Top 10 Causes of a Rough Idle

By March 17, 2017Engine

Does your vehicle seem to be roughing it lately?

Unfortunately, we’re not talking about enjoying the Great Outdoors, sleeping under the stars and breathing in the fresh air. No, we’re talking about a rough engine idle (much less pleasant, we know). You’ll know a rough idle because your car will feel rough or bouncy when the engine is running. Perhaps your car is idling below its regular speed, or you’re seeing inconsistent RPMs on your gauge. Or maybe you hear a skipping sound when the engine is running. All these are symptoms of a rough idle.

Incidentally, the mechanics at JL Motorworks are experts at quickly diagnosing the root cause of your rough idle. Call to schedule an appointment today! 

As with many car issues, the root cause of a rough idle can either be a quick, easy, inexpensive fix, or it can be something more serious and costly. It’s hard to tell without an accurate diagnosis, which is why it’s always important to 1) take your car in for regularly scheduled maintenance and routine checkups, and 2) get your car into the mechanic at the first sign that something could be wrong.

What is Idle Speed?

The idle speed of an engine is the rotational speed of the engine when it is not connected with the drivetrain and the throttle (gas pedal) is not being depressed. What’s the drivetrain, you ask? According to Mister Transmission:

The drivetrain includes the transmission, the driveshaft, the axles, and the wheels. Simply put, it works in conjunction with the engine to move the wheels. The drivetrain system is an essential component of a vehicle and the transmission is an integral part of the drivetrain.

When an engine is running at idle speed, it generates enough power to operate systems such as the water pump, alternator, and power steering — but not enough power to move the vehicle itself. A passenger car will usually idle between 600-1000 RPMs. A properly functioning idle should feel smooth — no bumps or skips.

So what causes a rough idle? Here are 10 potentials:

Top 10 Causes of a Rough Idle

1. Loose or broken vacuum hose/vacuum leak

Express Auto Inspections says:

While modern vehicles use a fuel injector system to burn gas, many older cars use a carburetor to process fuel. As your car rolls along, the engine takes in a mix of air and fuel that allows it to keep moving. This intake relies on a vacuum that is created between the carburetor and the piston tops… If there is a leak in this vacuum, the ability for your car to gain energy from the burning fuel is compromised, causing rough idling.

2. Clogged fuel filter

Fuel filters screen contaminants out of the fuel before it goes into the engine. If it hasn’t been cleaned out, it will become clogged and result in a rough idle.

3. Dirty fuel injector

Top 10 Causes of a Rough Idle

No idea what you’re looking at here? No need! It’s OUR job to know it, only yours to drive it.

Fuel injectors inject fuel into the cylinders to create a mix of air and fuel to burn. They need periodic TLC to keep gunk from building up. A dirty or clogged fuel injector will create a lack of fuel in the engine, which results in rough idling.

4. Incorrect idle speed setting

A rough idle could simply be the result of an incorrect idle speed setting. A trained mechanic can reset your idle speed setting easily.

5. Spark plug/spark plug wires

Spark plugs create the spark that is needed for your car to burn fuel. Damage to spark plugs or spark plug wires can cause a rough idle, as your engine is struggling to process the fuel it needs to run smoothly.

6. Distributor cap

The distributor cap and rotor pass the voltage from the ignition coils to the engine’s cylinders in order to ignite the fuel-air mixture inside and power the engine.

When a car’s distributor cap becomes faulty, the sequence of the functions of the engine will become out of order and will begin to fail. This can cause the car to have higher revolutions per minute, shake while idling, produce a high-pitched squealing noise and even begin to stall or backfire. (source:

7. Ignition control module

The ignition control module (ICM) is the heart of a vehicle’s ignition system. It is a computer that controls the ignition coil and regulates spark generation in the engine. The ignition coil “transforms the battery’s low voltage to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel.” (source)

When the ICM is defective, your engine might stall or sputter, you might smell gasoline, and you’ll experience a rough idle and possibly a decrease in gas mileage.

8. Defective/clogged fuel pump

The fuel pump pulls fuel from the gas tank to the fuel injectors. Over time it can become clogged, resulting in a rough idle, sputtering, stalling, and slow acceleration, as the engine is unable to get enough fuel.

9. Dirty oxygen sensor

Oxygen sensors measure how rich or lean the gasses are as they exit the combustion chamber. A dirty oxygen sensor will usually trigger the check engine light and can lead to a rough idle, lower fuel efficiency, and a failed emissions test.

10. Carburetor damage

Some older cars use a carburetor to process fuel, rather than a fuel injection system. Damage to the car’s carburetor and vacuum hoses can cause the engine to misfire, resulting in a rough idle.

Top 10 Causes of a Rough Idle | JL Motorworks