The car stalls when braking. What are the causes?

There are several reasons why the engine may stall when the brake pedal is pressed. Besides the inconvenience it makes, this issue can indicate a problem in the braking system which you need to consider. So, it’s best not to drive the car in this state until you fix the problem.

Engine stalling when braking was more common on cars that had a carburetor, but this problem may also be present on injection engines. To know what causes the car to stall, we will first go through the 5 main reasons for this problem. After that, you will be able to identify the true cause of the failure only after a detailed inspection.

What are the causes of engine stalling when braking?

  • A bad or failing vacuum brake booster;
  • Vacuum brake booster hose depressurization;
  • Malfunctions of the idle speed sensor;
  • Incorrect operation of the engine electronic control unit (ECU);
  • Bad or failing fuel pump.

A bad vacuum brake booster

The vacuum brake booster is located between the pedal and the main brake cylinder. Its role is to relieve the force that the driver creates when pressing the brake pedal. The vacuum brake booster is connected to the intake manifold through a vacuum hose. In its design, the vacuum brake booster includes besides other elements a membrane. If this membrane is damaged, then this is one of the reasons why the car stalls when braking.

If it’s damaged, the membrane will not have time to create the vacuum, and because of that, part of the air in the brake system enters the fuel mixture. This causes the engine to stall when pressing the brake pedal.

If the vacuum amplifier fails, we recommend you to change it, although it can be repaired, it is not advisable.

How to check the vacuum brake booster?

Fortunately, there are two easy ways to check if the vacuum brake booster hose is damaged.

First method

  • First, stop the engine;
  • Press and release the brake pedal several times. At first, the pedal stroke will be softer but after pressing several times, it will become harder.
  • Keep the pedal in the lower position;
  • Start the engine;
  • If after starting the engine, the brake pedal stroke becomes softer again, then there is no problem with the vacuum hose. But if it still remains still, then you need a detailed check.

Second method

  • After the engine has been running for some time, press the brake pedal and then stop the car
  • Keep the brake pedal pressed for about 20-30 seconds;
  • If during this time the pedal does not resist the foot, then everything is functioning properly with the vacuum brake booster.

Bad or worn vacuum brake booster hose

Another cause of why the car may stall when braking is a depressurization of the hose that connects the vacuum brake booster and the intake manifold. If this hose is damaged, then extra air will enter, leading to an incorrect air-fuel mixture that causes the engine to stall while pressing sharply on the brake pedal.

You can check this hose by detaching it from the vacuum booster. Next, start the engine and pinch the hole of the removed hose with your finger. If it is sealed, the engine will automatically increase RPMs, and after removing the finger, it will lower them. 

If the hose passes air through, the engine will run at constant speed while performing the above method.

vacuum brake booster and vacuum brake booster hose
the position depends on the make and model

Check also the valve that is installed at the end of the hose where it connects to the amplifier. It is important that through the valve air is not leaked, otherwise, it will have the above symptoms.

The easiest way to detect a malfunction of the vacuum cleaner is by touching. If it passes air, then the brake pedal will work very hard, and to press it you need more effort to press it.

The malfunction of the vacuum brake booster hose can also create a safety problem because pressing the brake pedal will require more effort, and will increase the stopping distance. Thus it’s best to resolve this issue fast.

A bad fuel pump or fuel filter

When braking the car’s Electronic Control Unit will give the command to not supply the engine with fuel. If a fuel pump is bad and has problems with the supply or the fuel filter clogged then the gas pump does not have enough power to supply or doesn’t supply the required amount of fuel to the engine, which causes loss of traction.

You can check the pressure in the fuel line using a pressure gauge. You will find the pressure values in the car’s manual.

If you however suspect that the fuel filter is bad, you will have to change it to be sure.

Another less common reason why the car stalls when you brake is bad high-voltage wires. You should also check the quality of the negative contact wire from the battery, if the battery has poor contact, there is a good chance that the engine stalls when you brake suddenly.

A failing idle speed sensor

The role of the idle speed sensor is to put the engine to idle and maintain its speed. If this sensor fails then the engine speed will begin to float and the engine will eventually stall. You should see the floating engine RPM at idle when you push the accelerator pedal.

Checking the sensor can be done with a good multimeter tester to check the resistance of the internal windings of the sensor. Depending on the design, you need to connect to different contacts, look into the car manual to see if you find the correct values to see if those matches yours.

Unfortunately, if the idle speed sensor is faulty you need to replace it with another sensor

A failing Electronic Control Unit (ECU)

Errors in the operation of the ECU are also a cause for this car stalling problem, this can be hard to find so it’s best to check the car in an authorized car workshop with a computer diagnosis.

Engine with LPG stalls when braking

You have a car with an older model of LPG gas without its own ECU then you may have an incorrectly configured gearbox. This happens at high speeds when you suddenly press the brake pedal, the throttle is closed, the flow of oncoming air depletes the mixture.

As a result, the vacuum mechanism of the gas reducer delivers a small dose of gas at idle, and the oncoming air flow impoverishes it even more.

Fixing this issue stands in reconfiguring the gearbox to run at idle so that the system delivers more gas.

Other reasons for cars with LPG for the problem above, represents a clogged filter on the electric valve. Although not all LPG systems have one of these filters, to fix the problem, you need to clean the filter or replace it. Otherwise, the counter-flow of air can also make the mixture poor.

Conclusion

Car stalling when braking can be very uncomfortable. The most common reason that causes this problem is the vacuum brake booster and its hose. So check these two parts. If they are OK, I would advise you to go to a car workshop and have a complete check for your car.

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