What Causes Rough Idling?

As your car ages, it’s normal to encounter a few issues, including rough idling. This can happen even in newer cars, but it’s more common in high-mileage vehicles. However, a rough idle isn’t a standalone issue; it indicates there’s an underlying problem with the engine or one of the related parts or components.

How Do You Know if Your Engine Has a Rough Idle?

Before we get into what can cause a rough idle, it’s important to be able to identify rough idling. When your engine is running, but your car isn’t in motion, this is referred to as idling—for example, when you start your car or when you’re stopped at a stop light. When your engine is idling, it’s only generating enough power to run essential systems, such as the alternator, water pump, and air conditioning.

Ideally, the idle should be smooth, consistent, and quiet. If you notice odd vibrations, skipping or shaking sounds from the engine, inconsistent idle speeds, or stalling or backfiring, your engine is experiencing rough idling. Although it’s generally safe to drive your car, you do risk the issue getting worse and potentially creating more expensive repairs if the cause is not corrected.

Common Causes for Rough Idling

While there are many issues that can cause a rough idle, here’s a look at some of the most common causes:

  • Clogged Engine Air Filter

Engines need clean air for proper combustion. The engine air filter prevents dust, dirt, sand, and other debris from entering the combustion chamber, but they’ll become clogged and lose their effectiveness over time. As the air filter becomes clogged, it affects the air quality and the amount of air entering the engine. If the air filter is clogged, it will need to be replaced or cleaned (if you have a reusable filter).

  • Fuel Pump, Fuel Filter, and Fuel Injectors

The fuel system comprises two essential components: the fuel pump and the fuel injectors. The fuel pump pressurizes the fuel in the tank and sends it to the fuel injectors, where it’s atomized and sprayed directly into the combustion chamber for ignition. Both the fuel pump and injectors need to operate at specific pressure levels and angles for the combustion process to occur optimally. Any issues with these components can lead to insufficient combustion and a rough idle.

Using poor-quality fuel can cause carbon to accumulate around the injector nozzles. Apart from a rough idle, this can also cause a sudden decrease in power and fuel efficiency. Another potential culprit is the fuel filter, which can block fuel flow if it becomes clogged.

  • Worn Spark Plugs

Spark plugs are a crucial part of the ignition system; they generate an electric spark that ignites the air-fuel mixture in the combustion chamber so the engine can produce power. When the spark plugs are faulty, the air/fuel mixture may not combust properly. This can cause numerous issues, including a rough idle, sluggish acceleration, poor gas mileage, and engine knocking.

  • Vacuum Leaks

A vacuum leak occurs when extra air enters the engine’s intake system without being detected by the engine control module (ECU). In a properly functioning engine, all the air that reaches the cylinders is routed through the throttle body. The mass air flow sensor (MAF) located on the intake measures the amount and temperature of the air passing through and relays this information to the ECU, which then injects the appropriate amount of fuel into the combustion chamber.

When there’s a vacuum leak, some of the air bypasses the throttle body and enters the intake manifold directly, which creates a lean air-fuel mixture that can lead to rough idling. In more severe cases, a lean mixture can cause engine detonations or knocking.

  • Defective Mass Air Flow Sensor (MAF)

The mass airflow sensor (MAF) helps determine the rate of airflow entering the engine and relays this information to the ECU. The ECU then adjusts the fuel delivery accordingly to achieve the correct air-fuel mixture in the combustion chamber. When the MAF sensor malfunctions, it can send inaccurate data to the ECU, resulting in either too little or too much fuel being delivered to the engine. In the case of too little fuel delivery, the engine will burn a lean mixture, which causes a rough idle and a loss of power.

If the issue is caused by dirt and dust accumulating on the sensor, cleaning it might temporarily resolve the problem. In some cases, however, it’s recommended to replace the MAF sensor entirely to keep your engine performing optimally and avoid potential issues.

  • Low Compression

Low compression is one of the more serious reasons for a rough idle in modern engines. Engines have a four-stroke cycle that includes intake, compression, combustion, and exhaust. During the compression phase, the piston moves inside the cylinder and builds pressure by compressing the air-fuel mixture. This higher-pressure mixture ensures the spark plug can ignite the fuel quickly and efficiently. However, when the piston fails to generate enough pressure/compression within the cylinder, it results in inefficient combustion.

Low compression is most noticeable when idling, but it can also cause misfiring, knocking, and may even prevent the engine from starting in severe cases. Low compression is most often caused by mechanical failures, such as worn piston rings and intake or more typically, exhaust valves.

  • Oxygen Sensor Failure

The oxygen sensor measures the amount of oxygen in the exhaust gases and relays this data to the ECU. The ECU then adjusts the air-to-fuel ratio to minimize the amount of unburnt oxygen in the exhaust gases. If the oxygen sensor is failing, it can cause a rough idle, poor fuel efficiency, and excessive carbon production—which can cause further issues for your engine.

  • Failing PCV Valve

The positive crankcase ventilation valve (PCV valve) is a one-way valve that prevents exhaust gases from mixing with engine oil. If the two do mix, it can cause harmful sludge to form, which can potentially damage the engine components. In addition, a faulty PCV valve can cause pressure to build up in the crankshaft, which can lead to blown gaskets and seals. If the PCV valve’s breather tube gets stuck in the open position, this can also allow an excessive amount of air to enter the intake manifold, causing a lean air-fuel ratio and rough idle.

Regular Maintenance: The Key to Preventing Rough Idling

While rough idling can be caused by various issues, many of them stem from damage, debris buildup, or normal wear and tear. To prevent problems that can harm your vehicle, it’s essential to keep your engine and its components clean and well-maintained. Stay proactive by keeping up with filter replacements, fluid flushes and top-offs, tune-ups, and scheduled maintenance. Having your vehicle regularly inspected by our technicians can help catch problems early—when they’re easier and less expensive to fix.

When dealing with rough idling or any other automotive concerns, you can always rely on Accurate Automotive Attention. As Yuma AZ’s only AAA-Approved Auto Repair and Service company, we’ve been providing friendly and reliable auto care since 1969. As a third-generation family-owned and operated business, we prioritize your needs and treat your car with the care it deserves. Accredited by the Better Business Bureau and voted Yuma’s Best Auto Repair for two decades consecutively, you can trust that you’ll receive trustworthy and competent automotive service from our team of service advisors and ASE Certified technicians.

Contact us today and discover why we’ve been named Yuma’s Best Auto Repair Shop for 20 consecutive years!

Want more great advice on saving money on car repairs? Order Russell McCloud’s book Knowledge is Power by clicking here.