Home
Blog Car Care Advice What Causes A Rough Idle In A Car? (11 Reasons + Fixes)
Car Care Advice

What Causes A Rough Idle In A Car? (11 Reasons + Fixes)

Looking for a mechanic near you for maintenance or repair? RepairSmith brings the shop to you. Get a free instant quote today.
Get a quote

You get into your car, crank it up, and wait for your engine to warm up.  

But as your car idles in the driveway, something feels off. 
It begins to shake and rumble unevenly.  

The symptom you just experienced is — rough idle. 

But what exactly is a rough idle? 
And what causes it? 

In this article, we’ll understand what rough idling is, the common and less common reasons that cause it, and how to prevent it

This Article Contains: 

Let’s get started!

What Is Rough Idling?

Rough idling or rough idle is the excessive shaking and vibrations you feel when your engine is running but not moving.

When your car is in “Park,” or you keep the brake pedal engaged, the engine idles at a constant speed. This rotational speed (RPM) allows the engine to power your vehicle’s components without turning off. In most cars, the idle speed is between 600 to 1000 RPMs. 

But when you have a rough idling engine, you’ll likely experience: 

So if you have a rough engine idle, you’ll know it almost instantly. 

But what events inside your car’s system lead to rough idling?

11 Causes Of A Rough Idling Engine (And Fixes)

Let’s look at the eight common causes of rough idling and three less common but possible reasons. 

1. Vacuum Leak 

Vacuum leaks are a pretty common problem that can allow more air to seep into the engine block. This mostly happens due to a loose, cracked, or damaged vacuum hose.

How?
Vehicle engines have several hoses to create a vacuum for both air and fuel. Over time, these hoses can wear out, giving way to vacuum leaks. 

When too much air gets inside the combustion chamber through the damaged hoses, it causes the engine to run on a lean airfuel mixture. This can lead to a misfire, slow acceleration, and rough idling at higher RPMs. 

How to fix it? 
Look for a hissing sound while your engine is running to spot a vacuum leak. Replacing the damaged vacuum hose should be an easy fix. 

2. Clogged Air Filter 

An air filter prevents dust, dirt, and other contaminants from entering the engine. Since the air filter is made of paper, it can easily get clogged and restrict airflow to the engine. 

When that happens, you’ll not only experience a rough idle, but your engine may also misfire or experience a hard start.

How to fix it? 
Changing the air filter as a part of regular vehicle maintenance can prevent rough idling incidents. 

3. Defective PCV Valve 

The Positive Crankcase Ventilation valve (PCV valve) directs the gases produced by the crankcase back into your engine’s combustion chamber. Over time, this valve can get clogged with dirt or sludge and remain stuck open. 

While a clogged air filter will restrict the airflow to the engine, an open PCV valve will allow more air into the engine. 

The result?
A lean airfuel mixture will burn inside the combustion chamber, causing a rough running engine. 

How to fix it? 
Cleaning or replacing a clogged PCV valve can fix a rough idling engine. 

4. Faulty Spark Plug 

Misfires caused by a dirty spark plug or multiple defective spark plugs can often result in a rough-running engine.

Spark plugs create the required spark to ignite the fuel mixture inside a gasoline engine. When a spark plug is damaged or a spark plug wire is corroded, the ignition timing (spark timing) will be off. Consequently, the fuel will burn inconsistently, causing your engine to misfire, stutter, or idle roughly. 

Apart from a bad spark plug, a cracked distributor cap or defective ignition coil could also cause rough idling. 

How to fix it? 
Get a good mechanic to inspect each spark plug, ignition coil,  and other parts of your ignition system. Ideally, they’ll replace all the spark plugs in your car if required.  

5. Faulty Fuel Injectors

Sometimes, dirty fuel system components like fuel injectors can cause a rough idling engine and lead to poor mileage.

Fuel injectors are designed to spurt fuel into the combustion chamber at a precise angle and quantity. When you have a dirty fuel injector, the fuel mixture gets affected, causing a rough idle or slow acceleration. 

How to fix it? 
Cleaning or replacing the fuel filter and adding a fuel injector cleaning treatment to the fuel tank should take care of clogged fuel injectors. If the rough idle persists, it’s time to take your car to an auto repair shop. 

6. Defective Fuel Pump

Apart from a fuel injector, a clogged fuel pump can cause a rough idling car and other engine troubles like stalling and diminished power.

Wonder why?
The fuel pump is responsible for supplying fuel to the injectors. But when it isn’t working correctly, your engine won’t get the right amount of fuel for ignition and won’t maintain the required engine speed. 

How to fix it? 
You can use a fuel system cleaner to remove any buildup or blockage in the fuel pump.

7. Damaged Oxygen Sensor (O2 Sensor)

An oxygen sensor measures the amount of oxygen in the exhaust gas being released through the exhaust pipe. The sensor helps your car’s ECU (Engine Control Unit) to maintain the correct amount of air inside the engine for combustion. 

But if an oxygen sensor is damaged or malfunctioning, it can affect the fuel mixture and the engine speed. 

When an oxygen sensor malfunctions, your car will register a related DTC code like P0131, P0134, or P0137, and your check engine light will come on.

How to fix it? 
Get a mechanic to diagnose the check engine code. Replacing an oxygen sensor should be a quick and cheap fix.

8. Exhaust Gas Recirculation Valve (EGR Valve) Stuck Open

The EGR valve helps prevent the production of nitrogen oxide tailpipe emissions by returning a controlled amount of exhaust gas back to the combustion chamber. 

Now, an EGR valve should be closed at idle, but if carbon deposits keep it stuck open, the engine will idle roughly and even stall. 

How to fix it? 
It’s best to allow a mechanic to take care of this fix. 

Now, let’s look at the other less common reasons that could cause a rough idle. 

9. Dirty Mass Air Flow Sensor

The mass air flow sensor measures the amount of air flowing into the engine. A dirty sensor can give false readings to the ECU and cause the engine to idle roughly. 

How to fix it?
You can use an aftermarket solvent to clean a dirty mass air flow sensor. 

10. Dirty Idle Air Control Valve (IAC Valve)

The idle air control valve does exactly what its name suggests. It regulates the amount of air entering the engine to control its idle speed. 

But, like many other engine components, carbon deposits tend to build up on the IAC valve, restricting the airflow. This could lead to rough idling or engine stalling. 

How to fix it? 
Cleaning the IAC valve and the throttle body with a carburetor or throttle body cleaner can fix this engine problem.

11. Defective Throttle Position Switch (TPS)

The Throttle Position Switch (or Throttle Position Sensor, usually mounted on the throttle body) monitors the movement and position of the throttle plate. This way, it helps the ECU adjust the air fuel mixture and ignition timing. 

But a faulty TPS can throw off the ignition timing (spark timing), leading to a rough idle and other drivability issues. 

How to fix it? 
You should hire a professional mechanic to diagnose and replace a bad TPS. 

Now you know the probable reasons for rough engine idle. Fantastic! 
But is there a way to prevent it? 

How Do You Prevent Rough Idle And Other Expensive Repairs?

Let’s be real —  you can’t foresee all the reasons causing a rough idling engine.

But you can take certain preventive steps to keep your vehicle running smoothly and avoid expensive engine repairs.

1. Keep Your Engine Tuned-Up

You now know that several engine components can alter your engine’s idle speed. Many of these components malfunction due to the buildup of debris or wear and tear. 

A good way to prevent a rough running engine is to keep your engine in top-notch condition with regular engine tune-ups

During an engine tune-up, a mechanic will change your oil filter, fuel filter, and air filter, perform fluid flushes and top-offs, and replace worn-out parts like spark plugs. 

Having your vehicle regularly examined by a mechanic will help identify an engine problem early on, which may be easier and less expensive to fix. 

2. Reach Out To A Mechanic ASAP

Your engine should run quietly and smoothly.

But if you’re hearing weird noises from the engine mount or are experiencing excessive vibrations, it’s time to call a trusted mechanic to inspect your vehicle. 

And for that, you can reach out to RepairSmith

RepairSmith is a convenient mobile vehicle auto repair and maintenance solution
Here’s why you should hire us:

Final Thoughts

A rough idle can be annoying but shouldn’t be ignored, as it could be your first sign of a deeper engine problem. If your car is experiencing a rough idle consistently, then it’s time to reach out to an expert mechanic.

Contact RepairSmith to get the correct diagnosis for your rough idling car and other repairs right in your driveway!