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Car Idling Is Rough – What Causes The Vibrating and Shaking? 

Car Idling Is Rough – What Causes The Vibrating and Shaking? 

If you find that your car is idling rough, it will be pretty easy to tell. The excess vibrations, rattling noises, and uncomfortable sitting will make it apparent to drivers and passengers that something is wrong under the hood. In this case, you should bring your car to a mechanic to evaluate why you are experiencing the car idling rough symptoms. 

Auto Repairs Are EXPENSIVE


Rough idling is a common problem, but diagnosing this issue’s root cause can be difficult and time-consuming. Several factors come into play when figuring out the culprit of this problem, making the labor costs quite expensive for this fix. How your engine idles is a good indicator and sign of its overall health. If you notice the car idling rough symptoms, chances are you have something wrong internally. 


By knowing the main reasons why your car may have a rough idle when cold, you can keep your transmission system and your engine system running for as long as possible without any issues. If you keep your car running rough in various conditions, this can lead to internal issues that are more severe, like engine replacements or transmission replacements. 

Causes of Car Idling Rough Symptoms

Eventually, your car, truck, sedan, or pickup might start running rough. You might be wondering – what is the cause and how can you fix it? Even if you are not the most tech-savvy person in mechanics and cars, you should know how to identify car rough idling symptoms in your car to prevent any further engine damage.

Shaking and Bouncing

You can identify the car idling rough symptoms if there is a shaking and bouncing sensation in the vehicle, and you feel like the car is vibrating too much while your car is not moving. Although some cases are less severe than others, the rough idle is usually pretty identifiable by both the driver and passengers.

Strange Sounds

In addition to the vibrations, you might also notice strange sounds when the car is running rough idle when cold. Drivers need to know what a bad engine sounds like to diagnose if they are experiencing car idling rough symptoms. 


One of the main sounds that drivers could hear is due to a faulty engine bearing.

  • An engine bearing is the mech device used to reduce friction between moving parts of machine elements to provide the necessary movement in the right way with minimum power and performance loss. 
  • The bearing material must be very strong and durable since the explosions’ stress inside the internal combustion engine can hurt the crankshaft. 
  • Engine bearings reduce the amount of friction between the rotating and moving part of the engine and the stationary part and the crank. The bearings prevent excessive friction, overheating parts, and the internal engine components from getting worn down, causing car idling rough symptoms. 

Inconsistent RPM

Along with the strange sounds coming from your engine and under your hood, there might be an inconsistent engine RPM while you are driving and accelerating. Typically, a healthy vehicle will have a smooth RPM that stays at a consistent level of around 1,000 revolutions. If your vehicle is going too far below this line or moving too far above, then you might be dealing with a car rough idling issue. 

Nine Causes of Car Idling Rough Symptoms 

  • Vacuum Leak

Most vehicles have numerous hoses throughout their internal engine system, creating a vacuum for both the car’s fuel and the outside air. In engines that are older models and have a carburetor, this vacuum is in charge of directing the fuel into the engine and powering the car. Most newer cars and modern vehicles have a throttle designed to regulate engine speed and airflow through the system.


This new modern car mechanism also creates a vacuum within the intake manifold. However, over time, these hoses can succumb to wear and tear and become damaged over time or develop a leak. If too much air gets mixed in with the fuel, this can cause the engine to misfire, resulting in the car’s idling rough symptoms at a higher RPM.

    • The average cost for a vacuum leak diagnosis in your car is between $88 and $111 to remedy the car idling rough symptoms. 
  • Depending on where the leak is in your car and vacuum system, the repair cost can range from as low as $150 to around $1,000 in most vehicles.
  • Spark Plugs

Car idling rough symptoms can be the result of faulty spark plugs or damaged spark plug wires. Spark plugs use the ignition coils’ electrical current to ignite the air to fuel mixture within the engine’s combustion chamber. A plug that has become damaged over time or is installed incorrectly can result in the fuel being burned at an improper rate.


If the engine’s damage and the spark plugs are severe, you might notice the car idling rough symptoms. Make sure when you're driving or sitting in your car that you pay attention to any jerks from your car or shaking while you are driving.

  • On average, the spark plugs can usually cost around $30 for you to install and buy yourself while going to a mechanic could add around $100 to $200 in labor costs for them to install the spark plugs in your vehicle. 
  • Dirty Fuel Injector

Sometimes the dirty parts that have accumulated debris in your car’s engine can also be the main cause of the rough idling engine. Fuel injectors disperse and transport fuel into your car’s engine at the right angle and in the right amount to fuel the car. 


The fuel injection is for optimal performance and preventing car idling rough symptoms. That being said, if the fuel injectors in your car are dirty, this can contribute to poor gas mileage or a bad fuel economy. 

  • Depending on your car’s make and model, the average fuel injector replacement cost is between that amount for the full replacement, with the parts alone costing between $600 to $1,200, and the labor cost being between just $200 and $250. 
  • Carburetor Concerns

Older vehicles that use a carburetor instead of a fuel injector can have various problems with the car idling rough symptoms. 

  • Black exhaust smoke is usually a strong indicator of an issue with the carburetor. Using a carburetor cleaner is a simple step to dissolve these deposits and prevent the car idling rough symptoms.
  • An engine should run smoothly without any excess noise. It is best to diagnose and figure out the issue quickly before it causes any other issues, like a possible engine repair or replacement that can be very costly for you and your car. 

Replacing your car carburetor is between $200 and $300 on average, with the range costing between $500 and $800 for high-end vehicles. 

  • Engine Coolant Temperature Sensor Malfunction

One of the main causes of car idling rough symptoms is a faulty ECT sensor. This sensor is in charge of signaling the car’s internal computer, the electronic control unit when the engine's coolant liquid is not the right temperature. The ECT has to tell the computer to deliver either more or less fuel to the engine.


An ECT Sensor is a variable resistor that responds to changes in temperature and can affect the rough idle when cold. On the other hand, a sensor that has an electric short will affect the driver’s ability to control the car and cause the car idling rough symptoms.

Test the ECT Sensor for Car Idling Rough Symptoms:

  • First, you will need to locate the ECT sensor around the cylinder head or close to the intake manifold, and then unplug the sensor’s electrical connector.
  • Using your ohmmeter, connect the leads across the connector terminals, and connect one lead to the terminal and the other to the sensor’s body. This allows you to get a reading to determine if the ECT sensor is causing the rough idle when cold. 
  • Now, set the transmission to Park if you are in an automatic car or Neutral if you are in a manual car. 
  • Turn on the emergency brakes, let the engine idle, and continue taking a reading of your ECT sensor every 2 minutes while your engine is running. 
  • If you see a gradual increase in resistance as the engine’s temperature goes down, this will work correctly. 

Replacing your ECT sensor is usually between $123 and $158, with the labor between $82 and $104, taking between 1 and 1.5 hours of work. 

  • Bad Intake Air Temperature Sensor

Like the ECT sensor, this IAT sensor can have a detrimental effect on the air to fuel mixture and ratio and the ignition timing. You can check the ECT sensor, so you can check the IAT sensor using an ohmmeter. 


The IAT sensor is supposed to decrease resistance as the incoming air temperature increases, and the resistance increases as the temperature outside declines. 


    • Symptoms of a bad intake air temperature sensor that can car idling rough symptoms is the engine stalling, the engine surging, the hard starting, the excessive fuel consumption, and the stumble and jerking while accelerating. 
  • Replacing the air intake temperature sensor in your car is between $95 and $106, with labor costs between $40 and $51 for between 30 and 45 minutes of work. 
  • Cold Start Injector Fault

Some car models come equipped with a cold start fuel injector. The main purpose of this fuel injector in your car is to add the right amount of fuel to the engine during the cold engine starting in winter weather. Typically, the fuel injector is operated through a time switch or the car’s internal computer, the electronic control unit. 


The cold-start injector is in charge of smoothing out car idling rough symptoms when the engine operates at a temperature below optimal levels. Once the engine reaches the right temperature, the cold-start injector turns off. If the injector is dirty or dealing with a bad fuel injector, it will not give the extra fuel needed. 

  • Fuel injectors typically cost between $800 and $1450 for a full replacement, with the parts alone between $600 and $1200. The labor price is between $200 and $250 for between five to six hours of work on average. 
  • Internal Coolant Leaks

Internal leaks can develop into driveability problems, like a car idling rough symptoms. A small coolant leak can move into an important mechanism, like the combustion chamber or the engine, and prevent a cylinder from firing. Once the engine reaches the correct operating temperature, the coolant evaporates and the combustion within the engine can return as normal. 


Unfortunately, this kind of fault is hard for a user or owner to diagnose since the internal and external leaks can produce various symptoms that can be hard to pinpoint, depending on where the symptoms occur or where the coolant and oil end up in the car. 

  • Expect to pay at least $100 for a coolant leak repair for an early-on diagnosis, while the average price is between $400 and $700 by the time most car owners notice the problems. 
  • Damaged O-ring Gaskets

On some car models, manufactures have been using O-ring gaskets to seal plastic intake manifolds. Eventually, the O-ring gaskets might become brittle due to the high engine temperatures, causing damage and break down over time. 


Once the damage has been done, the O-ring might fail to make the proper seal. As the engine warms up, the heat can expand the seal and the vacuum leak can disappear, reducing the car idling rough symptoms. 

  • Labor costs are between $699 and $883 for replacing the O-ring gasket seals, taking between 10 and 15 hours for this procedure. The parts of replacing O-ring gaskets are between $394 and $428 to fix the car idling rough symptoms.

The Bottom Line

As soon as drivers notice the car idling rough symptoms, car owners need to bring their car to a mechanic to diagnose the root cause of the issue. By checking for vacuum leaks. ECT malfunctions, damaged O-ring gaskets, coolant leaks, and faulty sensors, you can keep replacement prices as low as possible.