Car Starts Rough Then Smooths Out

When you spend a chunk of your savings on a car, you expect it to be a reliable and comfortable daily driver. No matter how well a car is built and how carefully you maintain …

car starts rough then smooths out

When you spend a chunk of your savings on a car, you expect it to be a reliable and comfortable daily driver. No matter how well a car is built and how carefully you maintain it, a problem can still occur.

One of the most common issues that car owners face is that their car starts rough and then smooths out. It is also known as “rough idling” which causes the car to shake and vibrate when started, and gradually settle down in a while.

As a car owner and driver, it is important that you know what causes the car to start rough, how to diagnose the issue, and what you can do to fix the problem. This comprehensive guide will drive you through the process in detail.

Car Starts Rough Then Smooths Out – Possible Causes

With so much going on under the hood, there could be many reasons behind your car starting roughly and then settling down. Keep an eye on these possible causes and fix them as soon as possible;

1- Faulty Spark Plugs

No car can start without a set of spark plugs, which are meant to provide the spark to ignite the air and fuel mixture inside the engine. If you’re experiencing rough idling in your car, the spark plugs on your car have probably gone bad.

Worn-out spark plugs or the ones that are not replaced after 75,000 miles, would not be able to function optimally and lead to rough starts and long idling intervals where the engine shakes abnormally.

2- Dirty Fuel Injectors

The fuel injector is the portal through which fuel passes from the gas tank to the combustion chamber. If the fuel injectors are clogged or leaky, the fuel will get spilled everywhere and not reach the engine in the optimal amount.

Fuel Injectors

This will lead to rough starts, eventually coming to a smooth functioning state. Keeping the fuel injectors clean and replacing them timely will prevent this headache.

3- Problematic IAC (Idle Air Control) Valve

When the car is idling and the gas pedal is not pressed, the air through the air intake manifolds is redirected via an IAC valve. During this time, the throttle valve is not functioning and is dormant.

If your car shows rough starts for a long time, even after attaining an optimal temperature, the IAC valve in your car is probably the culprit.

4- Malfunctioning Carburetor

Although modern cars use electronic fuel injection systems, carburetors were the mainstay of the automobile industry not so long ago. The function of a carburetor is to feed the engine with an air/fuel mixture and ensure proper combustion.


Over time, the carburetor gets choked due to the accumulation of dirt and debris. This can lead to rough idling and shaky starts that smoothen out eventually. If your exhaust is emitting too much black smoke lately, get your carburetor checked and fixed.

Other similar problem: Car Shakes When Accelerating But Not When Coasting

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5-  Faulty Coolant Temperature Sensor

An engine’s coolant temperature is relayed to the car’s ECU using a coolant temperature sensor. The system sends fuel based on the actual conditions; for instance, a cold engine requires more fuel and vice versa.

A defective coolant temperature sensor will give the computer false signals, which will result in an erroneous fuel flow. Hence, this results in rough starts that smooth out with time.

6- Leaky Vacuum

All the functioning parts of the engine are connected via relays and hoses, which keep the vacuum intact for optimal functioning. Over time, due to temperature changes and repetitive usage, these parts can get damaged and develop leaks and cracks.

If the intake manifold gets damaged and develops leaks, it can cause the engine to misfire and result in prolonged rough idling.

7- Faulty MAF (Mass Airflow) Sensor

The amount of air entering the combustion chamber is precisely measured by a mass airflow (MAF) sensor. The computer then receives the information and disperses the proper amount of fuel through the fuel injectors.

Mass Airflow Sensor

These sensors typically degrade over time or become partially occluded by carbon due to excessive usage, hampering the combustion process and leading to rough starts.

8- Problem with Transmission

If the gearbox is not properly calibrated with the engine or has developed a problem that doesn’t let it properly engage the gear, the car would not start optimally. The usual picture would be rough starts then smooths out, eventually attaining the normal temperature.

9- Bad Starter/Alternator

These components are responsible for starting the car and drawing power from the battery. If there’s something wrong with the starter or alternator, your car would show difficult/rough starts, eventually coming to a normal state with time.

What to do if you Experience this Problem?

If you’re experiencing rough idling in your car, you need to get it scanned and fixed by a professional technician. Don’t attempt to repair it DIY, as these are not layman problems and must be handled by an expert;

1. Get the OBD System Scanned – it is likely that if one of the components is faulty, your car will show a check engine light. Ideally, you get it scanned via an OBD-II scanner and read the error codes for easy diagnosis.

2. Replace the Faulty Sensor – Some sensors like MAF, and IAC usually causes the car to undergo rough idling. Simply replacing these sensors can get rid of this problem without any hassle.

3. Change the Spark Plugs/Fuel Injectors – The two main culprits behind your car showing rough starts and then settling down are the worn-out spark plugs and fuel injectors. Replacement of these parts is the easy and definite solution to the problem.

4. Check the Engine Thoroughly – A chocked carburetor or a leaky vacuum could be causing your car to start rough and then smooth out. Replacing the hoses and cleaning the carburetors can fix this problem.

Final Thoughts

A car is a complex piece of machinery, made of hundreds of parts working together to serve you flawlessly.

You should consult an expert technician for proper diagnosis and professional repair of your car if you ever run into problems like the above. Most probably, you’d have to replace the faulty parts to get rid of the issue permanently. Keep an eye on your car’s maintenance schedule and stick to the owner’s manual for a flawless ownership experience.