A car revving on its own is normal, but if it happens frequently, you should do something about it before the situation escalates. It indicates that some parts of the vehicle are malfunctioning and could disrupt the work of the engine.
When this happens, you might experience poor fuel economy, transmission failure, or engine misfire. This complete guide will discuss the reasons why your car will rev on its own and suggest solutions to rectify the problem.
- 1 What Causes Car Revving On Its Own When Driving or Idling?
- 1.1 – A Faulty MAF Sensor Sending False Readings
- 1.2 – A Clogged Engine Air Filter Obstructs Airflow to the Engine
- 1.3 – A Malfunctioning Idle Control Valve Allowing Excess Air
- 1.4 – Leaking Vacuum System in the Engine
- 1.5 – A Faulty Throttle Body Sending Wrong Signals
- 1.6 – Damaged Throttle Position Sensor Giving Wrong Readings
- 1.7 – Low Coolant Levels May Give Wrong Signals to the ECU
- 2 How To Fix Your Car When It’s Revving on Its Own?
- 3 Conclusion
What Causes Car Revving On Its Own When Driving or Idling?
The causes of a car revving on its own when driving or idling include a faulty MAF sensor, damaged engine air filter, and bad idle air control valve (IAVC). Vacuum leaks in the engine, low coolant levels, a bad throttle body, and a faulty throttle body sensor are also factors.
– A Faulty MAF Sensor Sending False Readings
The mass air flow sensor’s job is to measure the amount of air coming through. The engine control unit (ECU) will use that information to regulate the quantity of fuel necessary for combustion. A bad MAF sensor can’t measure the correct amount of air; thus, it’ll send a false reading to the ECU, which, in turn, will send wrong signals to the fuel injectors.
When the injectors receive these wrong signals, they may inject more fuel than the engine requires. When this happens, the engine revs high and low depending on the quantity of fuel; more fuel means higher revs, while low fuel means lower revs.
One way to solve this problem is to clean the MAF sensor, which we’ll discuss later. However, if your vehicle uses a manifold pressure sensor (MAP), you’ll have to replace it.
– A Clogged Engine Air Filter Obstructs Airflow to the Engine
An engine air filter gets rid of the debris coming from the air into the engine, which results in cleaner combustion and a healthier engine. Over time, the car engine air filter becomes clogged as the contaminants gather on its surface, and this obstructs the passage of air.
As a result, the engine doesn’t get the required amount of air to run properly, leading to revving. A clogged engine air filter is quite common; that is why we encourage drivers to change it every 3 years or 300,000 miles.
– A Malfunctioning Idle Control Valve Allowing Excess Air
Also known as the auxiliary air valve, the idle control valve is a device that allows air into the engine when the vehicle is idling. During idling, the butterfly valve of the throttle body (which allows air into the engine) barely opens.
Thus, the auxiliary air valve opens to allow air to pass the butterfly valve into the engine. When the IAVC starts to go bad, it allows more air than needed to enter the intake manifold, causing car revving up and down in park.
When the car revving in park check engine light, it means that there’s a problem with the idle auxiliary air valve. This device regulates the amount of air going into the engine for combustion. Thus, if it malfunctions, it’ll cause the car to rev, which will also illuminate the light.
– Leaking Vacuum System in the Engine
When you experience the car revving without your input while driving, a vacuum hose could be leaking. The leaks occur when the vacuum system is damaged and the engine sucks in air at the wrong time. This amount of air isn’t controlled by the MAF sensor, hence its technical name, unmetered air. Since the MAF sensor doesnt regulate unmetered air, the engine sucks more air than it needs.
When this happens, the ECU won’t know the exact amount of fuel to discharge into the combustion chamber, which leads to a rich air-fuel mixture. This causes a misfiring engine and affects the engine’s revolutions, causing car over revving when changing gears. The sources of vacuum leads include intake gaskets, brake boosters, throttle body gaskets and the air intake hose.
– A Faulty Throttle Body Sending Wrong Signals
When you experience constant engine revving, then a faulty throttle might be the cause. The throttle body valve is a device controlled either mechanically or electronically, depending on the type of vehicle. A mechanically controlled throttle body is linked to the accelerator via a cable. Now, if the cable to the throttle body is too tight, the car will rev on its own, even without stepping on the accelerator.
On the other hand, an electronically controlled throttle body works by sending signals to the ECU, which it uses to regulate the throttle valve. A damaged throttle body may send the wrong signals, forcing the ECU to open the valve wider than usual. This forces more air into the engine and leads to uncontrolled revving as the ECU injects more fuel to match the quantity of air.
– Damaged Throttle Position Sensor Giving Wrong Readings
As the name suggests, a throttle position sensor (TPS) monitors the position of the throttle plate and sends signals to the ECU. The ECU uses the information to monitor the position of the throttle valve to ensure it is in the right place. Simultaneously, the ECU uses signals from the accelerator pedal sensor, MAF sensor and TPS to calculate how wide the valve is opened.
Thus, if the TPS is bad, the ECU might not get the right information it needs to allow the right quantity of air for combustion. When this happens, the ECU might signal the injectors to spray more fuel and this could lead to higher revs. However, if the fuel/air mixture is lower, the revs may follow suit.
– Low Coolant Levels May Give Wrong Signals to the ECU
Low coolant levels can also cause fluctuations in the RPM, making the vehicle rev on its own. When the coolant level is so low that the coolant level sensor is exposed, the sensor won’t be able to read the temperature of the coolant.
Thus, the ECU won’t know the exact temperature of the coolant to adjust the time of the spark plugs and air/fuel mixture. When the ignition timing is off or the air/fuel mixture is wrong, it’ll cause the car to rev automatically.
How To Fix Your Car When It’s Revving on Its Own?
Fixing your car when it’s revving on its own depends on the source of the problem. If your MAF sensor is faulty or your engine air filter is dirty, you can clean them to see if they work. You can also refill the coolant if its level is low.
A dirty TPS will require cleaning and a malfunctioning throttle body may need replacement. You can do some of the repairs and cleaning at home but others will require the services of a mechanic.
– Cleaning the Mass Airflow Sensor With a Toothbrush
First, take out the negative terminal of your battery to avoid any electrical shocks. Next, locate the MAF on the air intake tube between the throttle body and your air filter box. Spritz some MAF sensor cleaner on the sensors you see without touching their wires.
Replace the MAF and reconnect the battery’s negative terminal. Now, turn on the engine and allow it to idle for 15 minutes to help the ECU get accustomed to the new idle speed.
– Cleaning a Dirty Idle Auxiliary Air Valve
Locate the IACV and remove all the connectors and the vacuum hoses attached to it. Unscrew the bolts and take out the IACV. Once out, generously spray the valve with an intake cleaner in and around the valve.
Next, remove the grime inside the valve with a toothbrush and allow it to dry. That’s how to fix over revving car caused by an idle auxiliary air valve.
– Cleaning the Engine Air Filter With Cleaner
Take out the air filter, ensuring that the dirt doesn’t fall into the airbox or intake. Once it is out, spray the air filter with a cleaner and allow it to sit for a few minutes. Next, place the air filter under a faucet and rinse out all the dirt. Dry the air filter in the sun and oil it to prevent rusting and malfunctioning.
– Clean the Throttle Body of Your Car
First, open the hood of your vehicle and locate the throttle body between the air cleaner and the intake manifold. Ensure your engine is off and disconnect the negative terminal of the battery to avoid injuries while cleaning the throttle body.
Next, disconnect all the hoses and coupling around the device and label them so that you won’t confuse them later. Disconnect the air duct and spray some cleaner on the throttle body.
Use an old toothbrush to remove any dirt or grime soaked by the cleaner, and wipe the area clean with a towel. Repeat the process on different parts of the throttle body until it’s clean. Remember to return all the hoses and couplings to their respective places and close the hood.
Many issues can cause car revving up and down while driving or idling; therefore, it is best to discover what the exact problem is and find ways to fix it. Here are other points discussed in this article that you should take note of:
- Some of the causes of a car revving high while driving automatic transmission or manual transmission include a dirty MAF sensor, a blocked engine air filter, or a bad auxiliary air valve.
- Other causes include a faulty throttle body position sensor, low coolant levels, leaking vacuum hoses in the engine, and a faulty throttle body.
- Fixing these problems requires spraying the parts involved with their respective cleaners, removing the grime with a toothbrush, and wiping them clean.
- However, if the parts are damaged, then you’ll need to drive to the mechanic for repairs or replacement.
- When you discover that the coolant levels are low, you might have to bleed out the radiator or top up the coolant levels.
Remember not to overfill the coolant, which could cause problems for your vehicle. If you try all the above but still experience car over revving when going uphill, contact your mechanic.
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