Why Is My Car Revving On Its Own While Driving, Stationary Or In Park?

Has your car ever seemed to take on a life of its own, with the engine inexplicably revving while you’re driving down the road, sitting at a stoplight, or even parked in the driveway?

This unnerving car revving on its own can leave you feeling anxious about a potential problem under the hood. In this guide, we’ll explore the common causes behind this issue and equip you with the knowledge needed to diagnose and resolve it.

So, why is car revving on its own? A car revving on its own often indicates an engine misfire, attributed to faulty spark plugs or an ignition system. Incorrect coolant temperature readings due to low engine coolant levels or a defective sensor can also cause this issue, particularly in park mode. Additionally, check for vacuum leaks, problems with the Idle Air Control Valve, and issues with the throttle body, including a tight, sticking throttle cable in mechanically controlled units.

Also Read: Is revving your car bad

Why Is My Engine Revs High Without Acceleration?

Your engine revs high without acceleration because of transmission slipping. If you have a manual transmission, it will also have a clutch plate that engages with the flywheel when you lift the clutch pedal.

If the friction material on the clutch wears down, the transmission will start slipping. The engine RPM will run as it should but due to transmission slipping, the power from the flywheel to the drivetrain will not be supplied smoothly.

In an automatic transmission, the transmission slip can occur due to a low level of transmission oil or when transmission has become dirty.

In an automatic transmission, oil pressure is used by the torque converter to transmit power from the flywheel to the driveshaft. If the transmission oil is dirty, it will also cause the engine to rev by itself without acceleration.

Moreover, in automatic transmissions, the gearbox band can also slip, which would cause your car to rev up while driving.

Also Read: Car jerks when shifting to reverse

Causes Of Car Revving On Its Own

Here are some of the most common causes of a car revving on its own when stationary or driving:

  • Vacuum leaks
  • Bad MAF sensor
  • Bad engine air filter
  • Bad Idle Air Control Valve
  • Bad throttle body
  • Bad throttle position sensors
  • Faulty coolant temperature sensor
  • Low coolant level
  • Damaged spark plugs
  • Bad fuel pressure regulator
  • Dirty fuel injectors
  • Stuck EGR valve
  • Bad cam phaser

1. Vacuum Leaks 

If your car is revving on its own while idling or driving, it can be due to vacuum leaks. Vacuum leaks are caused by a failure of the engine’s vacuum system. This is when the engine is sucking the air when it is not supposed to.

The air that enters the engine due to vacuum leaks is termed ‘unmetered air‘. That amount of air is not measured by MAF or MAP sensor. As a result, the ECU will not be able to decide the optimum fuel to be injected into the cylinder.

As a result, the rich air-fuel mixture will form which will result in the engine misfire and will cause the engine’s RPM to fluctuate.

Vacuum leaks can occur from cracked vacuum hoses, intake manifold gaskets, PCV hose, throttle body gasket, air intake hose after the MAF sensor, and brake boosters.

The easy way to find vacuum leaks in the engine is to use a propane torch with propane cylinder. Start the engine and let your car idle. Let the car idle.

Open the hood and turn on the propane. Aim the hose at all engine seals, around the air intake manifold after MAF sensor, and throttle body. If the RPMs bog in one spot, that’s where the propane is being sucked in and indicates a sign of vacuum leak.

2. Bad MAF Sensor

MAF sensor

A bad MAF sensor will not be able to measure the correct amount of air entering the engine air intake manifold. The mass airflow sensor sends signals to the ECU so that the ECU can adjust the amount of fuel accordingly.

If the MAF sensor is bad, the ECU will get inconsistent mass airflow readings from the MAF sensor, which will cause your car to rev up and down by itself while stationary or accelerating.

Keep in mind that older engines have a Manifold Pressure (MAP) sensor instead of a MAF sensor. So, it depends on which engine you have. The MAP sensor cannot be cleaned. So, you have to replace it.

MAF sensor gives more accurate readings to the ECU to calculate the correct amount of fuel. So, modern engines have a MAF sensor. Some engines even have both MAF and MAP sensors. So, you should check your owner’s manual.

To clean the MAF sensor, follow these steps:

  • Disconnect the negative terminal of the battery.
    Remove the MAF sensor, it is located on the intake tube between the air filter box and the throttle body facing upwards.
  • Spray all of the little sensors you see inside of the MAF carefully with this MAF sensor cleaner. Avoid touching the wires of the MAF sensor. It will damage it.
  • Reinstall and reconnect the battery terminal.
  • Start the engine and let the ECU relearn idle for 15 minutes.

To test the MAF sensor, you can watch the below Youtube video:

My Kia was revving on its own. So, after cleaning the MAF sensor, the issue was resolved. 

3. Bad Engine Air Filter

engine air filter

The air filter is a component of the engine that collects dust particles and other debris from the air before it is inhaled into the engine. A dirty air filter is often overlooked. It is essential to have it changed every 30,000 miles or 3 years to maintain the engine’s performance.

The air filter is responsible for keeping harmful contaminants out of the engine. When it is clogged, it stops working as efficiently.

In case of a faulty engine air filter, it may cause the engine to rev up spontaneously without your input. This may happen due to the accumulation of contaminants in the air filter as the airflow is disturbed due to the contaminants in the air filter. 

How to inspect?

A bad air filter looks dark in color or gey. The dirt in the air filter causes turbulence in the airflow. Due to disturbance in the airflow, the car may rev on its own.

4. Bad Idle Air Control Valve

A bad idle air control valve is also one of the most common causes of a car revving on its own. The idle air control valve (IACV) is ECU-controlled and is responsible for regulating the amount of air entering the engine at idle.

At idle, the butterfly valve of the throttle body is almost closed. So, the idle air control valve allows the air to bypass the throttle plate and enter the air intake manifold.

The following Youtube video is quite helpful to understand the working of idle air control valves.

IACV is located on the intake side of the engine, near the throttle body. When IACV starts going bad it either idles up and down, or it sticks in a certain position which will make your engine’s idle RPM vary due to changes in air density, temperature, etc.

IACV usually clogs with dirt and carbon deposits, due to which it is stuck and causes the car to rev on its own.

How to identify?

To identify a bad IACV, disconnect the IACV connector. If idle speed decreases or remains constant, it means the IACV valve is fine.

Another way to test the idle air control valve is to remove the air boot from the throttle body and cover the throttle body with something that will stop the airflow like your hand or tape.

During this process, your engine should be running. If the idle RPM does not decrease when you cover the throttle body, it means IACV is malfunctioning.

Another method to test a bad IACV is to check the resistance between the terminals of IACV and compare it with the specified resistance values.

testing of terminals of idle air control valve

From the above picture, B2 and B1 are terminals that receive a 12-V signal from the ECU. Set the multimeter to the ohm setting and measure the resistance between the terminals (B1 (or B2) to others).

Moreover, you should also check the wires of the harness connector of the IACV. Make sure the wires in the connector are not broken and the connector is firmly plugged into the IACV valve. If the wires are frayed or broken, you have to replace the harness connector.

5. Bad Throttle Body

mechanical and electronic throttle body

If your car is constantly revving, there are the chances that throttle body is bad. Now, keep in mind that the throttle body can be both mechanically and electronically controlled.

The mechanical throttle body is driven by a throttle cable that links the accelerator pedal to the throttle plate. If the throttle cable is too tight or sticking a little, it will not be able to have full control over the throttle plate. As a result, your car engine’s RPM will boggle without pushing a gas pedal.

The electronic throttle body is controlled by the ECU. The accelerator pedal sensor sends signals to the ECU so that it can compute how much the throttle valve should be opened.

You will see a port on which a harness connector is plugged into the throttle body. In that case, you have to test voltage signals at the harness connector of the throttle body and the resistance between the terminals of the throttle body actuator. In my guide on P1516 code I have explained the process of testing the throttle body. 

If the throttle body passes electrical tests, you should check for any carbon deposits inside the throttle body. You can use this throttle body cleaner to clean the throttle body. 

Make sure that the throttle plate of a throttle body is not binding when you try to move it with your finger. It should rotate freely under the action of a spring force. 

Moreover, you have to perform the ECU relearn procedure for an electronic throttle body after cleaning. I have explained this in my guide on the P1516 code I linked above.

6. Bad Throttle Position Sensors

The throttle position sensor is located on the throttle body and it relays information about the position of the throttle plate to the vehicle’s computer so that it monitors the position of the throttle valve and makes sure it’s in the right position.

The ECU finds a correlation between the readings of the accelerator pedal sensor, throttle position sensor, and mass airflow sensor to determine if the throttle valve is opened to a calculated position or not.

If the throttle position sensor malfunctions, the ECU will not be able to compute the position of the throttle valve. As a result, bad throttle position sensors will cause the engine to rev without the driver knowing.

The following Youtube video will help you understand the working of throttle position sensors:

7. Faulty Coolant Temperature Sensor

If the coolant temperature sensor (ECT) is malfunctioning, your car will rev up on its own when the engine is cold. After the engine warms up, RPM will be fine. Many users faced this issue and the root cause was a faulty coolant temperature sensor.

The ECT sensor is usually located near the thermostat and sends signals of the engine coolant temperature to the ECU. The exact location of the ECT sensor depends on the make, model, and year of the vehicle.

The ECU injects more fuel and richens the air-fuel mixture when cold and adjusts the fuel trim as the engine coolant temperature increases

If the ECT sensor is bad, it will send the wrong coolant temperature readings to the ECU. As a result, the ECU messes up the air-fuel ratio, which will cause your car to rev high automatically.

How to test?

The best way to diagnose the coolant temperature sensor is to immerse it in the water,  measure the resistance of the sensor at different temperatures and compare the readings to the specifications listed in the service manual.

Follow these steps to test the ECT sensor:

  • Immerse the tip of the sensor in the water.
  • Connect a digital ohmmeter to the two terminals of the sensor.
  • Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance
  • Repeat the resistance at other temperatures by heating or cooling the water.
  • If the sensor does not meet the specification shown in the temperature versus resistance chart, it must be replaced.

You can watch this video to learn more:

8. Low Coolant Level

The low coolant level in the engine can also cause fluctuations in the RPM of your car’s engine which results in revving your car automatically.

If the coolant level is low or dirty, the ECT will not be able to detect the correct temperature reading of the coolant as the sensor will not be completely submerged in the coolant.

As a result, the ECU will not be able to identify the correct air-fuel ratio. So, it will cause your car to rev up and down constantly.

The color of a clean coolant is green or blue. If you find a coolant discolored, you should flush the engine. 

To flush the coolant system, keep the radiator valve open and the drain pan in place, and then run water through the radiator. The drain valve of a radiator is usually located at the bottom. Flush the coolant system until you see the transparent water flowing out of the drain valve. If your coolant color is fine, you need to check whether the coolant level is low or not.

In my guide on P0128 error code, I have explained a detailed process of adding a coolant to the engine. You must read it.

9. Damaged Spark Plugs

A damaged spark plug can cause your car to rev itself on its own. Spark plugs can be fouled with the deposits of fuel, ash, oil and corrosion, and can cause engine misfire. I have found this amazing guide (bad spark plug signs PDF) that includes pictures of all possible damages with the spark plug.

The ignition coil is a very large transformer that is responsible for generating a high voltage pulse of electricity. This pulse is then sent to the spark plugs which ignite the air-fuel mixture in your engine.

A bad spark plug fails to generate and misses the firing point, resulting in the engine revving.

When an engine misfires, the air-fuel mixture is not burnt uniformly. As a result, sudden combustion events take place in the engine cylinders at the wrong time. This causes a sudden increase and decrease in the engine RPMs.

10. Damaged Fuel Injector

The fuel injector is what injects the fuel into the engine. When the fuel injector is damaged, it no longer injects the right amount of fuel into the engine, which causes the engine to rev on its own.

The fuel injectors get their signals from the computer which tells them when to open or close. If they are damaged, the signal is not getting to the injectors correctly, so you cannot get the required fuel to the engine. This could cause the engine to rev unnecessarily.

A fuel injector can be clogged with carbon deposits. Moreover, a fuel injector can also be stuck open. To test fuel injectors, you can read my guide on car won’t start after replacing the fuel injectors.

11. Bad Fuel Pressure Regulator

A fuel pressure regulator is responsible for maintaining fuel pressure inside a fuel rail so that fuel injectors inject fuel at a certain pressure.

A bad fuel pressure regulator will also cause an engine misfire as it allows fuel to enter the air intake manifold and damage the throttle body. You can read my guide on symptoms of bad fuel pressure regulator to learn more.

If you have a Gasoline Direct Injection engine, it will have a fuel pressure sensor instead of a fuel pressure regulator. This fuel pressure sensor sends signals to the ECU to control the solenoid valve of a camshaft-driven GDI fuel pump.

12. Stuck EGR Valve

EGR system

The EGR valve (exhaust gas recirculation) is a device that allows part of the exhaust gases to recirculate back into the intake manifold to utilize the heat of exhaust gases and to reduce the NOx emissions by re-burning exhaust gases. If the EGR valve becomes stuck, the car will rev up without any input from the driver. 

The EGR valve is controlled by the ECU. EGR valve is only opened when the engine is just started and is at idle so that heat of exhaust gases can be utilized for efficient combustion and to warm up the engine.

As more torque or power is required from the engine, the EGR valve is closed. This is because, when the EGR valve opens, exhaust gases take up the space of fresh air.

So, less oxygen is present in the air-fuel mixture. When high power is required, the EGR valve is closed to ensure as much oxygen enters the cylinder.

If EGR is stuck open, the vehicle’s air-fuel ratio will be disrupted. This will especially happen when you’re trying to accelerate your vehicle, but your engine’s RPM starts fluctuating.

13. Bad Cam Phaser

If your engine has a VVT (Variable Valve Timing) system, it will also have a cam phaser. When you notice your engine revving on its own, it could be a symptom of a faulty camshaft phaser.

The cam phaser is a device that is used to adjust the valve timing at high engine RPMs. The cam phaser is controlled by the ECU. When you start the engine, the ECU will detect the engine speed.

Based on the engine speed, the cam phaser will open or close the valves. If a cam phaser goes bad, it will also cause an engine misfire and cause your car to rev up and down automatically.

A cam phaser can fail due to several reasons. The most common cause of cam phaser malfunction is dirt and debris in the mechanism. Moreover, the locking pin of a cam phaser can be broken.

A damaged timing belt/chain could also be the cause of incorrect valve timings, which will result in the engine misfire and changing of engine’s RPMs without any control.

Car Revving On Its Own: Final Thoughts

In summary, a car that inexplicably revs its engine can be alarming and frustrating. Tracking down the root cause requires methodical troubleshooting, but is key to resolving the issue.

From vacuum leaks and sensor failures to problems with spark delivery, fuel supply, and airflow, there are many potential culprits.

Thorough inspection, testing of components, and process of elimination are needed. With persistence and logic, the source of the erratic revving can be identified and fixed, restoring normal engine function and driver confidence. Though daunting, solving this type of problem is very rewarding.

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