Why Is My Car Idling Roughly?

While idling in the driveway, your car starts shaking. If that shaking is rough idle, you've got a problem that needs fixing sooner rather than later.

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How your engine idles is a good indicator of how several of your car’s operating systems are working together. Rough idling is a common problem that can be difficult to diagnose. Understanding and recognizing how to identify rough idling, what can cause it and what you can do to fix it are critical to the overall health of your engine.

What Is Idling?

Idling is what an engine does when it’s running but your vehicle is not moving. Due to the transmission’s gearsets being engaged, engine speed (RPMs) are lower in “Drive” than when idling in “Park.” Except for accessories that are on, engine RPMs should remain smooth and steady because the engine is operating without any load.

What Is Rough Idling?

Rough idling is usually marked by shaking or vibrations felt in the vehicle. Depending on the cause, some of these sensations can be more severe than others. You may also notice or feel the RPMs randomly changing. A smooth idling engine indicates your engine’s air fuel mixture, fuel, ignition and emission systems are working exactly right.

When Does Rough Idling Occur?

Due to lower engine speeds, rough idle is usually more pronounced in “Drive” when sitting at a red light. Consequently, higher RPMs when idling in “Park” can help smooth out a rough idle. Depending on the severity of the problem, you may also experience stalling or backfiring.

Here are the most common reasons a car idles roughly, from the simplest and most likely to the more complex and expensive to repair.

Air filter/PCV valve

Always check the air filter and PCV valve first. A clogged air filter starves an engine of much-needed air, skewing the air/fuel mixture. On the other hand, a stuck or dirty PCV valve allows too much air into the engine. Both will cause your engine to idle roughly or stall when idling.

Replacing an air filter and PCV valve (they’re cheap, always replace, never clean) are basic, inexpensive, DIY maintenance tasks.

Vacuum leaks

Vacuum leaks from a loose, cracked, or damaged vacuum hose affect air/fuel mixture causing an engine to idle rough. A major vacuum leak will cause your engine to stall when idling. With the engine running, carefully listen for a hissing or sucking noise indicating a vacuum leak. Replacing vacuum hoses is typically an easy DIY fix.

Ignition system

A misfiring engine, caused by worn/dirty spark plugs, a cracked distributor cap, a defective ignition coil or damaged ignition wires, cannot fully burn all the fuel in the combustion chamber. This will cause your engine to idle roughly.

Replacing spark plugs, spark plug wires and the distributor cap (and rotor) are all DIYable tasks. However, diagnosing and repairing Coil Over Plug ignition systems (they have no spark plug wires) is best left to your mechanic.

Fuel system

Low or high fuel pressure from a dirty fuel filter, defective fuel pump, clogged fuel tank strainer, bad fuel regulator or dirty/clogged fuel injectors can cause rough idling. Try replacing the fuel filter and add fuel injector cleaning treatment to your fuel tank to clean and hopefully unclog your injectors. If this doesn’t fix the rough idle, it’s time to bring your car in for service.

Is the Check Engine Light (CEL) On?

Although obvious, when the check engine light is on, it means an engine sensor is sending abnormal data to the computer (ECM). The problem can be a dirty, failed or failing mechanical component or one of many sensors.

Without accurate data, the ECM cannot precisely control fuel delivery and spark timing (and automatic transmission shifting). That results in a rough idling engine. Besides turning on the CEL, the computer stores a “trouble code” in its memory that helps identify the source of the problem. Let’s look at some of the engine control devices that can affect engine idle.

Mass Air Flow Sensor (MAF)

The MAF measures the rate of air flowing into fuel-injected engines. A dirty or faulty MAF will cause an engine to idle roughly, as well as stall when idling. There are DIY aftermarket solvents you can use to clean a dirty MAF. Be sure to follow all label safety instructions and directions.

Throttle Position Switch (TPS)

The TPS measures throttle plate movement and position. Depending on engine load (sitting at idle, partial acceleration or full acceleration), TPS data helps the ECM adjust the air/fuel mixture and spark timing. Bad data from a defective TPS equals rough idle and other drivability issues. Leave it to the pros to diagnose and replace a bad TPS.

Idle Air Control (IAC)

The IAC regulates the amount of air entering the engine to control engine idle speed. Carbon gradually builds up on the IAC valve and threads, restricting air flow into the engine. That mimics a vacuum leak and rough idling, or worse, sticking and causing your engine to stall when idling. Cleaning the IAC and the throttle body with a brand name aerosol carburetor or throttle body cleaner is an easy DIY fix.

Coolant Temperature Sensor/Switch (CTS)

A CTS plays a significant role in how the ECM adjusts timing and fuel calculations for optimal engine performance. An out-of-calibration or failing sensor can cause a constant lean air/fuel mixture, causing the engine to idle roughly. Let your mechanic handle replacing the coolant temperature sensor.

Oxygen (O2) sensor

An oxygen sensor monitors and analyzes the amount of oxygen in a vehicle’s exhaust system after combustion. A dirty, damaged or malfunctioning O2 sensor (or sensors) affects the air/fuel mixture, causing the engine to idle rough. Replacing on oxygen sensor can be a DIY project. Be sure to purchase a sensor specific to your engine.

Exhaust Gas Recirculation (EGR) valve

An EGR valve helps prevent production of nitrogen oxide (NOx) tailpipe emissions. The EGR valve should be closed at idle. However, carbon deposits can keep an EGR valve stuck open. That causes the engine to idle roughly, misfire and stall. Let your repair shop take care of this fix.

Other Possible Causes

Low engine compression can also cause rough idling. And although unusual, collapsed engine mounts or a damaged crankshaft damper can cause vibrations that mimic a rough idling engine. Leave these problems to the experts to diagnose and repair.

Most of the problems listed here can be avoided by following your vehicle manufacturers’ maintenance schedule. A rough idling engine is a clue that some part in your vehicle isn’t functioning properly or may be failing.

Besides wasting fuel and other drivability issues, a rough running engine should be promptly diagnosed and repaired to prevent engine damage and ensure your driving safety.

Robert Lacivita
Bob Lacivita is an award-winning auto technician and career and technical educator and freelance writer who has written about DYI car repairs and vehicle maintenance topics, as well as writing state, federal and organizational foundation grants, and helped design a unique curriculum delivery model that integrates rigorous, relevant academic standards seamlessly into technical/vocational training, for more than 20 years. His work has been featured in Family Handyman, a Reader's Digest book and Classic Bike Rider magazine, among others. Bob and his wife lived through 20 years' worth of DIY home remodeling while parenting two (now grown) boys and now relax by watching their three fabulous granddaughters.