Car Turns Off When Coming To Complete Stop: Exploring the Oddity!

It can be inconvenient when your car turns off when coming to a complete stop. This sudden stalling can happen without warning, leaving you scrambling to restart the engine.

While this issue can be alarming, it is often caused by a minor mechanical problem. By identifying the specific cause, whether it’s the idle air control valve, fuel pump, or an electrical issue, you can troubleshoot the car stalling problem when coming to stop.

In this guide, we” will walk through some of the common culprits behind a car turning off when coming to a complete stop, and provide tips to diagnose the issue yourself as well as recommendations for potential repairs.

So, why does a car turn off when coming to a complete stop even in an automatic transmission?  The most common cause of a car turning off when coming to a complete stop is a stuck open idle control valve, clogged fuel filter, faulty fuel pump, bad ignition coil or spark plug, auto start-stop feature, vacuum leak, alternator or battery issue, transmission problem like a faulty torque converter or solenoid, or stuck EGR valve. Checking these systems can identify the cause.

Bonus Read: Car dies while driving but restarts

First Action You Should Do If Car Keeps Turning Off When Coming to a Stop

If your car keeps turning off or stalling when coming to a stop, you should check if there are any error codes stored in your engine’s memory. Even if the check engine light is not on, the ECU can still detect the problem and throw OBDII codes.

Analyzing OBD2 codes can help you readily identify the root cause of problems occurring in your vehicle. I would recommend Bluedriver scan tool. It is dirt cheap and works with every vehicle.

One of my friends was facing the same issue of a car turning off while coming to a stop. Upon scanning through the OBDII tool, he got the P0726 code. The code was related to communication problems in a transmission control module. So, from the scan tool, he easily identified the problem.

Causes of Car Turning Off When Coming To a Complete Stop

Here are the causes of a car turning off when coming to a complete stop:

1. Malfunctioning Idle Air Control Valve

The IACV is typically located in the throttle body assembly and is connected to the intake manifold. It consists of a valve, a motor, and a sensor. When the engine is idling, the throttle plate is closed, restricting the airflow. The IACV steps in to regulate the amount of air that bypasses the throttle plate, ensuring a stable idle speed. 

The IACV receives signals from the engine control unit (ECU) and adjusts the valve’s position accordingly.

The ECU takes into account various parameters such as engine temperature and load to determine the appropriate idle speed. By controlling the amount of air, the IACV helps the engine maintain a consistent idle speed regardless of external conditions. 

To better understand how the IACV valve works, let’s break it down into its components:

1. Valve body: This is the main housing of the IACV valve. It contains the internal components and is usually made of aluminum or plastic.
2. Solenoid: The solenoid is an electromagnetic coil that controls the movement of the IACV valve. It receives signals from the engine control unit (ECU) and adjusts the position of the valve accordingly.
3. Pintle: The pintle is a small needle-like component that protrudes into the airflow passage. It can move in and out, allowing more or less air to bypass the closed throttle plate.

You can watch below Youtube video to learn the working of the Idle Air Control Valve:

How malfunctioning IACV causes this problem?

When the IAC valve is functioning properly, it adjusts the engine idle speed based on various factors such as engine temperature, air conditioning usage, and electrical loads. However, if the IAC valve becomes dirty, clogged, or fails altogether, it can disrupt this delicate balance and lead to engine stalling.

Here are two situations that can occur due to malfunctioning IACV:

1. Inadequate Airflow: A malfunctioning IAC valve may not allow enough air to bypass the throttle plate when the vehicle is idle. This can result in an insufficient air-to-fuel mixture, causing the engine to stall when you come to a stop.
2. Erratic Idle Speed: Another common issue with a faulty IAC valve is erratic idle speed. You may notice that your engine revs up and down unexpectedly, especially when you release the accelerator pedal or when the vehicle is idle. This fluctuation in idle speed can eventually lead to the engine shutting off when you stop completely.

How IACV valve becomes bad?

IACV becomes bad due to the following reasons:

1. Dirt or Carbon Buildup: Over time, dirt or carbon deposits can accumulate on the IAC valve, causing it to become stuck or not operate as intended. This buildup restricts the flow of air, resulting in an unstable idle speed and, in some cases, the engine shutting off when the car comes to a stop.
2. Electrical Issues: The IAC valve relies on electrical signals to function correctly. If there are any problems with the wiring or the electrical connections, it can disrupt the communication between the valve and the engine control unit (ECU). As a result, the IAC valve may not receive the necessary instructions to maintain a stable idle speed, leading to the engine shutting off unexpectedly.
3. Mechanical Failure: Like any other mechanical component, the IAC valve can also experience wear and tear over time. The internal components may become worn or damaged, preventing the valve from operating properly. This can cause irregular idle speeds and, ultimately, the engine shut off when the car stops.

How to test?

To detect a malfunctioning IACV, perform the following steps:

1. Start the engine and allow it to run at idle.
2. Locate the opening in front of the throttle plate and place your finger over it to obstruct it.
3. Observe a decrease in idle speed as a result of this obstruction.
4. Remove your finger from the opening and expect a brief increase in idle speed before it returns to its original rate.
5. This momentary alteration in RPM occurs because the PCM recognizes the blocked bypass air port, retracts the IACV, and allows air into the bypass port.
6. The influx of air causes the RPMs to rise, which is detected by the PCM.
7. Consequently, the IACV extends to restore the idle speed to its typical rate.

Another way to test IACV is by measuring voltage and resistance across the terminals of its connector using the multimeter. You can watch the below Youtube video for a better understanding.

How to fix?

If the IACV valve is clogged to gunk, you can try cleaning it with carb cleaner.

2. Damaged Alternator

The alternator generates electrical power to keep the battery charged and to supply electricity to various components such as lights, radio, and the ignition system. The alternator is mechanically connected to the engine’s crankshaft via a serpentine belt, due to which the alternator rotates and produces electric power.

A bad alternator causes your car to turn off when coming to a stop in the following ways:

1. Insufficient power supply:  A damaged alternator may not be able to provide a sufficient power supply to the electrical components of the car when the engine is idling. When the car is stationary, the engine’s RPM drops. As a result, the electrical demand from components such as the air conditioning system, radio, headlights, and other accessories may overwhelm the limited power supply from the weakened alternator. This can cause the engine to stall or shut off as it struggles to maintain power to all the necessary systems.
2. Battery drain: As mentioned earlier, the alternator is responsible for charging the car’s battery. However, a damaged alternator may not be able to effectively charge the battery, resulting in a gradual drain. When the car is idling, and the electrical load is high, the battery may not have enough power to sustain the vehicle’s operation, causing it to turn off abruptly.
3. Voltage fluctuations: A damaged alternator can also lead to voltage fluctuations in the car’s electrical system. These fluctuations can disrupt the smooth operation of various components, including the engine control unit (ECU) responsible for regulating the engine’s idle speed. If the ECU receives inconsistent voltage signals due to a faulty alternator, it may not be able to maintain the engine’s idle speed, resulting in the car stalling when coming to a stop.
4. Ignition system not getting enough voltage: The ignition system requires a constant supply of electrical power to keep the engine running smoothly. When the alternator fails to provide sufficient power, the engine may experience a lack of spark. The spark plugs, which ignite the fuel-air mixture in the engine’s cylinders, may not receive the necessary electric current to generate a spark, resulting in engine misfires or even engine stalling. 

How does the alternator become bad?

The alternator becomes bad due to the following reasons:

• Damaged bearing of the rotor of an alternator
• Weak battery
• Bad battery connections (corrosion con battery terminals and damaged battery cables)
• Bad engine ground connections
• Faulty fuel pressure regulator
• Contamination from engine fluids like coolant, engine oil or power steering fluid

To learn more about each cause, you can read my guide on car keeps killing alternator.

The easiest way to test an alternator or bad battery

Next time your car turns off when coming to a complete stop, all you need to check the voltage across battery terminals with the engine off using a multimeter. The voltage of the battery should be 12V.

3. Torque Converter Is Not Unlocking

A torque converter is a fluid coupling that transfers power from the engine to the transmission. It consists of three main parts: the impeller, the turbine, and the stator.

The impeller is attached to the engine and spins with it. The turbine is attached to the transmission and spins with it. The stator is a stationary part that redirects the fluid flow between the impeller and the turbine.

The impeller and the turbine are filled with transmission fluid. When the engine is running, the impeller pumps the fluid into the turbine, causing it to spin. The faster the impeller spins, the more fluid pressure it creates, and the more torque it transfers to the turbine.

However, there is always some slippage between the impeller and the turbine, which means that they never spin at exactly the same speed. This slippage reduces fuel efficiency and performance, especially at higher speeds.

To solve this problem, some torque converters have a lock-up clutch that can lock the impeller and the turbine together when they reach a certain speed. This eliminates slippage and makes them spin at the same speed. This improves fuel efficiency and performance, as well as reduces heat generation and wear.

To visualize the working of each component of the torque converter, the following video is really helpful:

How does a lock-up torque converter cause the car to turn off?

A lock-up clutch of the torque converter is controlled by a TCC (transmission torque converter clutch) solenoid valve that receives signals from the engine computer.

The solenoid valve regulates the fluid pressure in the lock-up clutch, which determines when it engages and disengages. The engine computer monitors various factors, such as speed, load, temperature, and throttle position, to decide when to lock or unlock the torque converter.

Normally, when you are driving at a steady speed on a highway, the engine computer will lock the torque converter to improve fuel efficiency and performance.

When you slow down, it will unlock it to allow slippage and smoother shifting. However, if there is a problem with the solenoid valve, or if there is a low fluid level or dirty fluid in the transmission, the lock-up clutch may not unlock when it should. This means that when you come to a complete stop, the engine will still be connected to the transmission, which will stall it.

Why does it happen?

Insufficient or dirty transmission fluid can also cause the lock-up torque converter to remain engaged when it should disengage. Low transmission fluid levels can lead to poor hydraulic pressure, preventing the solenoid from operating correctly.

You should check if there are any transmission leaks.

Moreover, if the TCC solenoid becomes stuck or fails to operate properly, it can prevent the torque converter from disengaging. This can result in the engine stalling when coming to a stop. 

How to know if the torque converter is not unlocking?

The method to test a torque converter not unlocking is the same as testing a torque converter if it is locking or not.

To identify whether your torque converter is locked up completely, you can rely on your vehicle’s tachometer.

As you accelerate, there will be a noticeable pause in RPM fluctuations. This pause signifies the engagement of the lock-up clutch, mimicking the behavior of a manual transmission.

As your foot eases off the gas pedal, the RPMs remain stable, indicating that the torque converter remains locked. Conversely, when you accelerate again, the converter unlocks, and the RPMs fluctuate.

It’s important to clear up a misconception that often arises. While RPM will naturally vary with changes in vehicle speed, the increase or decrease won’t be as abrupt as when the torque converter is disengaged. This is a critical point of confusion that has led some to believe that lockup causes significant RPM changes when it’s not the case.

To provide a clearer picture, let’s consider an example. While driving in a higher gear at a constant speed, you may notice minor RPM fluctuations. However, during lockup, the RPM changes are proportional to your speed and are smooth, not jarring. This distinction becomes more apparent when transitioning between lockup and disengagement, leading to a more controlled driving experience.

Here are some key points:

1. Steady RPMs during acceleration: Locked-up torque converters maintain stable RPMs even as you accelerate.
2. Unlocking during acceleration: When the converter unlocks, RPMs fluctuate as you press the gas pedal.
3. Steady RPMs when foot off the gas: With the foot off the gas, locked-up torque converters sustain constant RPMs.
4. Unlocking when foot off the gas: Upon releasing the gas pedal, the converter may unlock, leading to RPM fluctuations.

Note: It’s essential to note that this analysis primarily applies to vehicles equipped with traditional automatic transmissions, ranging from 6 to 10 speeds. If you’re driving a car with a Continuously Variable Transmission (CVT), the behavior may differ. CVTs are designed to lock up early, sometimes as low as 10 miles per hour, resulting in improved fuel efficiency.

How to test TCC Solenoid?

You will find a TCC solenoid on the front side of the transmission near the valve body of the transmission.

A TCC  solenoid becomes bad when it is stuck closed or stuck open. This disturbs the flow of transmission fluid through the lock-up clutch due to which it does not disengage when a car comes to a stop.

Here is how to test TCC solenoid:

• Use a digital multimeter to measure the resistance of the torque converter’s solenoid valve.
• Connect the multimeter’s leads to the valve’s prongs.
• A reading between 15 to 25 ohms indicates a well-functioning valve.

Another test for the functioning of TCC solenoid valve is as follows:

• Apply power from a car battery or a similar power source to the solenoid valve’s prongs.
• You should hear a clicking noise, and the valve’s plunger should move back and forth.
• This test verifies the valve’s proper functionality.

4. Malfunctioning Transmission Speed Sensor

A transmission speed sensor, also known as a vehicle speed sensor (VSS) or output speed sensor, is an electronic device located on the transmission housing or the differential. Its primary function is to monitor the rotational speed of the transmission’s output shaft or the drivetrain’s components, depending on the vehicle’s design.

An automatic transmission has input and output shafts. The speed of the input shaft is the same as the speed of the turbine of the torque inverter and it transfers power to the drivetrain. The output shaft is basically the output of the drivetrain. It has the same speed at which the wheels of the vehicle are rotating. 

By measuring how fast the output shaft of the transmission is rotating, the sensor can accurately determine the speed at which the vehicle is moving. 

The transmission speed sensor uses a magnetic field to detect the rotation of a toothed wheel on the shaft, and sends an electrical signal to the powertrain control module (PCM), which is the computer that controls the engine and transmission.

The PCM uses the information from the transmission speed sensor to calculate the actual gear ratio of the transmission and compare it to the desired gear ratio.

Based on this comparison, the PCM can adjust the shifting of gears, the fuel injection, the ignition timing, and other parameters to optimize the performance and efficiency of the vehicle.

The transmission speed sensor comes in two configurations, depending on vehicle:

1. A gear is attached to the sensor that is driven by a drivetrain. The sensor rotates and generates signals.
2. The sensor is mounted with a certain gap on a reluctor wheel that rotates with the speed of the shaft.

Note: Just like a vehicle speed sensor, there is also a turbine speed sensor. It measures the speed of the input shaft of the transmission. The speed of the input and output shafts of the transmission are different, due to which the input shaft has a different speed sensor, which is called the turbine speed sensor. This sensor can have the same problem as VSS sensor. So, you should also check it.

How a Bad Transmission Speed Sensor Causes Engine Shutdown?

When you bring your vehicle to a halt, the ECM relies on the transmission speed sensor to detect that the wheels have stopped rotating. This information is crucial for the ECM to adjust the engine’s idle speed and maintain a stable operation.

However, if the sensor is faulty, it may not provide accurate data to the ECM, causing it to misinterpret the situation. As a result, the ECM might not adjust the idle speed correctly, leading to the engine stalling or shutting off completely. This can be quite frustrating, especially when you’re at a busy intersection or in heavy traffic.

How does the transmission speed sensor go bad?

Here are some common reasons why it may go bad:

1. Electrical Issues: The transmission speed sensor relies on electrical signals to function properly. If there is a fault in the wiring, connectors, or the sensor itself, it can result in inaccurate readings or complete failure. This can be caused by exposure to moisture, corrosion, or simply old age.
2. Mechanical Damage: The transmission speed sensor is often exposed to harsh conditions, such as extreme temperatures and vibrations. These factors can lead to physical damage, such as a cracked or broken sensor housing, which can impede its functionality.
3. Contaminants and Fluid Leaks: Fluid leaks within the transmission system can introduce contaminants that can damage the speed sensor. For example, if the sensor comes into contact with transmission fluid, it can cause internal damage or result in a buildup of debris that affects its performance.
4. Sensor Alignment Issues: The transmission speed sensor relies on a magnetic field to measure the rotational speed of the output shaft. If the sensor becomes misaligned or maladjusted, it can lead to inaccurate readings or complete failure.

How to check?

Here is how to test the VSS sensor:

1. To begin, you’ll need a voltmeter with fine probes, which will help you measure the sensor’s output voltage.
2. Insert the fine probes of the voltmeter into the tiny output wires connected to the speed sensor. Ensure a secure connection to accurately measure the sensor’s voltage output.
3. Change the voltmeter’s settings to AC mode. This is crucial because the speed sensor generates an AC signal, which is the type of signal required for proper speedometer and odometer operation.
   
4. Begin manually spinning the speed sensor by hand. Monitor the voltmeter’s readings as you spin the sensor.
   
5. Another way to check the transmission speed sensor is by bringing the magnet closer to the sensor and wiggling it back and forth. The sensor will generate voltage intermittently. Use this way if your speed sensor does not have gear attached to it.
6. Lastly, also check the resistance between terminals on the harness connector of the sensor and observer any signs of damage.

Also Read: Transmission shifts hard when cold

5. Malfunctioning Crankshaft Position Sensor

The crankshaft position sensor, also known as the CKP sensor or engine speed sensor, is usually located near the crankshaft or the flywheel.

Its primary purpose is to detect the position and speed of the crankshaft as it rotates. This information is then transmitted to the ECM, allowing it to make accurate calculations and adjustments to optimize engine performance. 

Here are the two functions of the CKP sensor:

1. Position Detection: The CKP sensor provides the ECM with real-time data about the crankshaft’s position in relation to the engine’s pistons. This information is essential for synchronizing the fuel injectors and ignition system.
2. Speed Measurement: By monitoring the rotational speed of the crankshaft, the CKP sensor helps the ECM precisely time the spark plug ignition and control fuel delivery, ensuring optimal combustion efficiency.

How does CKP sensor work?

The crankshaft position sensor operates based on the principle of electromagnetic induction. It consists of a magnetic coil and a reluctor wheel, which is a toothed wheel attached to the crankshaft.

As the crankshaft rotates, the reluctor wheel passes by the magnetic coil, creating an electromagnetic field. The changes in this field are detected by the sensor, which then generates an electrical signal. This signal is sent to the ECU for processing and adjusting various engine parameters accordingly. 

How does bad CKP sennor affect?

When the CKP sensor malfunctions, the ECU may not receive accurate information about the crankshaft position and speed, which can lead to incorrect ignition timing and fuel delivery. This can cause the engine to run poorly, stall, or turn off completely.

This problem may be more noticeable when the engine is idling or decelerating, as these are the situations where the ECU relies more on the CPS signal to maintain a stable engine speed.

How does CKP sensor become bad?

CKP sensor can become bad due to several reasons, such as:

• The CKP is dirty or contaminated by oil, dirt, or debris, which can interfere with its ability to sense the crankshaft rotation.
• The CKP is damaged or worn out by heat, vibration, or corrosion, which can affect its electrical resistance and output voltage.
• The CKP wiring or connector is loose, broken, or shorted, which can disrupt the communication between the CPS and the ECU.
• The CKP is misaligned or installed incorrectly, which can cause it to read the wrong position of the crankshaft.

How to test?

You should check the voltage across the terminals of the harness connector of the CKP sensor. Moreover, you should check the bolts of the CKP sensor through which it is mounted on the engine block. The bolts should be tightened to specs. 

I can’t discuss everything in detail here. For you, I have written a separate guide on car won’t start after replacing the crankshaft position sensor.

6. Vacuum Leaks

At its core, a vacuum leak refers to the presence of unmetered air entering the engine’s intake system that is not measured by the MAF sensor. This uninvited air disrupts the finely tuned balance of air and fuel that is necessary for an engine to run smoothly. 

Now, you will be wondering how a vacuum is produced in the engine. During the intake stroke, the piston moves downward in the cylinder, creating a larger space for the air-fuel mixture to enter. This movement causes a pressure drop in the cylinder, creating a partial vacuum.

The intake valve opens, allowing the air-fuel mixture to be drawn into the cylinder from the intake manifold. The vacuum created by the downward movement of the piston plays a crucial role in this process, as it pulls in the air necessary for combustion. 

As you press the gas pedal and increase engine speed, the throttle plate opens further which allows more to enter the engine cylinder. So, with an increase in acceleration, the vacuum starts decreasing. At idle, the vacuum is maximum in the engine.

When a car comes to a complete stop, the throttle plate in the engine closes to restrict airflow. Since the vacuum is maximum when the engine is idling, any vacuum leak will cause a large amount unmetered of air to enter the engine. This will disturb the air-fuel ratio to a greater extent which will cause your vehicle to stall or turn off.

Also Read: Car shakes at idle but smooths out while driving

How do vacuum leaks occur?

Here are the causes of vacuum leaks:

• Cracked or Leaking Intake Manifold: The intake manifold is responsible for distributing air evenly to the engine cylinders. If it develops cracks or leaks, it can introduce unmetered air, causing a vacuum leak. Inspecting the intake manifold for any signs of damage and replacing it if necessary is crucial to avoid this problem. 
• Worn or Damaged Intake Manifold Gaskets: Intake manifold gaskets play a crucial role in sealing the connection between the intake manifold and the engine block. Over time, these gaskets can deteriorate, crack, or become damaged, allowing air to escape or enter where it shouldn’t. This can result in a vacuum leak and all the associated problems.
• Faulty Vacuum Hoses: Vacuum hoses are responsible for carrying air between various components in the engine. If these hoses develop cracks, leaks, or become disconnected, they can introduce unmetered air into the system

How to spot the leak?

With the engine running, listen carefully for any hissing sounds. A vacuum leak often produces a distinct high-pitched hissing noise. If you hear such a sound, try to locate its source, as it can help you narrow down the problem area. 

Professionals often employ smoke machines to detect vacuum leaks. These machines introduce smoke into the intake system, making it easier to spot leaks as the smoke escapes from the affected areas. While this method may not be readily available to the average DIYer, it can be highly effective in locating even the most elusive leaks. 

You can also use carb cleaner to detect vacuum leaks in the engine.

7. Clogged or Stuck EGR Valve

To understand how the EGR valve works, let’s take a closer look at the combustion process in an engine. During combustion, a mixture of fuel and air is ignited, resulting in the generation of power. However, this process also produces harmful nitrogen oxides (NOx) due to the high temperatures reached within the engine.

The EGR valve comes into play by recirculating a portion of the exhaust gases back into the engine’s intake manifold. This process reduces the temperature inside the combustion chambers, ultimately lowering the formation of NOx emissions. By reintroducing these exhaust gases, the oxygen concentration is reduced, resulting in a slower and cooler combustion process.

The opening and closing of the EGR valve is carefully regulated by the engine control unit (ECU), which monitors various engine parameters such as engine load, RPM, and coolant temperature.

The ECU uses this information to determine the appropriate amount of exhaust gas to be recirculated. It then sends a signal to the EGR valve, which adjusts the valve’s position accordingly. The EGR valve can be fully open, partially open, or completely closed, depending on the engine’s needs at any given time. 

Why EGR valve becomes clogged?

Here are the causes of clogged EGR valve:

1. Unwanted Guests: Over time, carbon deposits can build up inside the EGR valve, causing it to become clogged. These deposits are primarily a result of the recirculated exhaust gases, which contain trace amounts of carbon. As these deposits accumulate, they can restrict the flow of exhaust gases through the valve, leading to reduced performance and increased emissions.
2. Dirty Engine Oil: Another culprit behind EGR valve clogging is dirty engine oil. Engine oil, when contaminated with dirt, debris, or carbon particles, can find its way into the EGR system. Once inside, these impurities can settle on the valve and hinder its proper functioning.
3. Incomplete Combustion: When your engine doesn’t burn fuel efficiently, it can lead to the formation of additional carbon deposits. These deposits can find their way into the EGR system and eventually clog the valve.
4. Moisture and Rust: Moisture and rust can also contribute to the clogging of the EGR valve. In regions where the climate is humid or where road salt is used, the EGR valve can be exposed to moisture, leading to rust formation. Rust particles can then break off and obstruct the valve’s operation.

If your EGR valve is malfunctioning, the OBDII scan tool will display P0400, P0401 or P0402 error codes.

To check if the EGR valve is working correctly, you can perform a vacuum test by using a hand-operated vacuum pump.

If the EGR valve can hold a vacuum, it indicates that the valve is functioning properly. However, if the vacuum pressure drops off after a while, it suggests a faulty EGR valve that needs to be replaced.

8. Malfunctioning MAF Sensor

The MAF sensor is an electrical component that measures the volume and density of the air entering the engine. It is usually located between the air filter and the engine’s throttle body.

The MAF sensor uses a heated wire or a thin film element to determine the amount of air passing through it.

As air flows over the heated element, it cools down, and the MAF sensor measures the current required to maintain the wire or film at a constant temperature, which is directly proportional to the mass of the incoming air.

The MAF sensor is exposed to dust, dirt, and oil particles in the intake air. Over time, these contaminants can build up on the sensor’s surface, affecting its accuracy and performance.

A malfunctioning MAF sensor can cause intermittent stalling or complete engine shutdown, especially when the car is decelerating or idling. The incorrect readings from the faulty sensor can disrupt the air-fuel ratio for the engine’s combustion process, leading to engine misfires and eventual stalling.

How to test?

As the mass flow rate of air through the MAF sensor increases, voltage output also increases.

When you push the gas pedal, more air goes through the MAF sensor. This makes the voltage go up. When the car is not moving much, the voltage should be less than 1.0V. When you make the car go faster, the voltage from the MAF sensor goes up from 1.0V to 1.7V.

If the voltage of the MAF sensor is fluctuating, you can clean that heated element of the MAF sensor using this cleaner. Be sure to never touch that wire of the MAF sensor. Let it dry completely before re-installing the sensor.

Here is a short Youtube video explaining how to clean the MAF sensor.

9. Fouled Spark Plugs

Spark plugs are small, yet vital components of a car’s ignition system. They are responsible for producing the electrical spark that ignites the air-fuel mixture in the combustion chamber, ultimately powering the engine. 

One of the primary reasons fouled spark plugs can cause a car to turn off when coming to a stop is the disruption of the air-fuel mixture combustion process.

When spark plugs fail to ignite the mixture efficiently, unburned fuel can accumulate in the combustion chamber, leading to unstable combustion and a loss of power. This can result in the engine shutting down abruptly, especially when the car is stationary or decelerating. 

How do spark plugs become fouled?

Spark plugs become fouled due to the following reasons:

1. Carbon Deposits: One common cause of spark plug fouling is the buildup of carbon deposits. Over time, fuel additives, oil contaminants, and incomplete combustion can result in the formation of carbon deposits on the spark plug electrodes. These deposits can interfere with the spark’s ability to ignite the air-fuel mixture, leading to misfires and reduced engine efficiency.
2. Oil Contamination: If there are oil leaks within the engine, it can result in oil reaching the combustion chamber. When oil comes into contact with the spark plug, it can form a layer of oil deposits on the electrodes, causing fouling. This situation can arise due to worn piston rings, valve seals, or a malfunctioning PCV (Positive Crankcase Ventilation) system. It is essential to address oil leaks promptly to prevent spark plug fouling.
3. Rich Air-Fuel Mixture: An excessively rich air-fuel mixture, where there is more fuel relative to the amount of air, can contribute to spark plug fouling. When the mixture is too rich, there is an increased likelihood of incomplete combustion, leading to the formation of carbon deposits on the spark plug. This situation can arise due to a faulty oxygen sensor, malfunctioning fuel injectors, or a clogged air filter.
4. Overheating: Spark plugs are designed to operate within a specific temperature range. If an engine runs hotter than normal, it can lead to the spark plug overheating. Excessive heat can cause the electrode to wear out faster and melt the insulator, resulting in fouling. Common causes of engine overheating include a malfunctioning cooling system, a faulty thermostat, a blown head gasket or coolant leaks. To learn more, you can read my guide on car won’t start after overheating.

To find out how the spark plug is fouled, you need to remove it using the spark plug socket. After that, visually inspect the spark plug to see how it was fouled.

You can take help from the spark plug trouble tracer chart (PDF download) to learn more.

How to fix?

Deciding whether to clean or replace spark plugs relies on their condition. If the spark plugs are old or damaged, they need to be swapped out. If they still look good, you can clean them and fix the gap between their tips. This gap adjustment is based on what the manufacturer suggests.

The process of cleaning spark plugs depends on how they are fouled. First, blow away dirt and junk using compressed air. After that, soak the spark plug in brake cleaner until all the gunk is gone. You can also use a soft brush to get rid of any remaining debris.

10. Dirty Throttle body

When the throttle body becomes dirty, it can impede the smooth flow of air into the engine.

Over time, dirt, grime, and carbon deposits can accumulate on the inner walls of the throttle body, affecting the movement of the butterfly valve. This build-up restricts the amount of air that can pass through the throttle body, leading to a disruption in the air-to-fuel ratio.

As a result, the engine may not receive the necessary amount of air to maintain smooth and consistent idle speed, causing it to stall when coming to a stop. 

How to fix?

Begin by visually inspecting the throttle body for any signs of physical damage or wear. Look for cracks, leaks, or loose connections that could be affecting its performance. Additionally, check the throttle plate for any signs of excessive carbon buildup, as this can hinder its movement.

Next, manually move the throttle plate back and forth to ensure it moves smoothly without any sticking or hesitation. If you notice any resistance or uneven movement, it could indicate a problem with the throttle body mechanism. 

Over time, the throttle body can accumulate carbon deposits and dirt, leading to poor performance. Use a throttle body cleaner and a soft brush or rag to gently clean the throttle body and remove any buildup.

11. Dirty Engine Air Filter

When we think about the various components that keep our cars running smoothly, the engine air filter may not be the first thing that comes to mind.

However, this seemingly small and often overlooked part plays a vital role in the overall performance of our vehicles. A dirty engine air filter can have a significant impact on how our cars operate, potentially causing them to turn off unexpectedly when we come to a stop.

An air filter is responsible for filtering the air that enters your vehicle’s engine. It removes impurities such as dirt, dust, pollen, and debris, preventing them from reaching the engine cylinders. This is important for several reasons:

• Protection: The air filter acts as a barrier, protecting your engine from harmful particles that can cause damage and wear.
• Performance: A clean air filter ensures a consistent flow of clean air to the engine, allowing for efficient combustion and optimal performance.
• Fuel efficiency: When the air filter is clean, the engine doesn’t have to work harder to pull air through, leading to improved fuel efficiency.

As we drive, the air outside contains various particles that can accumulate on the filter over time. When the filter becomes clogged, it restricts the airflow to the engine. This restriction disrupts the delicate balance between air and fuel, known as the air-to-fuel ratio, which is necessary for the engine’s combustion process.

How a Dirty Engine Air Filter Causes Stalling?

When we come to a stop or idle, the engine requires a consistent and steady airflow to maintain its operation. As you decelerate, the flow rate of air with which the air enters the engine decreases.

If the engine air filter is dirty and airflow is restricted, that small air flow at idle might now force through the constrictions in the air filter.

As a result, the engine may not receive the necessary amount of air to sustain its idle speed. This can lead to a rough idle or, in some cases, cause the engine to stall completely, resulting in the car turning off.

What to do?

You should always use a genuine air filter and replace it every 15,000 miles. Moreover, use compressed air to clean the air filter every 1000 miles.

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