Car Starts Rough then Smooths Out

Every car is made to last a certain amount of time and manufacturers pay special attention to the longevity of certain crucial equipment in order to make sure that quality control parameters have been met.

Some cars last longer than other cars, though the technical aspects of some cars make it near impossible for them to pass the test of time.

However, there are a few problems that are sure to happen in nearly every vehicle and one of such problems is that a car starts rough but then eventually smooths out. You may have tried to figure out the reason behind this and here we will be answering this exact question and its probable causes and solutions.

The phenomenon when a car starts roughly is commonly referred to as rough idle, meaning that the car shows abnormal behavior right after starting up and after some time it starts to behave normally as it gets warmed up.

Reasons Why your Car starts Rough then Smooths Out

There are a variety of reasons why a car would act in this manner and these reasons are listed below:

Spark plugs:

The number one culprit in case of a rough idle is the spark plugs or their connecting wires. An engine works when a mixture of gasoline and air is ignited with the help of a spark produced by spark plugs.

The vehicle will show abnormal behavior in case spark plugs are worn out or were installed incorrectly, therefore check your spark plugs first, they can be removed easily with the help of a socket.

If the gap looks greater than the usual, then the plugs are worn out, or if you haven’t changed the spark plugs in a long time then do it anyway as these are an important component. Remember to tighten them with the exact amount of torque as mentioned in the user manual.


Nowadays electronic fuel injection is used to feed the gas and air mixture to the engine. But in the past, cars used to have a carburetor that served the same purpose.

Over time, carburetors get dirty and can get choked. The narrowed pathways make it difficult to feed the correct mixture to the engine, hence the car idles rough.

Another symptom can be the black smoke from the exhaust, if this is the case or if your car has a carburetor and you have clocked several thousand miles and you haven’t cleaned the carburetor even once, then clean it with a good carburetor cleaner. Always stick with a carburetor cleaner as they are specifically made for this purpose.

Fuel Injectors:

Fuel nowadays is much cleaner than in the past and the inclusion of different additives has further enhanced its cleaning properties.

A dirty or a clogged fuel injector will not feed the engine with the required amount of fuel which will lead to a car that starts rough but eventually smooths out.

To clean a fuel injector, use a good quality fuel injector cleaner, after cleaning you will observe a noticeable difference in the idle behavior and fuel average as well

Vacuum leak:

You may have been surprised at some point in your life after opening the hood of a car and seeing an array of hoses. These hoses have a limited span of life, after that, they start to develop cracks and leaks, and this is one of the reasons behind a rough idling car.

The intake manifold can develop leaks which will lead to an engine misfire, which in turn will lead to your car starting rough and smoothing out after a while.

Bad temperature sensor:

A coolant temperature sensor sends feedback to the ECU regarding the temperature of an engine’s coolant. The computer sends fuel according to the conditions, for example, more fuel is required when an engine is cold and vice versa.

A bad coolant temperature sensor will send faulty readings to the computer which will cause an incorrect flow of fuel. This will ultimately lead to rough idle; the solution is rather simple for this problem as you will have to just replace the bad temperature sensor with a new one.

IAC valve:

The idle air control valve is located close to the throttle body and its function is to provide a smooth flow of air to the engine when idling. A rough idle can be caused by a dirty IAC valve, therefore cleaning it can lead to a smooth idle and your car will show a marked improvement in idle behavior.

MAF sensor:

Mass airflow sensor precisely measures the amount of air going into an engine. The data is then fed back to the computer, which then sprays an appropriate amount of fuel through fuel injectors. Over time, these sensors tend to go bad or are partially blocked by carbon, therefore cleaning the MAF sensor with MAF cleaner can probably solve your rough idle problem.

Conclusion on what it means if your car starts roughly then smooths out

A small thing in the engine can cause a rough start, I have listed the most common things that cause your car to start rough, I hope you find this information helpful.