10 Reasons Your Vehicle May Have a Rough Idle

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Hey all it’s Mike here. The other day I was cruising down the road in my 1999 Toyota Camry heading to get some food from my local Chick-fil-a. After placing my order for some tasty chicken nuggies, I’m sitting in the drivethrough waiting to pull forward when suddenly the ‘Yota starts having a rough idle! “dang” I thought to myself, as the car sputtered and smoked up the drivethrough. Thankfully I was able to get my lunch and make it back to the shop without the car completely crapping out on me. But it got me thinking we should do an article on what causes vehicles to shake and make strange sounds.

Here’s a closer look at 10 reasons why your vehicle has a rough idle.

Rough Idle

1. Dirty Fuel Injectors

Dirty fuel injectors are a one of the most common reasons.

Hydrocarbons, silt, and other components in the gasoline cause fuel injectors to clog. Ethanol is found in almost all fuels. Because ethanol is hygroscopic, it absorbs moisture from the air.

Internal components corrode due to water in the gasoline. When the gasoline filter is in terrible shape, rust residue gets into the injectors and this can prevent your car from getting enough power to operate.

Fuel injectors release fuel into your car’s engine at a precise angle and amount, to make sure the car gets optimal performance. However, when the engine starts to idle rough it’s reconditioning time.

Reconditioning dirty fuel injectors involves removing all removable parts such as the pintle caps, O-rings, and filter baskets.

dirty or clogged fuel injectors
Fuel injectors are often to blame for this — Img creds: Greg on Flickr

Then they are placed inside an ultrasonic cleaning bath to clean the fuel injector’s exterior component.

The dirty fuel injectors and the nozzle are then put in a cleaning solution and put in a second ultrasonic cleansing bath. The interior elements of the injectors are cleaned by opening and shutting its pintle valve, agitation by the cleaning solution, and ultrasonic waves.

If it is still dirty after several cleaning (flushing) attempts, a new fuel injector will be required to repair one of these causes of rough idling.

Cost of Fuel Injector Cleaning and Replacement

The average price of having a dirty fuel injector cleaning is $60 to $100 and this includes the price of labor. There is such a wide gap in price because some injectors only need to be put through a cleaning system once, while others may need several cleanings to get them back in good working order to stop the rough idle on your vehicle.

2.Vacuum Leak

A network of hoses runs throughout most automobiles, creating a vacuum for air and fuel. Most new vehicles, on the other hand, include a throttle that controls airflow and engine speed.

Vacuum leaks, in particular, causes a lean gasoline situation by letting excess air enter the system. As previously stated, lean gasoline can cause a misfiring engine, which can result in a rough idle. You may also notice black exhaust smoke and poor gas mileage. When the vacuum leaks in between the mass airflow sensor and the engine, a vacuum leak occurs.

Before your mechanic can fix the vacuum leak, they must find it, your engine computer will be hooked up to the diagnostic scanner and the vacuum system and vacuum hoses will all be checked until the vacuum leak is located.

If the problem is in the hoses, they can be sealed. If the holes are too large or there are too many, they will need to be replaced and that should put an end to your rough idle.

Cost of Repairing a Vacuum Unit or Vacuum Hoses

The diagnostic test to find out why you have a rough idling engine will cost you $50 to $100. To seal the hoses or replace a hose is about $125 plus labor.

To replace the entire vacuum unit will cost between $150 and $3,000.

3. Bad Spark Plugs/Plug Wires

The spark plugs and spark plug wires rely on the electrical current from the ignition coils to ignite the air fuel mixture in the combustion chamber. If that doesn’t happen, your vehicle will continue idling rough.

These parts will wear out over time, particularly if pollutants enter the cylinders. You may avoid this by replacing the spark plugs and spark plug wires on a regular basis. Since it is a scheduled maintenance task, the suggested interval can be found in the owner’s manual. This will also help prevent your vehicle from having a rough idling engine.

spark plugs tip
You don’t want your spark plug to look like this! Image creds: Zach Zupancic

Faulty spark plugs that are filthy or broken will not produce a spark and the engine may not start. Similarly, faulty wiring and broken distributor cap may cause spark plugs to malfunction.

Cost of Repairing Spark Plug and Plug Wires

Spark plugs and wires cost between $115 and $295 including labor.

4. Damaged EGR Valve

The EGR valve is also known as the exhaust gas recirculation valve. The intake manifold is connected to the EGR valves when it malfunctions, it can also lead to a misfiring engine, damaged oxygen sensors, poor gas mileage, and rough idling.

While idling, the EGR valve should be closed. Carbon deposits, on the other hand, can keep an EGR valve open. A burned valve or a damaged pistons or oxygen sensors can cause poor compression. When the exhaust gas recirculation valve is damaged it can also cause problems for the exhaust system and pump and it greatly affects the emissions system, causing the vehicle to idle rough.

When this part fails, it can affect oxygen sensors, coils, air fuel mixture, consistent speed, and carbon deposits that can all lead to reduced fuel economy and cause the check engine light to come on.

The mechanic will disconnect all electrical elements then carefully remove the valve without getting any debris in the cylinder.

Cost of Replacing

Depending on the make and model of the vehicle’s engine that needs a new valve, the cost ranges from $40 to $500 for the EGR valve. The average price with labor included is $150 to $700.

5. Clogged Air Filter

The volume of airflow in your engine is reduced when the air filter in your engine becomes clogged. Not only does this cause idle air control, it causes the engine to have a rough idle, and it also affects gas mileage.

If the vehicle’s air filter is clogged it deprives the engine of vital air, causing the air fuel mixture to be distorted. A clogged or dirty PCV valve permits too much air in the engine, causing it to overheat. These will cause a rough idling engine.

This reminds me I gotta clean the air filter on our 2001 dodge….

To fix the problem, replace the PCV valve and filter. This will eliminate the rough idle and you won’t have to worry about a vehicle that stalls or worry about having enough fuel.

Cost of Replacing a Clogged Air Filter or Dirty PCV Valve

Air filters cannot be cleaner. They must be replaced. The average cost of an engine’s air filter is $20 to $45 plus labor. A dirty PCV valve should also be replaced.

6. Dirty Fuel Filter/ Damaged Fuel Pump

If your vehicle is idling rough you may need to have, your fuel pump or filter looked at. If you look at your owner’s manual, you will find that the filter is in the engine compartment. Alternatively, it can run alongside or under the chassis structure that leads to the gas tank.

If your car’s fuel pump is damaged it will cause rough idling. The mechanic can hook your car’s computer up to the diagnostics scanner or do a compression test to get the trouble code to be able to identify rough idling. The emissions system should also be checked, as well as the exhaust system, because they can all cause your vehicle to have a rough idle.

Cost of Repairing/Replacing the Filter and/or the Pump

The cost of repairing/ replacing a filter or pump ranges from $750 to $1900.

7. Carburetor Problems Can Cause Rough Idling

Carburetors can be found on older vehicles. If there is a problem with the engine, it will not operate at optimal performance, you may notice black exhaust smoke, even airflow sensor problems can make an older vehicle start idling rough.

Having the carburetor cleaned out will remove the carbon deposits and your vehicle’s rough idling should stop. If the cleaning doesn’t stop the rough idle, you may need to replace the carburetor.

Cost of Repairing a Carburetor

The average repair or replacement of a carburetor is $60 to $300 plus labor.

8. Damaged Ignition Coil

If your car idles roughly and isn’t operating properly, your car’s check engine light will illuminate. The car’s computer should be hooked up to a diagnostic scanner. The vehicle may need new ignition coils. The ignition system is responsible for how the car starts, it can also affect the idle speed when running and if it causes a rough idling engine.

A car has three coils. If one is bad, it will be replaced and will help eliminate the vehicle’s rough idle.

Cost of Repairing a Damaged Ignition Coil

The average cost is between $264 and $376. Plus labor costs.


9. Low Compression

Low compression causes a rough idle in the engine which can be caused by several things including – an air fuel mixture that is produced and helps the vehicle ignite. Several things can cause low compression that leads your engine to a rough idle, including leaky valves, dirty spark plugs, bad wires, improper fuel and air exchange, and head gasket problems.

When the engine computer is connected to the scanner it will bring up a trouble code or a compression test will help the mechanic find the cause of the engine’s rough idle.

Once the cause of the rough idling is determined, it can be fixed. It will put an end to the car’s rough idling engine.

Cost of Repairing a Low Compression Issue

The compression test costs about $100. Depending on the cause of the low compression, it can cost between $100 and a whopping $8000 dollars!

10. Faulty Oxygen Sensors

A faulty oxygen sensor like the MAF or exhaust sensor are a common causes of rough idling. Combustion engines power the majority of cars on the road today. The engine burns an air to fuel mixture to produce the power required to operate the vehicle.

You don’t have to have a full failure of an oxygen sensor for it to cause problems. You will notice the following things if you have damaged oxygen sensors – the engine will have a rough idle, you may have trouble keeping the engine running, check engine light will illuminate, and poor fuel economy.

Mass Air flow sensor maf
Mass Air Flow Sensor
exhaust oxygen sensor
exhaust oxygen sensor

If your engine has a faulty oxygen sensor it will need to be replaced to prevent a wrong idle speed and prevent the engine from rough idling.

Cost of Replacing an Oxygen Sensor

Replacing an Oxygen sensor between $114 and $478. Plus the cost of labor. Some mechanics charge more if they have to work inside the engine compartment.

Frequently Asked Questions

Q. Why is my car chugging in idle?

A. A chugging engine is usually caused by one of three things – bad plugs and wires, a catalytic converter that is plugged, or a dirty fuel filter.

Q. Can a bad time belt cause the engine to have a rough idle?

A. Yes. A bad timing belt can cause your engine to have a rough idle.


If your vehicle has a rough idle, don’t panic. It could be something minor. Take it to your mechanic and find out the engine idling rough and go from there.






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