Car Starts Rough Then Smooths Out – Answer Here

If you’ve ever experienced the frustration of a car starts rough then smooths out, you’re not alone. This issue can be caused by a variety of factors, ranging from minor issues like dirty spark plugs or a clogged air filter to more serious problems with the engine or fuel system. 

In this article, we’ll explore some of the common causes of a rough start and provide some tips for diagnosing and fixing the problem so that you can get back on the road smoothly and safely.

Why Does My Car Start Rough

Why Does My Car Start Rough?

Car sounds rough when starting even though you haven’t pressed the gas it is a sign that the car is idling. At this time, it will be difficult for you to accelerate, sometimes the speed only reaches 1 rpm and is uneven.

In addition, you will hear an annoying sound coming from your car, the transportation suddenly becomes a lot more difficult. When this happens, it can cause additional fuel levels and potentially dangerous engine problems.

Usually, the condition of the car idling is caused by the long running time of the car. Some parts have worn out, causing the whole system to be severely affected. There are many reasons why this happens. The following section of the article will give you details of the causes that your car may encounter.

Car Starts Rough Then Smooths Out

Why A Car Starts Rough Then Smooths Out? 7 Main Reasons

If the car just shows up humming noise only when accelerating then you won’t need to worry too much about this problem. But when the car runs rough when cold or just starting, it is a sign that your car is having problems. Here are 7 reasons why this happens.

Spark Plugs

Spark plugs are the first cause of car idles rough when starting. This part is quite small in size, located inside the wheel. The main task of the spark plug is to take advantage of the ignition coil to generate electricity for the combustion chamber.

At this time, the electric current will change from the raw fuel to the pressure, which will act on the piston to push the force on the wheel’s bearing to start. Therefore, if the spark plug fails, the soot will clog the wire. This will lead to a faulty pressure regulator, making the car more difficult to start.

Then, if you do not fix the problem quickly, it will lead to many other serious problems in a short time.

Carburetor Problem

Carburetor Problem

Carburetor failure is the next cause engine sounds rough when starting. When you own a car with an older version, it can still run on a carburetor. This unit works by mixing air with gasoline in the right ratio to support the gasoline engine.

First, the air will pass through the filter, it will be narrowed at the ventilation duct. The fuel flow combined with the air will create a gas mixture for combustion. When this part fails, black smoke will appear from the exhaust pipe. You can solve it by using a specialized cleaner for the carburetor, which will help remove the deposits on the valve.

Fuel Injectors

Fuel Injectors

Fuel injectors are the next part that you need to take care of when the Car Starts Rough Then Smooths Out condition appears. Dirty injectors will cause your car to idling, reducing fuel economy.

Normally, when you start the engine, the ECU will scan the system. At that time, the “check engine” light will light up on the dashboard at that time. 

To be able to accurately calculate the amount of gasoline needed, the ECU will synthesize a lot of information: temperature, oil pressure, air temperature. Then, the injector will give effective fuel to start. engine.

Therefore, when the fuel injector leaks, the fuel flow in the system is distributed. At this time, the car will not be fully fueled, causing the car to idling when the RPM is high.

Vacuum Leak

The vacuum system is closely related to the spark plug. It helps restrict the flow of air entering the valve to aid in the intake air. When a vacuum leak occurs, it can come from a corroded valve cover gasket. 

In the event that the fuel pump draws in extra air and there is no control by the engine computer, the fuel mixing ratio will be incorrect. It is the premise that causes the rough idle when starting.

Bad Temperature Sensor

Bad Temperature Sensor

The temperature sensor is responsible for measuring the temperature of the cooling water, then it will transmit a signal to the ECU control system. Based on this data, the ECU will set the time for the injectors and spark plugs, so that these components can work correctly.

But when the temperature sensor fails, the information received will be inaccurate leading to the whole system being severely affected. 

In addition to spark plugs and injectors, radiator pipe will also be negatively affected. As a result, the car will experience vibration when operating.

IAC Valve

This set of valves has the main task of managing the engine’s airflow. When this set of valves is stuck, the air flow will pass through the ignition coil without being precisely controlled. This has caused the rough idling of cars to take place at high PRM speeds.

MAF Sensor

Air flow sensor is the final factor that makes your car Car Starts Rough Then Smooths Out. This part has the main task of supporting the creation of air flow inside the system. 

Through the analysis process, the ECU will accurately calculate the right amount of gasoline. Therefore, when the MAF sensor is not suitable, it will lead to the carburetor misbehaving, causing excessive fuel consumption.

What Are Other Causes Of Idling Cars

What Are Other Causes Of Idling Cars?

In addition to the 7 common causes, there are a few other factors that cause your car to have a Car Starts Rough Then Smooths Out condition including:

  • EGR valve: This part plays an important role in interfering with the operation of gasoline. It helps control emissions near the intake manifold. When carbon deposits appear on this valve set, it will cause the system to fail.
  • Teeth disappearing from the chain: This problem can cause serious problems with your vehicle.
  • Clogged air filter: It is the next cause of your engine running sluggishly.

How To Tackle The Problems?

Once you’ve identified what’s causing your Car Starts Rough Then Smooths Out, you can start working on fixing the problem. In case your spark plug is faulty, you should replace it with a new one. This job is quite simple and you can do it at home. Troubleshooting will take no more than 1 hour.

In addition, if the cause is a clogged carbon, you will need to use an injector cleaner. As for other parts such as vacuum or temperature sensor, it can be purchased at auto parts stores. However, this replacement work is not easy and you will hardly be able to do it yourself at home.

Bring your car to the service center, they will assist you to check and fix the problem most effectively.


How To Tell If A Spark Plug Is Broken?

There are 3 signs that your spark plug has failed:

  • The idling sound every time you start it up
  • Engine without ignition
  • The car consumes more fuel than usual

Why Does My Car Start Rough In Cold Weather?

In fact, when the weather is cold, the car will draw more air when it engages the gas system. At that time, the mixing ratio will be unbalanced, leading to incorrect ignition in the combustion chamber. The main cause of this situation is the ETC sensor.

Is It Safe To Drive A Rough Car?

At first, you will probably feel there is no problem with the car idling. However, in the long run, the car may have potentially dangerous problems that you can hardly foresee. It can happen unexpectedly when you are on the road. At this point, instead of trying to drive, you should take the car to a repair garage for assistance.


Hopefully, the article will help you better understand car starts rough then smooths out. You should not ignore this phenomenon. Because it is a sign that a certain part has failed in the engine. 

Therefore, take your car to the repair garage for assistance in checking and fixing the problem in time, helping to limit serious risks that may suddenly happen to you.

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