Rough idle - causes and remedy | mechanic base (2023)

When your car engine is running as it should, everything should be smooth and silent. If something goes wrong, the engine can start to run rough, especially when idling.

If you understand the causes of a rough idle car engine, you can easily solve the problem. Here are ten of the most common causes of a rough engine idle.

In this article, I will go through all the possible causes of rough idle, along with an explanation of how to test them. Let's start with a quick look at why.

Table of contents show

What causes rough idle?

The most common cause of rough idle is a vacuum leak or a faulty PCV valve. It could also be caused by other air/fuel mixture related issues such as: B. bad fuel injectors, EGR valves or low compression. In rare cases it is caused by misfiring from a bad spark plug or coil.

While these aren't all of the characters, it's a great place to start. Here is a more detailed list of the most common causes of rough idle:

1. Defective PCV valve

Rough idle - causes and remedy | mechanic base (1)

Your car engine's positive crankcase ventilation takes the unburned gases escaping from the cylinders and directs them out of the crankcase so they can return to the engine for complete combustion. Because the PCV valve works inside the engine, it is subjected to a lot of stress and is often filled with contaminants such as dirt or mud.

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Over time, the valve can clog and become unusable. This blockage can also create a leak that makes the air/fuel mixture lean, resulting in a rough idle.

To prevent this from happening, you can perform regular maintenance on the PCV valve. Once it's clogged, you may be able to clean it. Otherwise it must be replaced.

RELATED:7 Symptoms of a Bad PCV Valve

2. Vacuum leak

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The engine bay is filled with vacuum hoses, all of which can eventually wear out. Over time, these hoses will become brittle and tear. If you get a leak, it will cause a lean air/fuel mixture, resulting in a rough idle. It can also become oneEngine misfirebecause of combustion failure in one or more cylinders.

Vacuum leaks can also occur from leaking vacuum reservoirs, vacuum brake boosters, etcintake manifold gaskets. If you drive a car with a vacuum leak, you'll find that everything seems fine when the rpm and speed are high, but it runs rough at idle.

Checking the computer's codes should show a vacuum leak. The air/fuel mixture will be lean and the engine will not be able to correct it.


3. Dirty injectors

Rough idle - causes and remedy | mechanic base (3)

When the injectors get dirty, rough idle is just one problem you will experience. You will also see a sharp drop in fuel consumption.

When the injectors are restricted there is a significant lack of power, especially under acceleration. Dirty fuel injectors can be diagnosed by an emissions analyzer, as clogged injectors produce higher levels of hydrocarbons and carbon monoxide.

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By using ainjector cleanerAdditive regularly, prevent this situation. Once the injectors become clogged or restricted, an injectable solution needs to be introduced into the system.

Fuel injectors often contain a small filter that can be easily replaced if the injectors become clogged, and it's a lot cheaper than replacing.

RELATED:8 symptoms of a bad fuel injector

4. Carburetor problems

Rough idle - causes and remedy | mechanic base (4)

Older, high-mileage vehicles are the only ones on the road with carburetors. However, there are still enough of them for this to be a problem.

Carburetor problems create a signatureblack smoke from the exhaust. Normally, this smoke does not appear until the vehicle has warmed up.

The first thing to check is the choke. It should be fully open once the car is warm. If the choke is open, there is likely an internal carburetor problem that will require a rebuild. Sometimes it's just the ethanol in the gasoline that messes up the carburetor.

However, there can also be a problem with the throttle shaft at the carburetor base. This can wear out over time and cause a vacuum leak.

5. Bad EGR valve

Rough idle - causes and remedy | mechanic base (5)

The exhaust gas recirculation valve is an important exhaust gas purification device. It lets the exhaust gas enter the intake manifold with the sole purpose of reducing emissions from the engine.

However, this EGR valve should not operate at idle, at full load, or before the engine is warming up. If it is running at any of these times, the performance of the engine will be affected.

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When the EGR valve malfunctions, it doesn't play by the rules. Instead, it sticks open and causes a rough idle or worse, an engine stall.

The car's computer should set a code indicating that this component is defective. You can also try tapping it to see if you can solve the stuck position. Cleaning the dirty EGR valve may be enough to solve the problem. Otherwise, a cheap replacement will suffice.

RELATED:6 symptoms of a bad EGR valve

6. Faulty ignition coil

Rough idle - causes and remedy | mechanic base (6)

The ignition coil is essential to maintaining proper engine performance. It works in conjunction with the spark plugs to ignite the fuel/air mixture.

If the car has a bad coil, it will experience rough idling, misfire, loss of power, difficulty accelerating, and poor fuel economy. If the situation worsens, it can also cause the vehicle to stall.

7. Defective spark plugs/cables

Rough idle - causes and remedy | mechanic base (7)

The spark plugs and wires are also vital to the ignition system. Over time, these parts wear out, especially when contaminants get into the cylinders.

However, you can prevent this by changing the regularlyspark plugsand wires. As this is considered a maintenance task, refer to your owner's manual for the recommended maintenance interval.

8. Low compression

Rough idle - causes and remedy | mechanic base (8)

Low compression in your car engine is most noticeable at idle and therefore where you will first notice it. You can often notice low compression as a "miss" or misfire at idle.

The most common cars to experience a low compression induced fault are those with more than 100,000 miles on them. Typically, the error occurs when the compression in one of the cylinders is low. A compression test must be performed to determine if this is the cause.

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To solve the problem, the engine must be disassembled for repair. The valves may need to be replaced for proper compression, or the camshaft lobes may be worn. Either way, it's not a cheap solution.

Luckily, low compression isn't as common as it is in older cars and is becoming less common in newer car models.

9. Clogged air filter

Rough idle - causes and remedy | mechanic base (9)

Themotorluftfilteris critical for proper operation. It is designed to prevent dust, dirt and other contaminants from entering the engine. The paper element can easily become clogged, restricting airflow into the engine.

However, this is another maintenance task that should be performed regularly to keep the engine running as it should. Ideally, you want to choose a quality air filter that doesn't clog easily. For the low cost, it's worth protecting your engine.

10. Bad oxygen sensors

Rough idle - causes and remedy | mechanic base (10)

The oxygen sensors are vital to your car's emissions system. They are located in the exhaust system and constantly monitor the oxygen content of the gases. The information is sent to the engine computer where it is used to maintain a correct air/fuel mixture.

However, oxygen sensors degrade due to the high temperature they are exposed to. Over time, they wear out or suffer from contamination from carbon deposits. When this happens, the wrong information is sent to the computer, resulting in an incorrect air/fuel ratio. If the ratio becomes too lean, rough idle will occur.

This error generates an error code that can be read with your scanner. A digital oneMultimetercan also help you determine which oxygen sensor has failed.

Whatever is causing your engine to idle rough, it's important to diagnose and fix the problem quickly. Ignoring poor engine performance issues will only lead to more extensive and costly repairs later.


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