Did you recently experience your engine stall when you stepped on the brakes and want to know what you can do about it? You’ve come to the right place, for we have researched this question and have the answer for you.
There are five possible reasons why an engine would stall when you step on the brakes:
- A leak in the vacuum brake booster hose
- A failing valve in the vacuum brake booster
- Malfunctioning Idle Speed Control (ISC)
- Malfunctioning or failing Engine Control Unit (ECU)
- A bad or failing fuel pump
Learn more about these possible causes in the succeeding sections. Read on!
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What is a brake booster?
Imagine an object weighing 3,000 pounds or more moving at 50 mph. Do you think you will be able to stop it with just the force of your legs? Perhaps not. That is why cars need brake boosters.
A brake booster is a device that increases the force that you apply to your brakes. This makes your brakes more effective at stopping your car even when it is running at high speeds.
How does a brake booster work?
When you step on the brakes, a valve opens to allow air to get inside the brake pedal side of the booster. At the same time, the movement of air into the intake valve of the engine leads to a pressure drop that creates a vacuum. A vacuum hose connecting the intake manifold of the engine and the brake booster master cylinder grants the brake booster access to the vacuum inside the engine.
Once air gets into the brake pedal side of the booster, the vacuum from the engine extends into the master cylinder side of the booster. This creates a difference in air pressure, with the master cylinder side having the lower pressure. The lower pressure pulls on the brake diaphragm, and this amplifies the force that is applied to the brakes from your foot.
The increase in the braking force makes it easy to stop your car.
How to identify a bad brake booster?
Now that you have a working knowledge of how a brake booster works, you can apply that knowledge to understand the brake booster problem that causes your engine to stall.
A vacuum leak decreases the pulling force that the brake booster provides. This reduces its ability to amplify the force from your foot when you step on the brake pedal. As a result, you will feel that your brake pedal seems stiffer than usual.
Because the brake pedal is stiff, it is harder to stop your car. This means that you have a longer braking distance than normal. This can also be accompanied by a hissing sound.
Finally, since the engine is the vacuum source that the brake booster needs to amplify the braking force from your foot, a vacuum leak will draw more air into the engine than normal and compromise engine performance. Stepping on the brake pedal harder will draw even more air into the engine, and this can cause sputtering and stalling.
The problem could also come from a failing brake booster. A failing brake booster can leak air from the brake pedal side to the vacuum line side, which produces the same symptoms. Fortunately, you can use the same test for both.
How to test a vacuum leak?
To test this, park your car in a wide-open space where your car can run with weak braking power without hitting anything. Follow the steps below to test:
- Disconnect the vacuum line that goes to the brake booster.
- Start your car and make it move very slowly.
- Step on the brakes to stop the movement of your car. Keep in mind that your brakes will be very stiff, and you need a lot more force to stop your car. If the engine doesn’t shut off, then the problem is a vacuum leak or a failing brake booster. Disconnecting the vacuum line isolates the engine from the braking system. Whether it is a vacuum leak or a failing brake booster, the braking system will not be able to force more air into the engine.
- Bring your car to a professional mechanic to determine and fix the exact problem.
What is the Idle Speed Control (ISC)?
Even though it is called a gas pedal, stepping on it lets air and fuel inside the engine, producing combustion. However, if the engine relies only on the gas pedal for air and fuel supply, the engine will turn off when you let go of the gas pedal. This is where the ISC comes in.
The ISC takes over the air intake when you are no longer stepping on the gas pedal. It will let the right amount of air get into the engine through a bypass valve.
If your ISC system starts to fail, it will no longer be able to supply your engine with the right amount of air to keep it running at idle. And this causes the problem of the engine stalling when you step on the brakes and decelerate.
How to identify problems with the ISC?
Always check for a vacuum leak before checking the ISC. Once you rule out the possibility of a vacuum leak, you can proceed to troubleshoot the ISC. A vacuum leak can also affect the idle speed.
Check the status of the idle air bypass solenoid. In most cases, it will be completely out.
An idle air bypass solenoid in this state means that the ECU is constantly trying to close the air bypass to get the idle speed down. This behavior is often the result of an air leak in the engine.
Fixing The ISC
Bring your car to a professional mechanic to check your car and fix the problem for you. If the problem is due to an air leak in the engine, a mechanic will be able to know what to do to fix the issue.
What is the ECU?
The ECU is the brain of your car. It can be called the ECM or the engine control module. It is a PCM or Powertrain Control Module if the unit is also responsible for controlling an automatic transmission.
Its main function is to control the air and fuel that gets into the engine and the timing of the spark to ignite the mixture. It uses the information from the CPS or the crankshaft position sensor to determine the correct timing of activating the ignition system.
The ECU is also responsible for maintaining the correct air and fuel ratio. Too much air and the engine suffer from lack of power, causing it to stutter or stall. Too little air and the engine will produce too much heat, burn too much fuel (wasting fuel), and produce too much pollution.
How do you diagnose a bad ECU?
There are several symptoms to look out for to determine if you have a problematic ECU. The most common is a "Check Engine" light that stays on even after you do a reset. The engine suddenly turning off is another symptom.
Unfortunately, the problems that a failing ECU produces can also come from other failing parts of the engine. Thus, it is best to bring your car to a professional mechanic to check your car.
Do not drive your car to a mechanic, have a towing company bring it there. A problem with the ECU greatly compromises the safety of driving your car.
What is a fuel pump?
A fuel pump is responsible for maintaining the fuel pressure that is needed by the engine to keep fuel flowing.
Engines that use a carburetor use low-pressure mechanical fuel pumps. A fuel injector engine, on the other hand, uses an electronic fuel pump.
Identifying A Failing Fuel Pump
Fuel must have consistent pressure, and it is the responsibility of the fuel pump to maintain this. A failing fuel pump will lead to a drop in fuel pressure. This can cause the engine to run out of fuel, leading to stalling, hesitation, or misfires.
Replacing The Fuel Pump
A problematic fuel pump needs replacement. When you replace your fuel pump, it is also a good idea to replace the fuel filter.
The fuel pump is located inside the fuel tank in most modern vehicles. You can access it under the rear seats or under the compartment cover.
Even though the fuel pump is easy to access, replacing it is best done by a professional mechanic so that he can also check the fuel lines for problems and do the necessary check after he installs the replacement.
Bring your car to a professional mechanic to do this for you.
There are several reasons why your engine will shut down after you step on the brakes. Unfortunately, most of these causes are best handled by a professional mechanic so that you can be sure that the issue is fixed properly.
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