Idle Drops When Brake Is Pressed – What to Do?

Do you get idle drops when brake is pressed in traffic or while waiting at a stoplight? This article will help you diagnose the problem

Idle drop is actually a pretty common issue in most cars and motorbikes, especially when you press the brake. The idle drop makes your automobile stall instead of slowing when the brake is applied. 

In this article, I will discuss the primary reasons why your automobile’s idle drops when the brake is applied. But before that, let’s take a quick look at what is idle and how idle drop affects your car or bike. 

Idle Drops When Brake Is Pressed

What Is Idle?

Idle, which is also referred to as the idle speed, is the rotational speed of the engine of your vehicle when it is not in motion. It usually occurs when the drivers are waiting at a red light or even when you are in a traffic jam. 

Idle is usually measured in RPM (revolutions per minute) of the crankshaft. The crankshaft is what converts the up and down linear motion (reciprocation motion) of the piston into the rotational motion of the wheel. 

When a vehicle is in idle mode, the engine generates only enough power to operate the ancillaries (alternator, water pump, power steering, etc.) smoothly. But not enough power to perform any moving motion for the automobile.

Generally, most cars have an RPM of 800 when they are idle, but in case of an idle drop, the RPM drops to around 300-400 RPM.

Now back to the real question. Many people face the issue of idle drop or RPM drop when they press the brake, which results in static functions of the automobile not working smoothly. There are primarily four reasons for this issue, and pretty much all of them can be fixed.

Idle Drops When Brake Is Pressed

#1 Vacuum Leaks

Faults in the brake booster can cause vacuum leaks. Vacuum leaks cause many other issues, and low idle is one of them. It occurs due to the unmetered air that enters the engine. 

The primary reasons for vacuum leaks are usually a component fastened to a damaged or loosened vacuum hose or gasket, or it can simply be a torn intake manifold gasket.

Finding the leak source is usually a challenging task. You need to use some specific strategies, standard tools, and a vacuum pump to locate the vacuum leaks.

While checking, be attentive to the vacuum hoses and intake manifold gasket. Hoses have the highest chances of becoming loose, deteriorating, or developing tears.

Steps you need to take to locate the leak:

  • Put your ear against the end of a piece of the rubber hose.
  • Now scan gaskets and hoses with the hose’s other end.
  • Then most importantly, search for a hissing sound, the most common sign of a vacuum leak.

If you find the leakage, it’s usually a good idea to replace the master cylinder.

#2 Ignition System 

Burning the mixture of air fuel in the combustion chamber the right way requires a robust and healthy spark. When there’s weakness in spark or there is no spark at all in one cylinder, it can cause a rough or low idle.

The ignition system usually operates the brake lights, and therefore the engine needs a good spark in both cylinders.

Several issues cause weak sparks:

  • Damages or worn spark plug wire
  • Fouled or worn spark plug
  • Damaged or worn distributor rotor or cap (varies according to the vehicle model)
  • Ignition coil issues

If your engine ever has a random misfire, fix it as soon as you can to avoid any further performance issues, which can be even tougher to fix. 

The unburned fuel entering the exhaust system due to the misfire leads to catalytic converter damage.

This condition may require expert help. 

Idle Drops When Brake Is Pressed

#3 Faulty Idle Air Control (IAC) Valve

The idle air control is the primary segment of the vehicle that controls the idle speed on most fuel-injected models. The valve controls the air passing through the air flow sensor or throttle plates. It is mounted around the throttle body.

If you want to check the operation of the idle air control valve, using a scan tool can be helpful. Besides, you can also check the valve with a DMM (digital multimeter).

There are mainly two problems that affect an IAC valve:

  • The buildup of carbon in the bypass port. 
  • Circuitry faults in the Internal motor. 

These two things hinder the IAC motor’s operation or render the IAC motor inoperative.

In most vehicle models, the idle air control valve is pretty straightforward to detach for inspection from the engine.

Some of the steps you will have to follow:

  • First, unplug the idle air control valve.
  • Unscrew the two mounting bolts in the valve.
  • Inspect the valve for a buildup after removing it from the vehicle.
  • Carefully examine the throttle passages.
  • Remove buildup with carburetor cleaner.

Sometimes you may need to troubleshoot the IAC motor using your digital multimeter.

In the newer vehicles (vehicles made after 1996), the OBD-II (on-board diagnostic) system will set a DTC, also called diagnostic trouble code, when there’s an issue found IAC valve.

Idle Drops When Brake Is Pressed

#4 Throttle Position Sensor

Other than the already mentioned factors, faults in the throttle position sensor significantly affect the idle drop.

The throttle position sensor measures the throttle opening rate and sends the subsequent voltage signal to the vehicle computer. But just like all the other parts of a vehicle, a throttle position sensor can also fail after some time.

If the throttle position sensor sends a higher voltage signal than the standard signal when idle, the air control valve will close, predicting the throttle has been opened. 

This situation creates the same effect as a clogged or faulty idle air control valve.

You won’t find fixing the faulty throttle position sensor much challenging. You can simply fix a throttle position sensor using a digital multimeter.

Frequently Asked Questions

Why does my idle drop when I brake?

There are many reasons for your idle dropping when you press the brake. Faults in your vehicle’s components are usually the common problem. I have mentioned the primary reasons below:

  • Vacuum leak due to brake booster.
  • Problems in the ignition system because of brake lights.
  • Faulty idle air control valve.
  • Issues in throttle position sensor.

Idle Drops When Brake Is Pressed

What happens when your brake booster goes bad?

You will find many issues when your brake booster goes bad, which will affect the performance of your vehicle:

  • Applying the brake will get stiff.
  • The braking distance will increase.
  • Hissing noise.
  • Issues in engine function.
  • Power steering will fail.
  • Vacuum leak resulting in idle drop.

Why do your RPMs drop when you stop?

Dropping the RPM in your vehicle is the same thing as the idle drop. Basically, idle is measured as revolutions per minute of the crankshaft. When you stop, your vehicle does not require enough power to move. Usually, the reasons for RPM drops are vacuum leaks and faults in the IAC valve, TPS, and ignition system.

Can a bad brake booster cause a vacuum leak?

If you have power brakes, there is a possibility that the brake booster might have a leak in vacuum because its diaphragm is failing. Once this occurs, you might suddenly find it harder to press the brakes.

Idle Drops When Brake Is Pressed

Idle Drop Can Happen Due To Many Reasons, Its Best To Get Your Car Serviced Frequently To Avoid Them.

We discussed the different reasons why your vehicle’s idle drops when you press the brake. It is entirely dependent on the faults in the different components of your vehicle. Frequently scanning for issues is the best way to prevent an idle drop.

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Sean Mendez

Hi, I am Sean, a self-confessed petrolhead. I live in Boise, Idaho with a busy family of four and our energetic Labrador retriever. Thank you for visiting my website. You can find my email on the contact page.

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