bennet is the founder and owner of Kevin's Autos, a leading automotive service provider in Australia. With a deep commitment to customer satisfaction and years of industry expertise, Kevin uses his blog to answer the most common questions posed by his customers. From maintenance tips to troubleshooting advice, Kevin's articles are designed to empower drivers with the knowledge they need to keep their vehicles running smoothly and safely.