A rough idle is easy to detect because your car starts to perform poorly and the engine sometimes stops running when idling.
Often you can also feel strange vibrations in your car when you are sitting in the driver’s seat.
However, modern cars often have great engine mounts, and sometimes you have to open the hood to check if the engine vibrates or even jumps when idling.
There are several reasons why your car starts to judder in idle, and it can lead to terrible performance, loss of power, poor fuel economy and other serious engine-related problems. It is recommended that the problem of rough idling be corrected as soon as possible, otherwise, the repair may be very expensive.
Idling vs. Rough idling
If you start the engine and let it run without using the pedal, this is called idling. When the engine is left in the idle position, it maintains a constant but low speed of about 1,000 rpm. The speed needle remains constant, indicating that the car’s engine is getting the right amount of air and fuel.
On the other hand, if the engine does not receive the correct amount of air-fuel mixture, it will start to idle unsteadily. If your car starts to idle unsteadily, it will probably have difficulty maintaining a constant engine speed.
What Causes Rough Idle?
A rough idle can be caused by many problems, e.g. a damaged fuel injector, a worn air filter, a bad spark plug, etc. We will discuss some of these causes so that you can easily determine why your car is rough when idling.
Misfiring is probably one of the most common causes when it comes to rough idling. Misfires are often caused by a bad spark or a faulty air-fuel mixture.
If your engine misfires, the check engine light often comes on and error codes are stored. Check the error codes with an OBD2 Scanner to find out more easily on which cylinder the misfire is occurring.
If your engine control unit is clean of misfires, you should first remove and visually inspect the spark plugs to determine the damage. If you notice a lot of carbon deposit on the plug, this means that there is a rich air-fuel mixture. In contrast, oil deposits indicate poor piston rings or leaking valves.
The spark plugs, cables, and ignition coils are all connected. The spark plug delivers electric current from the ignition system to the combustion chamber and generates a spark that ignites the fuel-air mixture. Over time, the spark plugs are damaged due to carbon deposits or other waste products of the combustion process. This reduces the amount of current generated after the spark plug is ignited. This condition causes unsteady idling along with other engine-related problems. This is often referred to as misfiring.
If you notice ash deposits, it means that you are using a poor quality fuel or that oil is leaking. It is also recommended that you check the spark plug leads and ignition coils and replace the necessary components.
2. Faulty Idle Control Valve
An idle control valve is often installed on older vehicles. This is a valve that limits the air flowing into the manifold and thus adjusts the idle speed. If the idle speed control valve is defective, you may experience a rough idle.
3. Faulty Throttle Body or Throttle Body Sensor
On more modern cars you have a throttle body that adjusts the idle speed instead of the idle control valve. It is a common problem that the throttle valve gets dirty and causes rough idling. You can try to remove and clean it if it is dirty, but sometimes you will need to replace the entire throttle body. On slightly older cars there is also a throttle sensor that could become defective and need replacing.
4. Damaged Fuel Injector
Modern fuel injectors are highly efficient and capable of spraying just the right amount of fuel at the right time into each cylinder. These fuel injectors have a small hole from which the fuel is sprayed, and this hole sometimes gets clogged during the combustion process. This can cause the fuel injector to spray less fuel onto one or more injectors, resulting in a loss of power and unsteady engine idling.
You can learn more about fuel injectors here: Fuel Injector Symptoms & Diagnose
In such a case it is recommended that you take your car to a mechanic who will clean the injectors with solvents or other additives. If the injectors are extremely old, it is recommended to have them replaced.
5. Clogged Fuel Filter
Fuel filters can easily become clogged, as they filter the impurities in the fuel before it reaches the engine. A clogged fuel filter prevents the engine from receiving the correct amount of fuel, which leads to uneven idling of the engine.
Fuel filters are often quite cheap, and you can easily have the fuel filter removed and replaced by a mechanic near you or, if you have some knowledge, do it yourself.
6. Dirty Air Filter
The air filter does an excellent job of protecting the engine from dirt, dust, and other contaminants. It can become clogged or dirty after some time if it is not replaced or repaired. The engine needs not only fuel to function properly but also a sufficient amount of air. A clogged air filter will not allow clean air to pass through, resulting in rough idling.
To remedy this, have your air filter replaced with a new one and also have the casing serviced to remove any dirt deposits.
7. Dirty Oxygen Sensor
The oxygen sensor is located in an area with extremely high temperatures and its main function is to monitor the oxygen content of the exhaust gas. This information is sent to the ECU, which then determines the correct air-fuel balance for the effective combustion process. If the oxygen sensor is damaged due to carbon deposits, it sends incorrect information to the ECU, resulting in an incorrect air-fuel mixture.
A defective oxygen sensor triggers the check engine light and an error code in the engine computer. You can have the oxygen sensor replaced to remedy the problem.
8. Bad MAF Sensor/IAT Sensor
A poor MAF (mass airflow sensor) will cause the engine to run too rich or too lean, causing rough idling or misfiring. If you want to learn more about how to diagnose a MAF sensor, take a look at this: MAF Sensor symptoms
9. Bad Coolant Temperature Sensor
The coolant temperature sensor measures the temperature of the coolant and adjusts the fuel mixture according to the temperature. If the sensor sends incorrect values, it can cause a lean or rich mixture and cause rough idling.
10. Intake leak
Intake leaks often also cause rough idling. Check for broken vacuum hoses or a defective intake manifold gasket.
Cars with Carbureted Engine
Modern vehicles are equipped with an efficient fuel injection system and an advanced ignition system. However, older vehicles used carburetors and mechanical ignition systems. If you own an old car and have a rough idle, it is a good idea to clean the carburetor and check the hoses for damage or dirt accumulation.
Rough idling is a serious problem and should not be ignored for long. If the problem persists and is not corrected, it can also lead to engine damage. We therefore recommend that you take your car to a professional mechanic for inspection as soon as you notice any symptoms.
Rough idling is often caused by misfiring or a lean or rich mixture. The best way to find the reasons for rough idling is to use an OBD2 scanner; this can save you a lot of time and money. You can often find them quite cheap at Amazon, and it is certainly a significant investment. You can find our top list of the best OBD2 scanners to buy here.
If you have any further questions about the rough idling, feel free to comment below and we will answer your questions as soon as possible.
Hi, I’m Magnus, the owner and the writer of Mechanic Base. I have been working with cars for 10 years, specialized in diagnostics and troubleshooting. I created this blog because I was tired of finding false information on the web while looking for repair information. I hope you enjoy my content!