Why Is My Car Revving High On Its Own Idling Up and Down?

Does your car’s engine seem to have a mind of its own revving in park or neutral without you touching the accelerator?  If your engine is revving high without you touching the gas pedal or your engine can’t maintain a steady idle engine speed then this is something that needs to be addressed as if left unattended this little problem could grow into a big problem and leave you stranded on the side of the road.  So what are the most likely reasons that your engine is idling rough and revving up and down on its own?

If your engine RPMs are bouncing up and down and your engine can’t maintain a steady idle speed then there is likely a problem with one of the components attached to your engine or a problem with one of the sensors used to keep your engine running.

But lucky for you, most of the problems associated with a rough idle or your car revving up and down erratically can be fixed relatively cheap.  And good news for you, most engine idle problems can be fixed by most DIYers provided that you know how to turn a wrench and can troubleshoot different car components.

If your car has a check engine light this can also help point you in the right direction that way you don’t have to check multiple engine components to make sure that they are operating correctly.

Does Your Car Have A Check Engine Light?

Sometimes having a check engine light can be a blessing because it can drastically cut down on the time spent troubleshooting engine idle or rough idle issues.  If your car currently has a check engine light then I’d start with hooking up an OBDII reader to the car to get a better idea of what could be causing your rough idle issues.  The OBDII reader may point to an issue with a sensor or component which could be easily swapped out to see if it rectifies the issue.

If you currently don’t have a check engine light then you would need to start by troubleshooting each individual component or sensor to see which one could be causing your engine to rev up and down erratically.  If you are in this camp then keep reading.

1. Dirty Mass Air Flow Sensor

One reason that your car’s engine could be idling up and down on its own is a dirty mass air flow sensor. The mass air flow sensor attached to your car’s engine is designed to sense the amount of air coming into the air intake and through the throttle valve of your engine.

That data is then sent to the computer attached to your engine so that it can then calculate how much fuel to send into your engine cylinders to create the proper air/fuel mixture to combust.  Over time the mass air flow sensor or MAF sensor can become dirty or clogged with debris.  This dirt and debris can prevent the MAF sensor from getting a proper reading on how much air is flowing into the engine.

The good news is that most of the time you can just clean your MAF sensor to restore its air flow reading capabilities.  Just make sure that when you are cleaning your MAF sensor you are using a cleaner that is designed to be used on mass air flow sensors.

CRC makes a spray that is designed to be used on mass air flow sensors that won’t leave behind any film that could potentially damage the mass air flow sensor.  I’d recommend looking up how to clean your MAF sensor on your specific make and model of car before attempting to do it yourself.

Below is a video from ChrisFix showing how to clean the MAF sensor on a Mazda Truck.

2. Dirty/Sticking Throttle Body

Another issue that could be causing your car to rev high on its own is a dirty or sticking throttle body valve.  A car’s throttle body valve is designed to open and close based on how much throttle is applied to allow more or less air into the engine.  While cruising the throttle body valve will only open a portion of the way to allow a small amount of air to flow into the engine.  Under full throttle, the throttle body valve will open up all the way to allow max airflow into the engine.

Over time the throttle body valve can become dirty which can allow the throttle body valve to stick or not open/close all the way which can cause your car’s engine to rev erratically and can cause the engine idle to bounce up and down.

Usually, if your cars throttle body is dirty you can clean the throttle body with throttle body cleaner to allow the throttle body valve to move freely again.  If that doesn’t help out your engine idling issues then the throttle body may need to be replaced altogether.

I’d recommend checking out some youtube videos on how to properly clean a throttle body before attempting this yourself as you need to remove the throttle body from your engine intake to properly clean it.  I’ve included a video below showing how to properly clean an air intake.  You should also look up how to clean the air intake for your specific vehicle before attempting this on your car.

3. Bad Oxygen or O2 Sensor

Your car is a complex piece of machinery.  A car’s computer needs to know how much fuel and air are being output from the engine into your exhaust system.  It does this by using an oxygen sensor that is placed in the exhaust manifold or catalytic converter to determine how much air is flowing from the exhaust ports of your engine.  The car’s computer can then make adjustments to the air/fuel ratio to keep the engine running efficiently.

Over time your oxygen (commonly referred to as O2 sensors) can be gummed up with carbon deposits as the burnt air/fuel mixture passes over the O2 sensor.  Eventually, the O2 sensors can become so dirty that they can barely get a reading or they may stop getting an oxygen reading altogether.

The good news is that O2 sensors typically aren’t that expensive to replace and can be had for under a hundred dollars.  Usually, if your O2 sensors are going bad your car will display a check engine light.  You could then use an OBD2 scanner to read the check engine codes to determine if your O2 sensor is at fault.

4. Vacuum Leak

Another reason why your car could be revving on its own is a vacuum leak from one of the various systems connected to your engine.  Your car’s engine can be thought of as a large pump that sucks in air and then compresses it so that it can be combusted to transfer that energy to your wheels.  Vehicle manufacturers over time have learned that there is a vacuum that is created from your engine as sucks in air from the outside world.

Manufacturers could then use the vacuum created by the engine to power various systems under the hood.  Systems powered by vacuum lines include the brakes, EVAP, and EGR.  That’s why you may notice a bunch of vacuum lines running from the intake manifold of your engine.

These vacuum lines or hoses are made of rubber and can become brittle and crack over time.  When this happens it can allow air to escape through the cracked vacuum lines which can cause the air/fuel ratios of your engine to be different from what your mass air flow sensor is reading.

Replacing a vacuum hose is typically inexpensive because it’s just a small piece of rube hose. The bad news with a vacuum leak is that it can sometimes be tough to track down which hose is causing the vacuum leak.  You may have to perform a smoke test or spray carb cleaner onto your vacuum lines to listen for a change in your engine idle speed.

The video below will show you how to perform a DIY smoke test using a cigar.

The video below will show you how to check for a vacuum leak using carburetor cleaner.

5. Plugged/Malfunctioning EGR Valve

Another reason for a rough idle or your car revving up and down on its own can be a plugged EGR valve.  Your EGR valve is connected to your EGR (Exhaust Gas Recirculation) system which was developed to help manufacturers pass ever-increasing emissions standards.

Essentially your EGR system sends exhaust gasses that would have been sent out your exhaust pipe back through your engine.  This prevents some of the toxic gases from being emitted out into the environment.  Over time the exhaust gasses can cause a build-up of carbon which can prevent the EGR valve from opening and closing properly.  When this happens it can cause your engine to idle rough and cause it to rev up and down wildly.

The good news is that replacing your EGR valve on your car typically isn’t that expensive and can be replaced by most amateur mechanics provided that you have easy enough access to the part to replace it.  As always look up a replacement EGR valve for your specific vehicle before purchasing one as they are not all made the same.

Below is a video showing how to replace an EGR valve on a corvette but the process would be similar for most cars.

6. Bad Throttle Body Valve Position Sensor

There are two main types of throttle bodies, one type uses a physical cable to open and close the throttle body valve, this is called a drive-by cable system.  The other type uses sensors to determine the throttle position and then open the close the throttle body valve electronically, this is called a drive-by wire system.

If you bought a car manufactured in the past decade there is a good chance that your car uses an electronic drive-by wire system with a throttle body valve position sensor to determine if the throttle body valve is opened or closed.

Over time the throttle body position sensor can get plugged up with debris or may quit working altogether and needs to be replaced.  Sometimes it’s not always easy to tell that there is a problem with the throttle body position sensor and your throttle body will need to be replaced together.

You may get lucky and be able to remove the throttle body position sensor separately from the throttle body it just depends on how your specific throttle body was made.

7. Frayed or Disconnected Wiring

Sometimes the culprit for your car revving up and down isn’t a sensor or part at all but is actually the wiring harness that feeds data to your car’s computer.  Over time electrical wires can become hard and brittle which can cause them to fray.  This can result in inconsistent behavior or data from your vehicle’s various sensors.

If you are currently having problems with your car revving on its own check the wiring coming from the various sensors in your engine bay for signs of wear.  If you do notice that a wire seems to have signs of cracking or is frayed then it’s best to have the wiring replaced with new wires that are fully insulated.

8. Faulty Car Computer

Sometimes what’s really causing your car to idle rough is a bad engine computer.  Engine computers are commonly referred to as a PCM, ECU, or ECM but all do the same thing which is to interpret the data from all of the various systems of your car to keep your engine running.

If there is a problem with your ECU/PCM then your car’s computer can’t use all of the data being sent to it to keep your car running properly.  Problems with a car’s ECU can cause your engine to increase or decrease the idle speed wildly, your car may even have trouble staying running.

Unfortunately, if you do have problems with your car’s computer there isn’t an easy fix, it will need to be replaced and reflashed usually by a dealer.  This repair as you can imagine is not cheap and will likely come at the cost of hundreds if not into the thousands depending on your specific vehicle and manufacturer.