Why Does My Car Idle High And Low?

When your car is experiencing a rough idle, your engine may chug, vibrate, or shake while it is running. This will cause your overall driving experience to feel rough. And you will likely be bouncing a little bit in your seat from it.

When the engine idles, there will usually be a misfire that you can feel just from holding onto the steering wheel. Furthermore, the RPM of your engine will be higher than normal when the vehicle is idling.

The RPM will usually go back to normal as you accelerate the vehicle. But will increase again after you stop. This is definitely not a normal situation. To get the solution to follow the below cause of why does my car idle high and low.

What Is Idling?

Idling is the process of making a car standstill while the engine is on, but the vehicle is not moving. It happens when no gears are engaged, the acceleration pedal is not pressed, and brakes are activated. Idling is needed in many situations, such as when you wait for the traffic signal to turn green.

Or at any point when you don’t feel the need to turn off the engine. Sometimes idling seems to increase the RPM high until the engine is fully warmed up. But the problem arises when RPM is still set to high even after the engine warm-up is done. If you are encountering car idles high and accelerates on its own issue, here is your solution.

Why Does My Car Idle High And Low?

On a modern fuel-injected engine, engine idle is controlled by the engine control unit (ECU) and an electrically operated throttle control module. When the idle is high or low, it could be caused by a malfunction of this system or any of the individual components that comprise the system.

The idle air control valve controls the idling of the engine by allowing a determined amount of air to mix with liquid fuel. And create a vapor to be burned in the combustion chamber. When it fails or begins having problems, the impact will change the idle speed of the engine. There are a few other engine components that can have an impact on engine idle speed including the following

1. Intake Gaskets

The intake gasket is designed to maintain a consistent pressure inside the engine. When the gasket fails, it can allow pressure to escape, which tends to cause elevated idle issues.

2. Vacuum Lines

Used to control the internal vacuum pressure inside the motor, the vacuum lines, like the intake gasket above can cause elevated idle issues if they are broken or not properly attached.

3. Air Intake

The job of the air intake is to move air into the engine. When it is damaged, more air can pass through, which will cause the engine to idle high or low. A damaged throttle bore will result in the same problem. Because it is no longer able to regulate the volume of air flowing into the engine.

4. PCV Valve

The PCV valve or crankcase ventilation system removes vapors from the engine. When it doesn’t work correctly, the engine may idle at various speeds.

5. Oxygen Sensor

Since the oxygen sensor is part of the emissions system, its failure will usually trip a trouble code in the engine computer. A code reader can be used to identify the problem, and a digital multimeter may help to determine it. Whether the sensor has failed and needs leaks in these hoses and other parts of the vacuum system can be another cause of a rough idle. Often because of a lean fuel or air mixture from excess air entering the system. These leaks can result from a variety of causes

6. EGR Valve

A vacuum pump can be used to test whether the EGR valve is operating properly. A faulty EGR valve may also cause your Check Engine light to come on, in which case a code reader may reveal the culprit.

7. Carburetor Issues

Carburetors were previously used in vehicles before fuel injectors were invented. In case you have a vehicle that still has a carburetor in it.

If so, the carburetor itself could be having issues resulting in rough idling of the engine. A sign of this would be if your exhaust was blowing out black smoke.

8. Engine Timing

The idle speed also comes from engine timing. When it is not timed correctly, the result is an engine that idles too fast or too slow.

All of these components can cause a problem with idle speed when they do not work correctly. If your engine is idling too fast or too slow, a technician needs to inspect the system to determine which component is at fault.

These are the parts that are involved in why does my car idle high and low.

What Your Mechanic Will Do To Fix Your Rough Idle?

There are many possible causes for a rough idle. A great first step is to check the engine computer and see what codes have been triggered. These codes can lead your mechanic to the cause and suggest a fix.

In case the cause is not so obvious, your mechanic will check the usual suspects. This will include verifying that the relevant electronic parts are working properly. The valves and sensors are clean and operational, the hoses are not cracked or leaking. The spark plugs and their wires are in good condition, the air filter is in good shape, and so on.  The car vibrates when idle or your rough idle could be caused by one of these items, or possibly by a combination of them.

Once the trouble-causing items have been repaired, cleaned, or replaced, your vehicle should be idling smoothly and running well once again. To keep it that way, remember to do your scheduled maintenance at the intervals recommended by your vehicle’s manufacturer.


Your engine should normally get to around 700 RPM after you start the vehicle, depending on the model. If your idle is way off and you have one or more of the problems listed above, then you will want to diagnose the issue right away.

The cost of the repair or replacement job will depend on what the cause was. In case you take your vehicle to a repair shop soon, then your engine should be okay. So, follow each step of why does my car idle high and low above to check each part of the engine

But if you have continued to let it idle poorly for too long, then there could be irreversible damage to the engine. This in turn could cost you thousands of dollars in repair or replacement costs. So, don’t wait for this to happen.

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