Car Accelerates When Braking: 5 Causes and Safety Tips

Published by Marlo Strydom

You push down on the brake pedal expecting your car to slow down, but instead the engine revs up and the vehicle speeds forward. This terrifying situation, called unintended acceleration, happens when the throttle plate stays open while you are trying to stop. Your foot is pressing the brake pedal, but the engine is still sending power to the drive wheels through the transmission and drivetrain components.

Understanding what causes this problem and knowing how to respond can prevent accidents and save lives. The issue usually involves either mechanical failure (stuck throttle cable, jammed accelerator pedal assembly), electronic problems (faulty cruise control module, engine control module errors), or human factors (pedal misapplication by pressing the wrong pedal by mistake). The brake system and throttle system are designed to work independently, but when the throttle malfunctions while you apply the brakes, the hydraulic brake system must fight against engine torque.

5 Main Causes of Unintended Acceleration

1. Stuck Accelerator Pedal

The accelerator pedal controls how much power your engine produces. When you press it down, it either pulls a throttle cable through the cable housing (older vehicles with mechanical throttle linkage) or sends an electronic signal from the accelerator pedal position sensor to the throttle body (newer cars with drive-by-wire systems). Both systems open the throttle plate or throttle butterfly valve, which lets more air into the intake manifold and engine. The engine control module sees this extra air through the mass airflow sensor and injects more fuel through the fuel injectors to match, increasing engine speed and power output to the crankshaft.

When the accelerator pedal gets stuck in a pressed position, the throttle plate stays open even when your foot moves to the brake pedal. The most common cause is a floor mat that slides forward and jams under the gas pedal arm, preventing it from returning to the closed idle position. This can also happen if the pedal pivot point or pedal bracket gets sticky from dirt and grime, or if the throttle return spring breaks or weakens over time.

In cable-operated systems, the throttle cable itself can bind inside its cable housing due to internal rust, kinks in the cable, or fraying steel strands. In electronic throttle systems (drive-by-wire), the accelerator pedal position sensor might stick or send faulty voltage signals, telling the engine control module you are still pressing the pedal when you are not. Some systems use dual accelerator pedal position sensors for redundancy and safety verification.

When the throttle plate stays open, your engine keeps producing power and sending it through the transmission and driveshaft to the drive wheels. Your hydraulic brake system has to fight against this engine torque, it is like trying to stop while someone else is pressing the gas. The brake calipers and brake pads can usually overpower the engine, but it takes much more stopping distance, and in some cases the brake rotors and brake pads can overheat from the excessive friction heat and lose effectiveness (brake fade or brake glazing).

2. Pedal Confusion

Pedal misapplication, accidentally pressing the accelerator pedal when you mean to press the brake pedal, is more common than people think, especially in stressful situations like parking maneuvers or sudden traffic stops. Your brain thinks you are pressing the brake, but your foot is actually on the gas pedal. When you press harder (because the car is not slowing down), you are actually increasing the throttle plate opening angle and making the problem worse.

This happens most often when drivers switch between vehicles with different pedal layouts and pedal spacing. For example, if you normally drive a manual transmission car with three pedals (clutch pedal, brake pedal, gas pedal), your muscle memory is trained to that specific pedal arrangement. When you switch to an automatic transmission with only two pedals (brake and accelerator), your foot might land on the accelerator when reaching for where the brake used to be in your regular vehicle.

The pedal spacing and pedal height varies between vehicles too. In some cars, the brake pedal and accelerator pedal are farther apart; in others they are closer together. If you wear thick soled shoes or boots, it is even easier to accidentally press both pedals at once or hit the wrong pedal entirely.

Emergency situations increase the risk because stress and adrenaline affect your fine motor control and reaction time. When panicked, drivers often press whichever pedal their foot is near without verifying it is the correct one. They feel the car accelerating and press harder, convinced they are on the brake pedal.

To prevent pedal misapplication, always look down to verify which pedal your foot is on before pressing in an unfamiliar vehicle. Adjust your driver seat position and seat height so your right foot rests naturally in front of the brake pedal, not between both pedals. And wear appropriate driving footwear, flip flops, high heels, and thick boots all increase the risk of hitting the wrong pedal.

3. Vacuum System Problems

Your brake booster uses engine vacuum to multiply the force you apply to the brake pedal. The brake booster is a large canister (usually black, about the size of a dinner plate) mounted on the firewall between your brake pedal linkage and the brake master cylinder. Inside the booster, there is a rubber diaphragm that uses the vacuum from your intake manifold to provide power assist or vacuum assist, this is why your brakes feel light and easy to press compared to non power assisted brakes.

The brake booster connects to the engine intake manifold through a large vacuum hose and vacuum check valve. When this vacuum hose develops a crack, split, or comes loose at the connection points or hose clamps, it creates a vacuum leak. This vacuum leak has two effects: First, it reduces the vacuum available to the brake booster diaphragm, making your brakes feel harder to press and less effective at stopping. Second, the leak lets unmetered air into the intake manifold, which can cause the engine idle speed to rev higher and produce more power.

Here is what happens: the vacuum leak pulls extra air into the engine combustion chambers without passing through the throttle body or mass airflow sensor. The engine control module does not account for this extra unmetered air in its fuel injection calculations. The engine may run lean (not enough fuel) or the computer may try to compensate by adding more fuel through the fuel injectors. Either way, idle speed increases and the engine produces more power than it should at normal idle.

When you press the brake pedal with a vacuum leak present, you are fighting two problems: weak brakes due to low vacuum assist in the brake booster, and an engine that is revving higher than normal idle RPM. The combination can feel like the car is accelerating when you brake, even though it is actually just not slowing down as much as it should.

A certified mechanic can diagnose vacuum leaks by listening for a hissing sound from the engine bay, checking intake manifold vacuum with a vacuum gauge, or using smoke testing equipment to find the exact location of the leak. Replacing a cracked brake booster vacuum hose is simple and cheap, the replacement hose costs $10 to $30 and takes minutes to swap out.

4. Cruise Control Malfunction

Cruise control maintains a set speed by automatically controlling the throttle position. When you set cruise at 65 mph, the cruise control module monitors your actual vehicle speed through the vehicle speed sensor or wheel speed sensors. If the car slows down (like when going uphill), the cruise control module opens the throttle plate to add more engine power. If the car speeds up (going downhill), it reduces throttle opening.

Normally, cruise control disengages immediately when you tap the brake pedal. There is a brake pedal position switch or brake light switch that tells the cruise control module the driver is braking, cancel cruise now. But if this switch fails or the electrical wiring develops a problem, the cruise control system does not know you are braking. It keeps trying to maintain the set speed, fighting against your hydraulic brakes.

In older cable operated cruise control systems, the cruise control actuator physically pulls the throttle cable. If this actuator sticks or the throttle cable jams in the cable housing, the throttle plate stays open even after you press the brake pedal. The actuator might have a stuck servo motor, seized throttle cable, or vacuum diaphragm failure inside the cruise actuator.

In modern electronic cruise control systems (often integrated with adaptive cruise control or radar assisted systems), the cruise control module communicates directly with the engine control module through the vehicle CAN bus network or data bus. If the cruise module malfunctions, it might send continuous accelerate commands or throttle commands even when cruise should be off or deactivated.

A diagnostic scan tool can read cruise control system diagnostic trouble codes and monitor the brake pedal switch signal in real time. A mechanic can test if the brake light switch is sending the proper voltage signal when you press the pedal. If the cruise control actuator or cruise control module has failed, it needs replacement, costs range from $100 to $400 depending on the system type and vehicle make.

5. Engine Control Module Issues

The engine control module (ECM) or powertrain control module (PCM) is your car main computer brain that manages the entire powertrain system. It reads dozens of sensors and controls fuel injection, ignition timing, throttle position, and transmission shift behavior. In modern cars with electronic throttle control (drive-by-wire systems), the ECM directly controls how far the throttle plate opens in response to your accelerator pedal input through the throttle body actuator motor.

When the ECM malfunctions, it can send wrong commands to the electronic throttle body. For example, corrupted software code, bad memory chips, or electrical problems inside the ECM circuit board might cause it to command the throttle open even when your foot is on the brake pedal. The ECM might misread the accelerator pedal position sensor voltage and think you are pressing the gas when you are not.

Another scenario is when the ECM fails to recognize brake pedal input. Modern cars have a brake pedal position sensor or brake light switch that tells the ECM the driver is braking. The ECM uses this electrical signal to cut engine power, disengage cruise control, and prepare the transmission for stopping. If the ECM software glitches or the brake sensor circuit fails, the computer does not know you are braking and keeps sending power to the drive wheels.

ECM problems can also cause the throttle to stick open electronically. In drive-by-wire systems, there is no physical cable connection between your gas pedal and the throttle body, it is all controlled by the ECM. If the ECM throttle control software crashes or gets stuck, the throttle plate might stay at whatever opening position it was in, ignoring new commands from the accelerator pedal or brake pedal.

Diagnosing ECM issues requires professional diagnostic scan tools that can read live sensor data, monitor throttle position commands, and check for stored diagnostic trouble codes. The automotive technician can see if the ECM is receiving correct voltage signals from the accelerator pedal sensors and brake light switch, and whether it is sending proper throttle commands to the electronic throttle body actuator. ECM repair or ECM replacement is expensive ($500 to $2000), and the new ECM unit usually needs to be programmed with your vehicle VIN and security codes.

Warning Signs and Symptoms

Recognizing these symptoms early can prevent dangerous situations. If you experience any of these signs, avoid driving until the problem is professionally diagnosed and repaired.

Increase in RPM

When unintended acceleration happens, your tachometer needle will climb even though your foot is on the brake pedal. RPM (revolutions per minute) measures how fast the crankshaft is spinning. When the throttle plate stays open, more air and fuel enter the combustion chambers. Each cylinder fires more powerfully, spinning the crankshaft faster and increasing the RPM reading on your instrument cluster dashboard.

You will notice the engine sound gets louder and higher pitched as RPM climbs. The normal idle range is around 700 to 800 RPM, but with a stuck throttle or cruise control malfunction, RPM might jump to 1500, 2000, or even higher. The engine is producing more horsepower and torque, which flows through the transmission input shaft and transmission gears to the drive wheels.

Your hydraulic brakes have to fight against this increased engine power. Even though modern disc brakes are strong enough to overpower the engine, stopping distance increases dramatically. The brake pads are creating friction heat to slow the wheels while the engine is pushing to speed them up. This battle generates excessive heat in the brake system, the brake rotors and brake pads can get hot enough to cause brake fade, where braking power temporarily decreases due to friction material degradation.

If you have an automatic transmission, the torque converter is still connected and transferring engine power to the transmission input shaft even with your foot on the brake pedal. In manual transmissions, the clutch disc stays engaged if you do not press the clutch pedal, so engine power keeps flowing to the wheels through the flywheel and transmission gears. This is why shifting to neutral is so important, it physically disconnects the engine crankshaft from the drive wheels, letting the brakes work without fighting the engine torque.

Brakes Not Responding As Expected

When your brakes feel weak or take longer than normal to stop the vehicle, it might seem like the car is accelerating when you are actually just not slowing down as fast as expected. Your hydraulic brake system works by converting the force from your foot on the brake pedal into hydraulic pressure. The brake master cylinder pumps brake fluid through steel brake lines to the brake calipers (disc brakes) or wheel cylinders (drum brakes), which press the brake pads or brake shoes against the brake rotors or brake drums to create friction.

Worn brake pads reduce stopping power because there is less friction material left to grab the rotor surface. When pads wear down to the metal backing plate, you get metal on metal contact that is much less effective at stopping. Low brake fluid level (usually caused by brake fluid leaks or worn brake pads) means less hydraulic pressure reaches the brake calipers, making the pedal feel spongy and reducing braking force.

A failing brake caliper can stick in the partially open position, preventing the caliper piston and brake pads from clamping firmly on the rotor. The brake master cylinder might have internal seals leaks that prevent it from building proper hydraulic pressure in the brake lines. If the brake booster (which uses engine vacuum to multiply your pedal force) fails, braking requires much more physical effort and feels weak.

When weak brakes combine with unintended acceleration, like a stuck throttle or active cruise control, the situation becomes dangerous. The engine is producing power while the compromised brake system struggles to overcome it. This is especially noticeable when the brake fluid is old and absorbed moisture, which lowers its boiling point and can cause brake fade under heavy braking use.

Unusual Sounds or Vibrations

Strange noises during braking often signal worn or damaged brake components. Squealing or squeaking sounds usually mean the brake pad wear indicators are touching the rotor surface, these are small metal tabs designed to make noise when brake pads get thin. Grinding or scraping sounds indicate the friction material is completely gone and the metal backing plate is scraping against the rotor, which damages both the rotor and backing plate.

Brake calipers need lubrication on their caliper slide pins to move smoothly. When these slide pins dry out or corrode, the caliper sticks and causes uneven brake pad wear, pulling, and sometimes clicking or clunking sounds when braking. In drum brake systems, worn return springs or rusty brake shoes can cause rattling or squeaking noises.

Pulsating or vibration in the brake pedal usually means warped brake rotors. Brake rotors can warp from excessive friction heat (caused by hard braking or dragging brakes). When the rotor surface is not perfectly flat, the brake pads make inconsistent contact, creating a pulsing feeling through the brake pedal and steering wheel.

Air in the brake lines compresses when you press the pedal, making the brake pedal feel spongy and reducing hydraulic pressure at the brake calipers. This happens when brake fluid leaks let air into the hydraulic system, or after brake repair work if the brake system was not properly bled. Old rubber brake hoses can also deteriorate internally, allowing air bubbles to form or collapsing under hydraulic pressure.

Car Lurching Forward Unexpectedly

A lurching or surging feeling when you brake means the vehicle is fighting your brake inputs. This jerky motion happens when engine power pulses while you are trying to slow down, creating a tug of war between the brakes and the powertrain.

In automatic transmissions, a bad torque converter can cause lurching. The torque converter uses automatic transmission fluid (ATF) to transfer power from the engine crankshaft to the transmission input shaft. If the torque converter lock up clutch malfunctions, it can grab and release repeatedly, creating a shuddering or lurching sensation. A failing shift solenoid might also cause the transmission to hunt between gears, unable to decide which gear to be in.

A dirty transmission filter restricts ATF flow, which affects shift quality and hydraulic pressure. Low transmission fluid level from transmission leaks has the same effect, the transmission cannot build proper hydraulic pressure to hold gears smoothly.

Severely warped brake rotors create a strong pulsation that can feel like lurching, especially if one rotor is warped worse than the others. The uneven braking force side to side makes the vehicle pull and jerk. Misaligned wheels also cause pulling during braking because the suspension geometry is off, making the tires fight each other.

If the brake master cylinder has internal leaks or worn master cylinder seals, hydraulic pressure can fluctuate as you hold the pedal down. You will feel the pedal sink or pump up and down, and braking force varies, creating a lurching sensation as the brakes grab and release.

Serious Safety Risks

The primary dangers include colliding with other vehicles, pedestrians, or fixed objects like buildings and barriers. You can also lose vehicle control entirely if brakes overheat or if panic causes you to make sudden steering inputs. Understanding proper emergency response - especially shifting to neutral - is critical for all drivers.

Increased Risk of Accidents

Unintended acceleration puts you in an emergency situation where split second decisions matter. When your car does not respond the way you expect, your brain has to process what is happening, realize the car is doing the opposite of what you want, and then remember the correct emergency procedure, all while the vehicle is speeding up instead of slowing down.

The danger is highest in confined spaces. In parking lots, you are surrounded by other cars, shopping carts, pedestrians walking to their vehicles, and concrete barriers. On residential streets, there might be kids playing, people walking dogs, or cars backing out of driveways. When your car suddenly accelerates in these situations, there is very little room to maneuver and almost no time to avoid a collision.

Many drivers instinctively stomp harder on what they believe is the brake pedal. If they are actually on the accelerator (pedal misapplication), this makes the vehicle lunge forward violently with maximum engine power. In drive-by-wire vehicles where the electronic throttle responds instantly to pedal input, the car can reach dangerous speeds in seconds.

The key to preventing accidents during unintended acceleration is knowing the emergency procedure beforehand. Practice shifting to neutral in your driveway so the motion becomes automatic. Know where your ignition switch is and how to turn it off without locking the steering wheel. These skills need to be muscle memory because you will not have time to think during a real emergency.

Damage to the Car's Engine and Transmission

When the engine fights the brakes, several components take a beating. The brakes themselves overheat from constant friction while trying to overcome engine power. Brake rotors can warp from excessive friction heat, brake fluid can boil (which creates air bubbles and reduces hydraulic pressure), and brake pads can glaze over, developing a hard smooth surface that reduces friction instead of increasing it.

The transmission endures stress too, especially in automatic transmissions. The torque converter is designed to slip a little during normal operation, but when you are holding the brake while the engine revs, the torque converter is trying to transfer high power while the transmission output shaft (connected to the wheels through the transmission gears) is being held stationary. This creates tremendous heat in the automatic transmission fluid. If this continues for too long, the ATF breaks down, losing its lubricating properties and ability to transfer hydraulic pressure.

In manual transmissions, holding the clutch pedal down while the engine revs can wear the throw out bearing or release bearing. If the clutch stays engaged (foot off the clutch) while braking, the clutch disc gets excessive friction heat from slipping between the flywheel and pressure plate.

The drivetrain components, CV axles, driveshaft, differential, and wheel bearings, all experience shock loading from the contradictory forces. The wheels want to rotate (engine power pushing through the transmission) but are being held by the brakes. This creates twisting forces in the axle shafts and can damage universal joints or constant velocity joints.

Engine wise, sustained high RPM without a load (like when in neutral) is not ideal but will not cause immediate damage. The bigger risk is if the throttle sticks wide open and you keep driving, the engine can over rev beyond the red line, which can cause valve float, connecting rod damage, or in extreme cases, throw a connecting rod through the engine block.

Prevention and Safety Measures

Regular Maintenance of the Car

Regular car maintenance is essential to ensure that it runs smoothly and safely. By regularly inspecting and servicing the brake system, drivers can help prevent unintended acceleration when braking.

This includes checking for wear and tear on the brake pads, brake rotors, brake calipers, and other brake components. Additionally, brake fluid should be checked to ensure it is still at optimal levels in the brake master cylinder reservoir.

Drivers must know their vehicle braking system performance to reduce the risk of excessive wear and tear on transmission parts resulting from accelerating while braking. Any signs of sudden or abnormal jerking or vibration should be addressed immediately by a certified automotive mechanic.

Drivers should also be mindful of their driving habits and not over applying the brakes. This can lead to increased wear on brake components and fuel consumption due to the added acceleration from harder braking.

It is also important for drivers to ensure they are using the correct brake fluid specification which provides adequate hydraulic properties and protection against breakdowns in extreme temperatures or terrain conditions.

For optimal safety, drivers should keep up with regular inspection and maintenance schedules for their cars as the vehicle manufacturer recommends to keep their vehicles running efficiently and reliably for years to come.

Keeping an Eye Out for Symptoms

Keeping an eye out for symptoms of a car accelerating when braking is essential for drivers. It is important to be aware of sudden or abnormal jerking or vibrations when the brake pedal is pressed.

Additionally, drivers should pay attention to changes in sound, such as grinding noises, squealing noises, and other indicators of faulty brakes or inadequate brake caliper lubrication.

Drivers should also watch for excessive wear on the brake pads, brake rotors, brake calipers, and other brake components. By regularly inspecting these parts and ensuring they are in good condition, drivers can reduce the risk of brake system failure resulting from unintended acceleration when braking.

Additionally, they can reduce wear on transmission parts due to harder braking and fuel consumption from added acceleration due to more aggressive braking.

Finally, it is important to ensure that the correct brake fluid specification provides adequate hydraulic properties and protection against breakdowns in extreme temperatures or terrain conditions. This will help reduce the risk of unintended acceleration when braking and keep cars running safely and efficiently.

Avoiding regular sudden braking

Avoiding sudden braking is an essential part of driving safely and efficiently. When a driver brakes suddenly or slams on the brakes, it can cause excessive wear on the brake system due to increased heat and stress.

This can be dangerous as it can increase the risk of colliding with other cars or objects around them.

Drivers should ensure their vehicles function properly by conducting regular maintenance and inspection schedules to avoid sudden emergency braking situations.

This will help keep the brake system in good condition and reduce the likelihood of brake system failure resulting from unintended acceleration when braking.

In addition, drivers should practice proper defensive driving techniques such as maintaining a safe distance between themselves and other vehicles, anticipating potential hazards on the road ahead, and paying attention to their speed and how much time they need to stop at intersections and turns.

Furthermore, drivers should always stay aware of their surroundings by using their side mirrors and rearview mirror regularly and checking blind spots before changing lanes.

The more prepared a driver is, the less likely they are to make sudden maneuvers or engage in risky behavior, which can lead to unnecessary stress on the brake system.

Finally, drivers should use gradual braking techniques when possible to minimize wear when coming to a stop. Gradual braking will also help reduce wear on transmission parts and fuel consumption due to harder braking.

Pulling Over and Turning Off the Engine if the Problem Persists

If a driver experiences persistent unintended acceleration when braking, they should immediately pull over and turn off the engine.

This will help reduce the risk of further acceleration and potentially hazardous situations. Additionally, it will give the driver time to inspect their vehicle for any underlying issues that might be causing the problem.

The driver should check for brake fluid leaks from the brake lines, worn brake pads, loose or disconnected brake lines, or other problems that could lead to sudden acceleration when braking.

If any of these are detected, the driver should take their car to a qualified automotive mechanic as soon as possible for professional repair.

Moreover, drivers should inspect their tires for excessive tire tread wear, which can lead to traction problems and poor braking performance. Worn tires can also cause the car to slip and lose control when coming to a stop.

As such, drivers must maintain their tires regularly and replace them with quality replacement tires.

Taking proper precautions when driving can help reduce the risk of sudden unintended acceleration when braking.

By maintaining regular vehicle maintenance and practicing safe driving habits, drivers can ensure that they experience smooth braking without any unexpected acceleration or other dangerous situations.

Conclusion

Unintended acceleration when braking is a serious safety issue requiring immediate emergency response and professional automotive repair. While relatively rare, understanding the causes and proper emergency response procedures can prevent accidents and save lives.

The key is knowing how to react: shift to neutral position, apply brake pedal firmly, steer safely, and turn off the engine ignition. Regular vehicle maintenance and awareness of common automotive problems help prevent these dangerous situations.

Never ignore signs of unintended acceleration. If your car has ever accelerated unexpectedly, have it inspected immediately by a qualified automotive technician before driving again.