IS YOUR CAR SHAKING WHEN STOPPED? 3 COMMON CAUSES OF A ROUGH IDLE
If your car only has a rough idle and doesn’t experience any other symptoms, then keep on reading. You’ll notice your vehicle vibrating or wanting to turn off when you’re parked, at a red light, or at a stop sign. As you begin to drive, the shaking will go away and it will seem as if your car doesn't have any problems. In this article, we will discuss the most common causes for this issue.
causes
idle air control valve
The first potential cause for a rough idle is a clogged or stuck-closed idle air control valve. This little valve is responsible for controlling the amount of air that enters your engine while your car is idling, or in other words stationery. However, not all cars have one, especially the newer models. They use an electronic throttle body to open up a tad bit while idling to control the air coming in. When the idle air control valve gets clogged or stuck-closed, it restricts the airflow needed for efficient combustion. In order for proper combustion to occur, the engine strives to achieve the perfect air-fuel ratio. A faulty idle air control valve restricts the correct amount of air needed, throwing off the air-fuel mixture and causing combustion issues that create a rough idle.
The location of the idle air control valve will be near the throttle body. If you suspect a clogged idle air control valve, you can just clean it with this cleaning chemical.
dirty throttle plate
The second potential cause for a rough idle is a dirty throttle plate.
When you press down on the gas pedal, the throttle plate opens up to allow more air to come in. However, at idle, the throttle plate is just slightly open to let a small amount of air come in. Over time, there can be build-up around the throttle plate, which can restrict the airflow. This causes the air/fuel mixture to become imbalanced, and as a result, your car could have a rough idle. You might notice that your vehicle is vibrating or shaking when it's idling.
Fortunately, cleaning the throttle plate is a relatively EasyAutoFix. You can use a carburetor cleaner to clean the throttle plate and restore proper airflow. This can help smooth out your car's idle and improve its overall performance.
vacuum leak
The third potential cause for a rough idle is a Vacuum leak. If you’re not sure what a vacuum leak is, in simple terms, it’s the air that enters the engine (marked with blue arrows in the picture) that is not noticed by the mass air flow sensor or the manifold absolute pressure sensor. These sensors are used by the engine control module to read the correct amount of air coming into the engine. The goal is to make the air-fuel ratio the best it can be, and these sensors are crucial to achieving that. Now, when there is air entering the engine that isn't being noticed by the engine control module, then that’s when the rough idle starts to occur. This extra air entering the engine disrupts the air-fuel mixture, leading to a poor running engine. The reason why a vacuum leak will affect the vehicle more in idle vs while it's moving is because at idle the throttle plate is open less, so the ECM commands less fuel. As a result, the difference between air and fuel is greater. On the flip side, when the vehicle is moving, the throttle plate is open more so the ECM commands more fuel. This is why you feel your car performs better when it's moving, and rough when it's idling.
Some of the locations of a vacuum leak could be from a torn intake manifold gasket or a cracked intake manifold, any torn vacuum hoses, a stuck open PCV valve, anything torn after the MAF sensor, a stuck open EGR valve, a ruptured diaphragm inside of the brake booster, or any torn brake booster hoses.
A quick tip for finding a vacuum leak is to use a carburetor cleaner. While the car is on, spray around the areas I described and if you hear the engine change in sound or speed then you found the problem.
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