Why is My Car Idling Rough 7 Reason You Don’t Know

why is my car idling rough, car idles rough but drives smooth, car idling rough when stopped, car idles rough and shakes, what causes rough idle at low rpm, what does a rough idle sound like

Your car is idling rough because of an abnormal air intake because of a clogged or dirty air filter. Another reason could be the presence of water on your fuel tank which has made its way up to your carburetor or fuel injector.

Idling refers to how an engine runs when you are not pressing on the throttle and the gear is not engaged. The best time to observe your car’s idle is in the morning, the first time your start it, or generally after it has been off and the engine is cold.


7 Common Why Your Car is Idling Rough


Like I said before rough idles could be caused by a lot of reasons. However rough idle is related to the engine’s air intake, plugs spark, and fuel flow.

Rough idle is a sign that could even cause the check engine light to come up since it affects the overall engine performance. Rough idling could increase your car or vehicle’s fuel economy and overall engine performance.

Below is a detailed highlife as to why your car might be idling rough.


why is my car idling rough, car idles rough but drives smooth, car idling rough when stopped, car idles rough and shakes, what causes rough idle at low rpm, what does a rough idle sound like


  • Bad or failing spark plugs

In most cases, spark plugs are the most common cuprite behind a rough idling car. Continue reading to find out as this could be the answer to your question why is my car idling rough.

The function of the spark plug in the engine is to create a spark that ignites the air/fuel mixture, creating the explosion which makes your engine produce power. Don’t get things twisted just yet, spark plugs are electrical devices.

When spark plugs are not giving enough sparks to burn the air mixed with fuel in the combustion chamber, rough idling will kick in. Signs of a bad spark plug itself include engine misfire, rough idling, poor acceleration, bad fuel economy, and difficulty in starting.

  • Dirty fuel injector

If you have checked the spark plugs and they are all fine, the next step is to check the fuel-injected if it is what your vehicle is using. A dirty fuel injector can be in the forefront of a rough idling car.

Fuel injection is the introduction of fuel into the internal combustion engine, this is done by the injector. The fuel injector is responsible for spraying fuel into the vehicle’s engine via the means of special valves capable of opening and closing many times a second.

These valves can get clogged and dirty over time as a result of tiny carbon files present in the fuel. When they get clogged, there would not enough fuel supply with much oxygen to burn, which will cause the engine to start rough idling.

If you are the type that usually runs your fuel tank empty before refueling your fuel injector might just be at risk. the best precautionary action to this is to never run your fuel tank empty before refueling.

  • Dirty Air filter

Just like most engine parts some get old and worn out over time and would need to be replaced at some point. Well, the air filter is not left out on the list, air filters get dirty and would need to be replaced in your car.

The engine needs air to burn fuel in the combustion chamber, this air is sucked in through the air filter. The primary function of an air filter in the engine is to clean the air that the engine breaths.

When an air filter becomes dirty it can cause your car to start rough idling. If you have checked your spark plugs and fuel injector to be working fine then the air filter might be the answer to your question why is my car idling rough.

If you found out that the air filter is dirty, please do your engine a favor and get it cleaned or replace it with a new air filter. Replacing a new air filter will as well help to increase the overall engine performance.

  • Clogged fuel filter

A clogged fuel filter can as well be the cause of rough idling in your vehicle. Fuel filters become clogged by dirt and debris just like in the case of the injector.

The fuel filter is found in a vehicle’s fuel line, the primary function of a fuel injector is to clean fuel delivered to the injector or carburetor. Fuel filters are usually situated inside the fuel tank beside it.

Signs of a bad or failing fuel filter include misfiring engine, stalling, difficulty ins starting, and rough idling itself. If you notice any of these symptoms please visit the mechanic as quickly as possible to avoid disappointment on the day you least expect.

  • Dirty carburetor

The carburetor is another form of the injector, the function of the carburetor is to deliver fuel into the combustion chamber. Basically, the carburetor is a device that mixes air and fuel for internal combustion engines in an appropriate air-fuel ratio for combustion.

The carburetor can become dirty over time as a result of dirt and debris accumulated over time. When the carburetor becomes dirty, t fails to deliver the right amount of fuel and air required by the combustion chamber.

If your car uses a carburetor, and it has not been cleaned in a while, you should consider having it checked. Carburetors can be very complex to fix sometimes so it is advisable that you should visit an auto repair shop for proper checks.

  • Incorrect timing

An incorrect timing is another reason why your car could be idling rough, but this case is not common unless you have recently had it replaced. The timing belt or chain functions as the link between the crankshaft and the camshaft.

The timing belt or chain synchronizes the movement of these two most important engine parts so that the engine’s valves open and close at the proper times during each cylinder’s intake and exhaust strokes. Here is a simpler explanation, the timing belt is a rubber belt that connects the camshaft and the crankshaft to make the engine work in harmony all through each cylinder’s intake and exhaust strokes.

A wrongly installed timing chain or belt can cause a car to be idling rough too. When timing is installed incorrectly, it affects the overall engine performance and expensive damage to the engine.

  • Incorrect ignition timing

While cam timing is mechanically controlled by a timing belt or chain, ignition timing is controlled by the ECU and the distributor. Some vehicles have a crank or cam angle sensor instead of a distributor that can be manually adjusted to advance ignition timing in much the same way.

If ignition timing is incorrect, the car could run rough and often try to stall. You should be able to check your own ignition timing using a timing light. The procedure is different for every vehicle, so check a repair manual if you’re not sure how to do this yourself.


What does it mean when your car is idling rough?


A dirty air filter, failing spark plug and a worn-out engine seat is factors that can cause a car to be idling rough. When spark plugs become too sooty they lose their ability to spark and ignite fuel and air in the combustion chamber.


why is my car idling rough, car idles rough but drives smooth, car idling rough when stopped, car idles rough and shakes, what causes rough idle at low rpm, what does a rough idle sound like


A dirty filter can also be the culprit, as a dirty air filter decreases the amount of clean air that is reaching the engine. Air filters should be cleaned or even replaced between intervals if found dirty.

Motor mounts or engine seats can also be the reason why your car is idling rough. Engine mounts, also known as motor mounts, are designed to secure your vehicle’s engine and transmission to the subframe.


What causes rough idle and stalling?



The fuel pump, which is responsible for pulling fuel from the gas tank to the fuel injectors, can become clogged or defective. If this happens the engine will not get enough fuel, which can cause a rough idle, sputtering, stalling, and even slow acceleration.

A rough idle is one symptom of a clogged fuel filter. The fuel filter can get clogged or even blocked over time due to the presence of dirt and debris in the fuel or fuel line.

People who run their fuel empty tanks stand a greater height of danger when it comes to clogging the fuel filters. Do not wait until the fuel tank is empty before refueling, this practice will increase your fuel filter lifespan.


How to Adjust Your Car Idle Speed


why is my car idling rough, car idles rough but drives smooth, car idling rough when stopped, car idles rough and shakes, what causes rough idle at low rpm, what does a rough idle sound like

Car engines need air to take in and burn fuel. But too much air will cause the engine to run too hot and damage it. There’s a small needle valve on your carburetor that controls how much air gets into the engine. When you turn the key on, the needle valve closes off until you give it gas; then it opens up to let in more air.

  1. Remove the Engine Air Filter. The air cleaner and filter assembly must be removed for you to access the carburetor.

  2. Locate Adjustment Screws. Before beginning, you should familiarize yourself with the location of the idle mixture and idle speed screws.

  3. (Optional): Hook up the Vacuum Gauge. If you are using a vacuum gauge for this process, you should find and connect it to a manifold vacuum port before starting the engine.

  4. (Optional): Set Baseline. If the carburetor is new or has been rebuilt, you should set it to “factory” adjustment. You can do so by adjusting the idle mixture screws to 1.5-2 turns out, and the idle speed screw to 1-1.5 turns in.

  5. Warm the Engine Up. The engine’s running temperature directly correlates with proper air and fuel mixtures. So, be sure to let the engine run and reach normal operating temperatures before proceeding.

  6. (Optional): Adjust Idle Speed Screw. If you set a baseline in step 4, chances are idle is a little high. Be sure to adjust it so that you start with the engine’s normal running speed. You may need to perform this during warm-up if the idle speed is too high after reverting to the “factory” spec.

  7. Adjust the Air-Fuel Mixture. Begin by adjusting the idle mixture screws by 1/8 turn increments in or out. Be sure to adjust each in direct relation to the other as you proceed if you have more than one idle screw (most two and four-barrel carburetors do).



A rough idle is usually characterized by a shaking and bouncing sensation in the vehicle. You may also notice odd sounds, as well as inconsistent RPM, and counts whether the idling is severe or little.

Did you find this article useful by any means? if yes please share it. If you have questions or need more clarification about why is my car idling rough, please ask using the comments section.

Be the first to comment

Leave a Reply