[SOLVED] Car Starts Rough Then Smooths Out

So you’ve noticed that your car is really rough when you start it, but then quickly recovers after you drive for a minute or two. This also means that your car is idling rough which means it rocks or vibrates aggressively if you leave your car idle with the engine switched on. 

Lucky for you, this isn’t a serious issue and can be quickly solved! Read on to find out the top 4 reasons why your car starts rough then smooths out. 

A car starts out rough and then smooths out because of any of these four reasons: 

  1. Vacuum Leak
  2. Spark Plug
  3. Dirty Fuel Injector
  4. Carburetor Problems

However, if you’re facing the issue of your car starting out rough and then staying that way (never smoothening out), you might want to take a look at the shocks of your vehicle. 

Top 4 Reasons Why Your Car Starts Rough Then Smooths Out

Here are the top four reasons why you might be facing the issue of your car starting out rough, but then quickly smoothening out in a few minutes.

Car Starts Rough Then Smooths Out

Reason #1: Vacuum Leak

To understand this problem, you have to first understand how your car works. Most cars have a lot of hoses (and you would’ve seen this if you’ve ever checked under the hood), and your engine will have a carburetor that pulls fuel into the engine itself. While rare, it’s possible that there is a vacuum leak in this kind of engine. 

More commonly, newer vehicles have a different system that utilizes a throttle. This regulates the speed of the engine as well as takes care of the airflow throughout your car’s system while balancing the vacuum. Sometimes, a singular hose can wear out, causing a leak and air to seep into the fuel. 

Solution: Having air mixed with your fuel almost certainly leads to engine misfiring. The misfiring is also why your car is starting out rough. To solve this problem, you’ll have to replace the hose which is responsible for the vacuum leak and even the O-rings. We recommend sourcing the parts yourself according to your car model and then getting your local technician to make the replacement since it can be a little tricky to do yourself!

Reason #2: Spark Plugs

Faulty Spark Plug

Another very likely explanation behind your car starting out rough, but then quickly smoothening out is the spark plugs are at fault. It could even be the spark plug wires rather than the spark plugs themselves. 

What are Spark Plugs? They are a component of your car that utilizes the electrical current from your ignition coils. After this, they ignite the air/fuel blend inside the combustion chamber. You can see why Spark Plugs have such an important role. 

If the plug is installed incorrectly or it has worn out, it can have immediate repercussions on the fuel, most often making it burn at an inconsistent rate. Not only is this harming your engine and wasting fuel, but causing the car to sputter while accelerating and idle rough when you first start it. 

Solution: The only way to repair this problem is to confirm that the spark plugs or spark plug wires are faulty. You will have to do a couple of checks to confirm they are indeed at fault, and possibly get a second opinion. Once this is confirmed, the only course of action is to replace the spark plugs. Luckily this is a mostly inexpensive fix, and shouldn’t be too difficult to do yourself! Take care to ensure you are selecting the correct spark plugs that are compatible with your vehicle. Enter the year, model, and make of your car in the directory. 

Reason #3: Dirty Fuel Injector

Fuel Injector

The third most common reason and explanation behind this particular issue is that your fuel injector is dirty. It’s difficult for all the components inside your car to stay clean over long periods of time, which is why maintenance is so important. When we say ‘dirty’ we refer to common road grit and dust that can sometimes weasel its way in.

Additionally, rust is the bane of every car owner’s existence. Fuel injectors have an important role in ensuring the smooth functioning of your car. They disperse fuel into the engine of the car with the specific quantity, force and angle. If the injector is dirty, this will significantly impair its ability to do its job properly. 

Solution: The good news is that out of all the reasons we’ve listed in this article, this one is the easiest to fix! All you need to do is clean the fuel injector, and you don’t have to work on replacing any parts of your vehicle. However, you can’t plug out the fuel injector and give it a good rinse in your kitchen sink! The correct way to clean the fuel injector would be to use an injector cleaner gas additive, such as the Gumout High Mileage Fuel Injector Cleaner.

Reason #4: Carburetor Issues 

Car Carburetor

As we mentioned before, your car either has the old system with a carburetor or the new one with a throttle. There are still many cars that work with carburetors. One of the reasons why your car might start out very rough but then quickly smoothen out is because of the carburetor. 

One of the most clear-cut signs that something is up with your carburetor will be tell-tale black smoke. If you pop the hood and are greeted with a gust of black smoke, it’s almost certainly the mark of a faulty carburetor. 

Solution: So what’s wrong with the carburetor? Most commonly, there are too many carbon deposits, and just like the fuel injector, it needs to be cleaned. You can purchase a carburetor cleaner and do the job yourself! This fluid will get rid of all the excess carbon deposits and have your car working like it is brand-new. 


As you can see, there are a couple of things that could be at fault when your car starts out rough but then quickly smoothens the ride out. It could be either the Vacuum Leak or Spark Plugs that need to be replaced and looked at. Alternatively, you could get lucky by simply having to clean your fuel injector or your carburetor! Whatever the reason, we’re sure your car will be back to normal in no time.

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