For most vehicles a good idle speed is between 500 to 1000 RPM after the engine has a chance to warm up. A car revving on its own in Park is a serious problem that requires the immediate attention of a mechanic to avoid costly repairs.
If the RPMs are bouncing up and down, this could be an issue with the idle speed or transmission. No matter the culprit, it requires the help of a professional mechanic for a proper diagnosis to avoid the possibility of an accident. It’s a problem that could require a simple fix or extensive repairs.
1. Idle Speed
In most cases, a car revving on its own in Park only requires an adjustment of the idle speed which is a simple fix. The idle control system is responsible for keeping the engine at a constant idle when the car is not in motion. If it’s not working properly the engine will rev on its own without input from the accelerator.
The idle speed could malfunction due to something as simple as a dirty air filter. Or something more serious like a faulty throttle position sensor. A faulty idle speed control system can also wreak havoc on your fuel economy.
A change in idle performance is common as a car gets older. For instance, when I start the engine in my eight-year-old Ford Fiesta, it tends to idle at higher RPMs than my two-year-old Toyota Corolla.
To adjust the idle speed, locate the screw on the throttle body after the engine has had time to warm up. Then disconnect the idle air adjust valve and turn the screw to the right to lower the RPMs.
2. Idle Air Control (IAC) Valve Issues
An idle air control valve is located on the throttle body of fuel-injected engines. It works with the ECU to regulate airflow to the engine for smooth idling. Idling issues in Park can come from a clogged idle air control valve.
It’s an easy part to replace. I did it myself right outside an AutoZone on a Hyundai Elantra my wife and I owned just after we got married. The whole replacement took less than ten minutes.
Since air is constantly flowing in and out of this valve, it’s totally possible for it to become clogged, jammed or damaged internally resulting in an improperly functioning valve.
If the solenoid inside fails or the seals wear down the valve no longer receives an electrical signal. Along with causing a high revving engine. You could also experience an engine that starts, then dies; backfiring due to residual fuel on the spark plug that wasn’t burned off; and issues with just being able to start the motor.
3. Transmission Issues
An engine that revs on its own puts undue stress on components. It’s only a matter of time before effected components need to be changed due to wear and tear. Just like the engine, a transmission requires regular maintenance. If the transmission fluid is dirty, it could cause the engine to rev higher than it should when in Park.
In an engine, the crankshaft produces torque. The torque converter transmits torque from the engine to the drive wheels. This is how you get forward and backward motivation. Fluid pressure is used in the transmission of torque. But if the transmission fluid is dirty, the transmission will slip causing the engine to rev high.
In a car with manual transmission, the clutch plate engages with the flywheel when you lift your foot off the clutch pedal. But if the clutch wears down, the transmission will start to slip causing the engine to rev high in neutral.
4. Faulty Oxygen Sensor
An issue with an oxygen (O2) sensor will likely cause your RPMs to bounce up and down. An O2 sensor helps the electronic control unit (ECU) balance out the air-to-fuel mixture necessary for an internal combustion engine to provide the best performance.
An oxygen sensor measures how much unburned oxygen is in the exhaust. The O2 sensor tells the ECU what this mixture is so that the fuel injection system can be adjusted accordingly.
Over time, an oxygen sensor can become clogged by excessively rich fuel mixtures; or by oil blow-by as an engine gets older; or by engine coolant being burned in the combustion chamber due to a head gasket that needs replacing.
When this happens, it won’t be able to read the fuel-to-air mixture accurately and the ECU won’t be able to adjust engine components accordingly. The result is an engine that bounces all over the rev range, even when in Park.
5. Bad Mass Airflow Sensor
Similarly, a bad mass airflow sensor can cause the same issue. You can find a MAF sensor on the intake side of an exhaust system, the air intake duct or air filter housing. It measures air density as air moves into the engine intake where it mixes with fuel to create combustion.
Whereas an oxygen sensor can be found on the exhaust side of an exhaust system where it detects how much unburned oxygen is present in the exhaust as exhaust gases exit the engine. A dirty mass airflow sensor will cause your engine to idle up and down when the transmission is in Park.
A MAF sensor becomes dirty over time, making it hard for it to read the flow of air accurately. But you can easily fix this by cleaning it off yourself. Make sure that whatever you choose for cleaning is specially formulated for MAF sensors to avoid destroying the sensor.
6. Clogged EGR Valve
A car that revs on its own in Park can also be the fault of a clogged EGR valve. The exhaust gas recirculation valve (EGR) is an integral part of an engine’s management system. It recirculates finely metered quantities of exhaust gas to the engine intake system to increase engine efficiency, reduce fuel consumption and lower NOx emissions.
The science behind this is interesting. 80% of the air we breathe is nitrogen. But when exposed to extreme temperatures, like the type of high temperatures that an engine experiences. This inert gas becomes reactive creating a harmful oxide of nitrogen – NOx. It could escape into the atmosphere causing air pollution.
By allowing precise quantities of the exhaust back into the intake, the EGR changes the chemical make up of these otherwise harmful gases actually using them to lower temperatures in the combustion chamber. And reducing the production of NOx for a cleaner exhaust emission.
7. ECU Issues
Also known as an electronic control module (ECM), or engine management system. The ECU (electronic control unit) in an engine controls the operations of your engine. There are several ECUs throughout a car all responsible for controlling different functions of a car.
But the ECU in an engine is the one responsible for controlling the amount of fuel that enters a cylinder. So far, you’ve just learned about at least two sensors that talk with the ECU so that it can do its job properly – the MAF sensor and oxygen sensor.
But if a malfunctioning ECU can’t read the information from these sensors, it could be the reason why your car is revving on its own in Park.
There are many reasons why an ECU fails: water damage from heavy rain; corrupt software which happens when the ECU is interrupted during programming or coding; or if the ECU is flashed with incorrect firmware. Failed injector or ignition coils can short out an ECU.
8. Exhaust Leak
A car is home to a series of interrelated systems all working toward the common goal of getting you where you need to be. If one component isn’t working the way it should, it usually sets off a chain reaction that eventually corrupts other parts.
Which is why the best advice is to get your car checked out by a professional mechanic right away any time something goes wrong with your car and you start experiencing issues.
A leak in an exhaust manifold can allow air to enter the engine at the wrong time. Remember, it’s the ECU’s job to decide the best time to allow air into the engine for the right fuel to air mix. But if air goes into the engine at the wrong time, it can cause a lean air to fuel mixture, which can cause a variety of issues including high RPMs in Park.
9. Throttle Body Issues
A problem with the throttle body or an electronic throttle control system is a serious issue because it will need to be repaired or replaced. Similar to an ECU, a throttle body regulates the amount of air coming into an engine. It uses a flat valve that rotates in a tubular housing to adjust the amount of air coming into an engine.
Fuel injected engines have a throttle position sensor and air-flow sensor that communicates with the ECU which is responsible for supplying the correct amount of fuel to the injectors. The throttle body is located between the air filter and the intake manifold. It’s also connected to the accelerator either mechanically by a cable or electronically by wire.
When your foot presses the accelerator, the butterfly opens allowing more air into the intake manifold. Filth, carbon and dirt can build up on the housing of a throttle body. This creates problems with air flow which could be the reason why your car is revving high in Park. This buildup of gunk creates an imbalance that interrupts the air and fuel mixture which can cause the butterfly to become stuck.
10. Vacuum Leak
Vacuum leaks can cause an engine to rev high in Park. An engine is like a big air pump. If there’s a breach, it can’t work efficiently.
A vacuum leak is anything behind the MAF sensor that allows more air than the ECU regulates to enter the engine. Like an exhaust leak, this causes the engine to run lean, resulting in a high revving motor. An engine is an enclosed system. So, if you hear a hissing noise. You know you have a vacuum leak. A vacuum leak can cause your car to fail a state inspection because the engine runs hotter than normal.
You might even experience a loss of power since the air-to-fuel ratio is unstable. A vacuum leak can happen as a result of just about anything. A broken gasket, loose hose connection and cracked engine block are just some of the common causes of vacuum leaks.
11. Faulty Ignition System
An engine’s ignition system is what helps the car create the combustion necessary for locomotion. Today, there are many types of ignition systems depending on the car in question. The purpose of an ignition system is to generate a very high voltage from a car’s battery. Once created, this high voltage is sent to each spark plug igniting the fuel and air mixture in the combustion chamber which is necessary for a car to move.
As such, spark plugs are a crucial part of that ignition system. If the spark plugs become less effective at doing their job due to wear and tear that comes from combustion, contamination, overheating or other electrical ignition system problems. To compensate, this will result in issues like the engine revving on its own in Park. The leading cause of spark plug failure is engine oil leaking onto the spark plugs. The primary culprit is usually leaky O-rings.
12. Frayed Wiring
Engine wiring harnesses are a combination of electrical cables or an assembly of wires connecting all the electrical components in a car like sensors, ECUs, the battery and actuators.
If any of them should become frayed or disconnected, it can cause issues such as your car revving higher than normal when in Park. You want to be especially careful with this one because faulty wiring can lead to serious issues like fires.
Typically, wiring harnesses last 15 to 20 years and even longer. Some of the reasons why they go bad prematurely include manufacturer defects, corrosion, vibration, physical damage, heat, thermal degradation and damage cause by animals that crawl in and around your car.