Is your car jerking or shaking excessively while stationary?
That’s called rough idling — a sign your engine is struggling to maintain a smooth combustion process.
Rough idling is usually caused by a clogged air filter or failing sensors, but only a proper diagnosis can confirm your suspicions. Without timely inspection, a rough idle can lead to severe engine problems or even complete failure!
To avoid future trouble, let’s understand rough idling and explore some possible fixes. We’ll also discuss why driving your car with this issue is a risk you shouldn’t take.
This Article Contains:
- What Does a Rough Idle Mean?
- 10 Key Causes that Make Your Car Idle Roughly
- Can You Drive with a Rough Idling Engine?
- How Do You Diagnose a Rough Engine Idle?
- How to Fix a Car that Idles Roughly
What Does a Rough Idle Mean?
Rough idling indicates your car’s engine isn’t running smoothly at rest, leading to noticeable vibrations, erratic behavior, and fluctuating RPMs.
A healthy engine typically runs at a stable idle speed (or engine speed), which is around 600 to 1000 revolutions per minute (RPM). To maintain this idle speed, a small amount of fuel must combust continuously. But when your engine struggles with this combustion, it’ll start exhibiting rough idling signs, like:
- Excess bounce and vibrations
- Odd noises, like sputtering or knocking sounds, from the engine
- Erratic movement of the tachometer needle
If you notice these symptoms during a cold start or after the engine runs for a while, call a mechanic to schedule an inspection ASAP.
Wondering what causes this issue?
Let’s find out.
10 Key Causes that Make Your Car Idle Roughly
Here are some possible reasons your car has a rough idle:
1. Fuel System Problems
A common cause of rough idling in modern vehicles is a faulty fuel injection system.
For example, a failing or clogged fuel injector, pump, or filter may not deliver the right amount of fuel, causing an imbalanced air-fuel mixture.
The result?
When you start the car, you’ll notice excessive shaking or weak vibrations — clear signs your engine isn’t receiving adequate fuel to idle smoothly.
2. Damaged Spark Plug, Wire, or Ignition Coil
Fouled spark plugs might produce weak or inconsistent sparks, which can lead to incomplete combustion.
Likewise, worn spark plug wires or ignition coils may not deliver sufficient electrical current to the plugs and will cause a misfire. This makes your engine perform poorly and idle roughly.
3. Clogged Air Filter
An air filter allows clean air to enter the engine by blocking dirt and debris. However, when the filter gets clogged with dirt, it restricts airflow, depriving the engine of the oxygen it needs for proper combustion. As a result, your vehicle fails to maintain stable engine speed, leading to rough idling.
4. Vacuum Leak
A vacuum leak occurs when unmetered air enters your engine’s vacuum system. It’s usually caused by a damaged hose, brake booster diaphragm, or intake manifold gasket. This imbalance can lead to symptoms like rough idling and high-pitched squealing sounds.
5. Faulty Mass Airflow Sensor (MAF Sensor)
The MAF sensor measures the amount of air that enters the engine for ignition.
However, corrosion or dirt accumulation can prevent the sensor from accurately measuring the airflow. When this happens, the air-fuel mixture may be either too lean (excess air) or rich (excess fuel). This results in misfire and rough idling.
Moreover, the failing sensor can send inaccurate airflow data to the engine control module, triggering a diagnostic trouble code and a check engine light.
6. Damaged Oxygen Sensor
This sensor monitors the oxygen in the exhaust gases and sends this data to your engine computer to adjust the air-fuel ratio for proper combustion.
However, a damaged oxygen sensor can send incorrect data to the engine computer, causing your engine to consume less or more fuel. This can trigger issues such as stalling and rough idling.
7. Failing Positive Crankcase Ventilation Valve (PCV Valve)
The PCV valve controls gas emissions and regulates air pressure in the engine crankcase. However, carbon deposits or debris can cause the valve to stick in one position.
Here’s what happens when the valve gets:
- Stuck open: Excess air can enter the engine during ignition, resulting in a lean air-fuel mixture.
- Stuck closed: Gas vapor can get trapped inside the crankcase, increasing the air pressure. The excess pressure can force the engine oil into areas like the spark plug wells. Oil-soaked spark plugs can fail to ignite the air-fuel mixture effectively.
In both situations, your car will misfire and idle poorly.
8. Malfunctioning Exhaust Gas Recirculation Valve (EGR Valve)
The EGR valve recirculates exhaust fumes back into the intake system to reduce emissions. Over time, carbon residue or debris can make the valve stick open. As a result, exhaust gases replace air in the combustion chamber, creating a lean air-fuel mixture. This hampers the combustion process and causes a rough idle.
9. Cracked or Broken Motor Mount
A motor mount keeps the engine secure and absorbs shocks and vibrations when the car is running. When a motor mount breaks or cracks, it can no longer stabilize the engine properly.
Soon, your car will shake excessively and create odd noises when idling.
10. Carburetor Issue
A failing carburetor is a common cause of rough idle in an older vehicle manufactured before the 1990s. The carburetor mixes air and fuel for ignition. However, when it becomes clogged with grime, the air-fuel ratio in the combustion chamber is affected.
Once the carburetor malfunctions, it’ll cause black exhaust emissions and make your car idle roughly.
NOTE: A malfunctioning coolant temperature sensor, throttle body, or crankshaft damper can also result in poor idling.
Knowing the causes, if you’re still considering driving a rough idling vehicle, here’s why it may not be the best idea.
Can You Drive with a Rough Idling Engine?
Short answer: You shouldn’t.
Whether your car idles rough during a cold start or once it warms up, it’s a sign that an essential car component needs attention. Ignoring this issue can lead to serious engine problems such as:
- Increased exhaust emissions and poor fuel economy
- Premature wear of engine components
- Risk of stalling or complete engine failure
Curious how to identify the main culprit of a rough idle?
We’ve got you covered.
How Do You Diagnose a Rough Engine Idle?
Diagnosing a rough idle requires good knowledge of the engine components, as multiple parts like dirty fuel injectors or loose spark plug wires can be the root problem.
Moreover, the causes can be different for older and modern vehicles, so you should call a qualified mechanic to avoid misdiagnosis.
Here’s how they’ll determine the issue:
- Drive the vehicle to check if it’s bouncing or shaking more than usual due to worn engine mounts.
- Listen for odd squealing or knocking sounds while the car idles.
- Use an OBD-II scanner to verify if any fault codes are active.
- Look for dirty fuel injectors or a damaged filter and pump.
- Verify whether the spark plugs, wire, ignition coil, or coil pack (collection of ignition coils) are faulty.
- Inspect key sensors like the oxygen and MAF sensors.
- Check the PCV and EGR valves to see if they are stuck open or closed.
- Perform a smoke test to identify vacuum leaks in the air intake system.
- Use a compression testing kit to check the cylinder’s performance (especially for cars with high mileage.)
Depending on what your mechanic finds, they’ll repair or replace the faulty parts.
How to Fix a Car that Idles Roughly
Here are some basic car maintenance and repair services your mechanic may perform:
- Install new motor mounts on your vehicle.
- Remove the clogged fuel injector, pump, and filters to clean them.
- Replace the faulty spark plugs, wire, ignition coil, and coil pack.
- Fix vacuum leaks by repairing the intake system, replacing the intake manifold gasket, and changing the hose.
- Clean the throttle body and install a new PCV or EGR valve.
- Swap out the oxygen or MAF sensor.
- Wipe away the dirt or fix a new carburetor (for cars with a high mileage).
- Clear active fault codes using an OBD-II scanner.
Help Your Car Idle Smoothly with AutoNation Mobile Service
Ignoring a rough idling car will not only disrupt your driving experience but lead to expensive repairs in the long run.
However, fixing rough idle-related problems can be tricky, so let AutoNation Mobile Service mechanics take over.
We’re available seven days a week and offer easy online bookings. Whether your vehicle needs fuel system repair or brake service, our mechanics can perform the job right from your driveway at your convenience.
Contact us today to fix the rough idle on your vehicle and enjoy stress-free car maintenance.