Cars are pretty amazing when you think about it, right? I mean something with that many intricate parts that last for thousands and thousands of miles is absolutely mind-blowing! Almost as if they’re as complex as the human body…Sometimes they even act as if they were alive and have a mind of their own – Like all of a sudden revving while in park or at a stop!
If you’ve ever experienced something like this, rest assured, there’s an explanation…
So what’s happening? Why is my car revving in park? No one wants to pull up to a stop light revving their car like it’s a race!
The most likely explanation is a problem with the air/fuel mixture. With modern fuel-injected cars, this is often due to faulty sensors sending bad data to the computer system. Older cars lack these sensors, but the problem generally still stems from something disrupting the air/fuel mixture.
That’s the quick answer and will get you on the right track to solving this problem. But we’ll take a much closer look to help you figure out what’s causing this and what to do about it.
Understanding Revving Vs. Idling
Before we dive into the explanation, let’s clear up one important distinction: the difference between idling and revving.
Idling is the state where the engine is kept running while the vehicle remains stationary, not moving forward or backward. Essentially, it’s when you have your engine on, but you’re not driving. On the other hand, revving occurs when the engine speed rises, signifying an increase in the rotations per minute (RPM) of the engine.
While both involve the engine being active, idling is a static state, whereas revving indicates a surge in engine activity- not something you want when you’re parked.
Reason 1: Vacuum Leak
A very common reason for a car to rev up and down while in park is a vacuum leak. This is usually the first thing a technician might look for when dealing with an idle that is revving up and down…
So what the heck is a vacuum leak? My car isn’t sucking up dirt! Well, it kind of is and an engine is actually a pump that sucks in air.
It’s almost like an extremely powerful vacuum cleaner!
Remember when we were talking about all the sensors that control this crazy thing of an engine? Well, these sensors measure a bunch of things, for instance, how much air is coming into the engine. When you have a leak in the system that delivers air to the engine (Intake system) that the sensors can’t read it starts to throw everything off…
- Damaged, worn, or kinked hoses
- Faulty gaskets
- Broken intake manifold.
- Faulty brake booster diaphragm
There are many reasons why unintended air can enter the engine. In response, the engine can rev up and down as it tries to find its equilibrium.
Reason 2: Idle Air Control Valve (IACV) Issue
You know how sometimes you need to adjust the volume on your TV to get the sound just right? Well, your car’s engine has something similar, called the Idle Air Control Valve (IACV). The name pretty much says it all and it’s a valve that helps control how much air goes into the engine when you’re not moving (Like when you’re in park or at a stoplight).
Now, here’s where the trouble starts. If the IACV isn’t doing its job correctly, it’s like having a volume knob that’s acting wonky. When it’s supposed to keep the engine running smoothly at idle, it might go a bit haywire, letting in too much or too little air- or with our TV example, turning up or turning down the TV.
- Dirty or clogged valve
- Stuck in a certain position
- Electrical or wiring issue
- Vacuum leaks (Again!)
- Engine sensor issues
When your engine gets confused and starts revving up and down – The idle air control valve might just be the suspect.
Reason 3: Faulty Sensors
Your car relies on a BIG network of sensors to function efficiently. These sensors constantly provide important data to the engine control unit (ECU), AKA pretty much the car’s “brain”. The ECU uses this information to make precise adjustments so the engine can run smoothly under various conditions, including idle.
However, when any of these sensors develop issues, it can throw off the entire engine’s stability, leading to revving problems. Let’s touch on some of these sensors that can cause revving issues. Keep in mind these are just some of the main ones and not the entire picture.
Throttle Position Sensor (TPS)
The TPS informs the ECU about your throttle input. If it malfunctions and sends incorrect data, the ECU may struggle to maintain a stable idle.
Mass Air Flow Sensor (MAF)
The MAF measures incoming air, crucial for the correct air-to-fuel mixture. A malfunctioning MAF or a dirty sensor can lead to incorrect fuel delivery and revving.
Engine Coolant Temperature Sensor (ECT)
The ECT monitors the engine’s temperature which plays a role in the fuel demands. Inaccurate readings can cause the ECU to adjust idle speed incorrectly, resulting in revving issues.
Oxygen Sensor (O2 Sensor)
The O2 sensor measures the amount of oxygen in the exhaust. It feeds that information to the engine control unit, which then adjusts fuel. It pretty much makes sure the combustion is operating correctly and can make adjustments as needed
The engine control unit, or car’s “brain,” can develop internal faults or programming errors causing multiple different issues.
Reason 4: Faulty EGR System
Your car’s EGR system, which stands for Exhaust Gas Recirculation, can sometimes cause your engine to act a bit weird when it comes to revving.
The EGR system’s primary job is to reduce emissions by recirculating a portion of the exhaust gases back into the engine’s intake manifold. This process helps lower the combustion temperature, reducing the formation of harmful nitrogen oxides (NOx).
It pretty much runs a small amount of exhaust gas back into the engine for a round 2…
Now, when the EGR system encounters problems…well you probably guessed it by now, it can make your car rev abnormally. These problems can include:
EGR Valve Sticking:
The EGR valve can sometimes become stuck open or closed. If it’s stuck open, too much exhaust gas recirculation can change the engine’s idle.
Clogged EGR Passages
Over time, carbon buildup can clog the passages leading to and from the EGR valve. When this happens, the EGR system can’t operate properly, leading to erratic idling.
Faulty EGR Solenoid
The EGR system often includes a solenoid that regulates when the EGR valve should open and close. A malfunctioning solenoid can cause issues with idle control.
Reason 5: Sticking Throttle Cable
In the world of modern cars, many have adopted advanced technology, employing a “drive by wire” system where the throttle is controlled electronically based on sensor inputs like the accelerator pedal. However, there are still some cars out there that rely on a mechanical linkage known as a throttle cable.
This trusty cable connects the gas pedal to the throttle body, and its purpose is simple: When you press the gas pedal, it pulls the cable, opening the throttle to let in more air for acceleration. But like any of these many components we’ve been talking about, it can sometimes act up.
In the worst-case scenario, your throttle cable won’t return after pressing the gas pedal. While this would usually happen while driving – If you pressed your gas pedal down while at a standstill and it was binding or sticking it would be stuck revving however far the point of sticking is.
Reason 6: Exhaust Leaks
An exhaust leak is a situation where there are unintended openings, gaps, and cracks in your car’s exhaust system. These leaks can occur in various parts, such as the exhaust manifold, gaskets, pipes, welds, or clamps. When these leaks happen, it can lead to issues with your car’s idling.
Here’s how an exhaust leak can cause revving problems:
False Sensor Readings
In the faulty sensor section we were talking about how the oxygen sensors (O2 sensors) monitor the levels of oxygen in the exhaust gases. This information is crucial for determining the right air-fuel mixture. However, when an exhaust leak occurs before the first O2 sensor, it introduces additional oxygen into the exhaust stream.
This extra oxygen can give false readings to the O2 sensor, which may indicate that the air-fuel mixture is getting too much air. In response, the ECU may try to compensate by increasing the fuel delivery, resulting in the engine revving higher than it should during idle.
Reason 7: Wiring Issues
There are certain wiring issues that can happen on your vehicle that will cause not only idling problems, but other strange things to happen…
Over time, your wiring harness can encounter wear and tear while on the road. Factors like vibrations, heat, and constant flexing can cause wires to fray or suffer insulation damage. When these wires break or lose their protective covering, they can short-circuit or send incorrect signals to the ECU. This may lead to erratic idling.
Weather and road conditions can introduce moisture and salt to your car’s wiring. Corrosion can set in, eating away at the wires and connections. When corrosion occurs, it can interfere with electrical signals and cause idling issues.
Rodents tend to find it cozy in your car’s warm engine bay and think it’s a great place to nest! Unfortunately, they tend to gnaw on wires which can lead to severe electrical damage. Some wire insulation is made of soy, peanut oil, rice husks, and various plant-based materials which attracts them. Rodent-chewed wires can disrupt the communication between sensors, affecting idling performance among other things.
My vehicle’s fluids are just in my engine, right? Theoretically but sometimes coolant and oil can actually find it’s way into the wiring harness. This wicking into the wiring harness usually comes from sensors and runs throughout the entire harness. This not only can cause issues with revving but is followed by all sorts of problems. Notorious on some Mercedes Benz it’s pretty bizarre when you unplug an ECU and dump a little oil out of it.
Each of these wiring problems can cause a disruption in your car’s communication lines, leading to hiccups in its idling behavior. If you suspect wiring issues are the root cause of your car’s revving troubles, it’s worth checking out.
Sometimes It’s Normal!
Wait, you’re telling me about something that actually isn’t messing up on my car! Sheesh! Finally, something that is positive. All this talk about car problems has my head spinning!
So when you first initially start your car before it’s warmed up at all, the car has to warm up quicker so the O2 sensor and everything can work properly. It does this by having the engine idle higher than normal to get to the correct operating temperature.
The O2 sensor even has a heater inside it (That sometimes also fails, sorry) in order to get the entire system where it needs to be. However, once your car reaches this normal operating temperature it should drop down to a normal RPM.
OK, My Car Is Revving In Park – What Should I Do Now?
With such a complex operating system it at least has the reliable check engine light to guide you or the technician working on your car. As stated before sometimes you get lucky and the code the check engine light gives you is straightforward – other times, not so much.
Here is the typical route you or the technician may take to figure out what’s going on here.
Verify The Problem
This one might sound silly but you verify the problem. If the car isn’t acting up and/or there is no check engine light. You can only wait it out until it isn’t intermittent.
Run The Codes
Run the fault codes for the check engine light and see what triggered the light to come on. You may get a code like P0101 indicating a Mass Air Flow issue pointing you to this. Or you may get one that is a little trickier to pinpoint like p0171 Code – Fuel Trim System Too Lean meaning the car is getting too much air making the diagnosis process a little more involved.
Don’t Forget A Visual Inspection
Pop the hood open and start inspecting the intake system. You might get lucky and just see something that is obvious like a broken hose causing a vacuum leak.
Smoke Your Car (Not What You Might Think!)
A technician might have to smoke your car (No this isn’t some kind of slang term for killing your car off). This is when the vehicle has smoke pumped into the intake system to locate exactly where a vacuum leak may be coming from.
Look up Technical Service Bulletins and Recalls
May as well look into these for your car to see if someone else ran into this problem. No sense in giving yourself a headache when other people have run into something similar. Google is a good resource sometimes. Just don’t trust everything you hear.
Your car is such a marvel of engineering. It’s a reliable companion that transports you from place to place with precision and power. It’s sometimes easy to take for granted what is going on underneath it all while on the road. But as you know, it isn’t perfect and can sometimes exhibit quirks that leave you scratching your head, especially when it comes to revving in park.
In this article, we’ve unraveled some of the mysteries behind this peculiar behavior, exploring six common reasons why your car might rev in park or at a stop. Hate to say this too, but these are just scratching the surface of some of the possibilities.
No matter the cause, there’s always a solution. At the end of the day, it’s just nuts & bolts. Whether it’s inspecting and replacing vacuum hoses, addressing sensor issues, lubricating a sticking throttle cable, or sealing an exhaust leak (which can cause other acceleration issues), your car can return to the idling state you expect.
The key is to stay attentive to your vehicle’s behavior, listening and observing for any unusual signs. When you notice revving that seems out of sync with your car’s idle, don’t fret. Instead, consider the insights you’ve gained from this article and take action to diagnose the issue or get some help.
By doing so, you’ll ensure that your car continues to serve as the reliable companion it was designed to be. After all, understanding and maintaining your vehicle’s health is the best way to keep it running smoothly for countless miles and years to come.
Kris’s journey in the automotive world began straight out of the Mercedes Benz training program at Universal Technical Institute in 2010. He has since worked for both Mercedes-Benz dealerships and independent shops as a technician. Along the way, he has honed his skills under the guidance of multiple master technicians and has earned ASE certifications.