Here’s Why Your Car Turns Off When Coming To Complete Stop or Slowing Down

It can be very annoying when your car stalls unexpectedly as you slow down or come to a stop. All of a sudden, the engine cuts out, meaning you need to start it up again quickly.

Though alarming, this usually happens for minor reasons that can be fixed—it might be something like the idle air control valve, fuel pump, or small electrical problems causing it. By figuring out exactly why your car stalls when stopping, you can look into solutions.

In this guide, we will explore some common causes for car stalling at stops. We will also give you some tips for testing things so you can identify the problem and get your car fixed and running nicely again.

You can also read my guide on Car dies while driving but restarts.

For your convenience, I’ve developed an interactive tool that aids in diagnosing car problems. It provides straightforward steps. Ensure to check it.

What is My Personal Experience With Car Stalling Intermittently at Low Speeds?

Car stalling problem happened with my sister’s 2010 Toyota Corolla a few months ago. The car would turn off every time I stopped at a red light or a stop sign. It was very frustrating. I decided to check the IACV myself.

I found out that the IACV was clogged with carbon deposits and needed to be cleaned. I used some carburetor cleaner and a toothbrush to remove the dirt and sprayed some WD-40 to lubricate the valve.

After reinstalling the IACV, I started the car and let it run for a few minutes. The idle was smooth and stable, and the car did not die when I drove it around the block.

First Action You Should Do If Car Keeps Dying While Idling

If your car keeps turning off or stalling when idling, 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 sputtering and dying when slowing down. 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.

What Are the Causes of Car Cutting Out At Low Speeds?

Here are the causes of a car stalling at idle:

1. Malfunctioning Idle Air Control Valve

Think of your engine as a living, breathing thing. When it’s just sitting there idling, it still needs air to breathe properly. That’s where the Idle Air Control (IAC) valve comes in.

You’ll usually find the IAC valve chilling in the throttle body assembly, hooked up to the intake manifold. It’s made up of a few key parts – a pintle (like a little needle), a solenoid (basically an electromagnet), and a motor.

When your engine is idling with the throttle plate closed, airflow is restricted. The IAC valve’s job is to bypass some air around that closed throttle plate, so your engine can breathe easy at idle. It uses that needle-like pintle to open or close the bypass passage as needed.

The engine control unit (ECU) is constantly monitoring things like engine temp and load to figure out the perfect idle speed. It tells the IAC valve to move its pintle in or out to hit that sweet spot. That way your engine hums along smoothly no matter what the conditions are outside.

So in a nutshell:

• Valve Body – the main IAC valve housing, usually aluminum or plastic
• Solenoid – the electromagnet that controls pintle movement
• Pintle – the needle that opens/closes the bypass air passage

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

How malfunctioning IACV causes this problem?

The IAC valve helps control how fast or slow your engine idles. It’s job is to change the idle speed based on stuff like the engine temperature or if you have the A/C on. But if the valve gets dirty or broken, it can mess up the idle speed and make your engine stall out.

Here’s two ways that can happen:

• Not Enough Air: A bad IAC valve might not let enough air get into the engine when you’re idling at a stop light or sign. Without enough air, the engine doesn’t get the right fuel mixture and could stall out.
• Up and Down Idle: You might also notice the idle speed going up and down a lot, like the engine revving itself on its own. This up and down idle speed from a faulty IAC valve could cause the engine to cut out when you come to a full stop.

How IACV valve becomes bad?

IACV gets clogged due to following reasons:

• Dirt Buildup: The IACV can get clogged up with dirt or carbon over time. This blocks the airflow and makes it hard for the valve to keep the idle speed steady. Eventually, the engine might shut off when you stop the car.
• Electrical Issues: The wiring that tells the IACV what to do can go bad. If the signal gets messed up between the engine computer and valve, the valve won’t know how to control the idle. The engine can get real finicky and cut out when you hit the brakes.
• Mechanical Failure: Like other parts in the engine, the IACV pieces inside can wear out from use. If the valve can’t move right anymore, it won’t be able to keep the RPMs steady. The idle will get weird and the motor might konk out at 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. Alternator Malfunction Preventing Proper Charging During Engine Idle

Making juice keeps a car moving. But when the juicemaker breaks, your car can shut off at stop signs.

Let me explain. The alternator makes electricity to charge the battery and power things like lights. It spins around while connected to the engine. More spins makes more juice.

When the alternator goes bad, your car might stall because:

• Not enough juice when idling: When you stop, the engine slows down. Things like A/C still want power. But broken alternator doesn’t give enough, so car stalls.
• Battery draining: Good alternator keeps battery charged. Bad one doesn’t and battery loses power. Not enough leftover when idling to keep car on.
• Up and down power: Busted alternator gives uneven power that confuses electronics. Engine computer can’t maintain idle speed. Car dies at red light.
• Ignition lacks oomph: To start burnin’, spark plugs need steady power. Weak alternator fails to deliver. No fire in the cylinder means no vroom at idle.

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.

How to test?

Next time your car dies at idle, 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 like a fluid coupling that transfers the engine’s power to the transmission. It’s made up of three main pieces:

• The Impeller is attached to the engine and spins around with it.
• The Turbine connects to the transmission and turns when the transmission spins.
• The Stator just sits there and doesn’t move. It helps redirect the fluid between the impeller and turbine.

Now, the impeller and turbine are filled up with transmission fluid. When the engine fires up, the impeller starts paddling that fluid into the turbine, making the turbine start spinning.

The faster the impeller goes, the more transmission fluid pressure it makes, and the more twisting force (torque) it hands off to the turbine.

But here’s the thing – there’s always some slippage happening between the impeller and turbine. So they never spin at exactly the same speed. This slippage cuts into fuel efficiency and performance, especially when you’re going fast.

To fix this, some torque converters have a lock-up clutch. At certain speeds, this clutch locks the impeller and turbine together so they turn at the same rate. Now there’s no more slippage, so fuel economy and performance improve. Less heat and wear happen too.

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?

The torque converter has a handy little lock-up clutch that connects the engine directly to the transmission. This gives better fuel economy when cruising on the highway. A TCC valve controls the lock-up clutch by regulating fluid pressure based on signals from the engine computer.

The computer keeps an eye on speed, workload, temperature and more, deciding when to lock or unlock the clutch. Normally, it locks while at steady highway speeds for efficiency and unlocks when slowing down for smooth shifting.

But if that TCC valve acts up, the fluid is low or dirty, the lock-up won’t disengage properly. So when you stop, the engine stays tied to the transmission and stalls your ride.

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?

As you press down on the accelerator, keep an eye on those RPMs. When the lock-up clutch engages, mimicking a manual transmission, you’ll notice the RPMs pause before continuing their rise.

Conversely, as you ease off the gas pedal, a locked converter keeps RPMs stable. Accelerate again and the unlocking action allows RPM fluctuations to resume.

Clearing up a common mix-up: while speed changes cause subtle RPM variations, lockup itself doesn’t dramatically throw off RPMs. You’ll notice the difference between a smooth increase while locked versus abrupt jumps when unlocked.

For example, light RPM fluctuations are normal when cruising in a higher gear. But the locked converter keeps these changes gradual – no jarring shifts to worry about.

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 Confusing ECM During Low RPM Operation

Ever wondered how your car knows how fast it’s going? There’s a little device called a transmission speed sensor ((also called VSS sensor or output speed sensor) that keeps tabs on things.

Think of your car’s transmission as having an input shaft and an output shaft. The input shaft takes power from the engine and transfers it to the wheels (the output). The output shaft spins around at the same speed as your wheels – so if you’re cruising at 60mph, that shaft is spinning like mad too!

The transmission speed sensor uses some clever magnetic tricks to detect how rapidly that output shaft is spinning. It sends this speed data off to your car’s onboard computer.

Your car’s ECU then compares the actual transmission speed to the speed it’s trying to be in the current gear. This allows it to make small tweaks to things like when to change gears, how much fuel to use, and ignition timings to keep your ride running smooth.

Some cars have the sensor directly turned by the transmission gears. Others have the sensor read off a toothed wheel on the shaft that spins at the same rate. As long as it can detect the spin speed, the system works!

There’s also a similar sensor on the input shaft called, you guessed it, the turbine speed sensor. Same idea – it reports back spin speeds to keep things in harmony.

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: These sensors use electrical signals to work their magic. If there’s a wiring problem, bad connection, or the sensor itself starts to fritz out, you’ll get weird readings or a dead sensor. Could be moisture, corrosion, or just old parts wearing out.
2. Mechanical Damage: Being stuck near hot engines and rattling transmissions is rough. Eventually the housing can crack or break, gumming up the works.
3. Contaminants and Fluid Leaks: Leaks let gunk inside that damages sensors. Transmission fluid kills ’em too – either trashes the internals or leaves a lovely debris coating that causes trouble.
4. Sensor Alignment Issues: These sensors use magnetic fields to check the transmission’s RPMs. If they get knocked out of whack, the readings go screwy or die completely.

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.

If your Transmission shifts hard when cold, you can read my guide on that.

5. Malfunctioning Crankshaft Position Sensor

You can think of CKP sensor as the nerve center, continuously feeding the engine computer critical details on what your crankshaft is up to. Its location varies but is often found chatting it up near the flywheel.

The CKP has twin responsibilities:

• Detecting Position: Tracking precise crankshaft positioning compared to the pistons. This intel allows for properly syncing fuel injection and sparks.
• Measuring Speed: Keeping close tabs on how fast that crankshaft is spinning. This data point helps set timing of combustion events.

CKP gathers all this through a reluctance wheel – essentially a toothed gear – attached to the rotating shaft. As it spins, an electromagnetic field is created that the sensor interprets into an electrical signal. This then speeds over to the ECU to make any adjustments needed.

When a crankshaft position sensor goes bad, the ECU is left flying blind without reliable intel on engine speed or positioning. Cue rough idling, stalling, or worse. Its failure is most obvious at idle or deceleration when the ECU leans heavily on that CKP signal to keep things stable.

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 Disrupting Proper Fuel Mixture Calibration Near Idle Speeds

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.

If your car shakes at idle but smooths out while driving, you can read my guide on that.

How do vacuum leaks occur?

Here are the causes of vacuum leaks:

• First up – that intake manifold. It’s supposed to evenly distribute air to all the engine cylinders. But if it springs a crack or starts leaking, it lets in unwanted air. Not good! You’ll need to give that manifold a good inspection. Any cracks or damage? Better replace it before you end up with a vacuum leak.
• Don’t forget about those intake manifold gaskets either. Their job is to seal the connection between the manifold and engine block. When they get old and worn out, cracks can form, or they get damaged. Then you have air escaping or entering places it has no business being.
• And we can’t leave out the vacuum hoses. They carry air between engine parts. But the hoses crack over time or get disconnected. Then bam – unwanted air gets pulled into the system.

How to spot the vacuum 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. Blocked EGR Valve Failing to Recirculate Exhaust Gases for Stable Idle Control

When fuel and air mix and ignite in the engine, power is created. But the high heat also produces some bad stuff called nitrogen oxides (NOx).

This is where the EGR valve comes to save the day. It takes some of the exhaust and sends it back into the engine’s intake. This exhaust chillin’ in the engine lowers the temp so less NOx is made. It also lowers the oxygen levels so combustion happens slower and cooler.

The engine computer decides when the EGR valve should open or close. It’s watching stuff like engine load, rpm, coolant temp. Then it tells the EGR valve to adjust itself to let in more or less exhaust. The valve can be fully open, partly open, or totally closed depending on what the engine needs.

Why EGR valve becomes clogged?

Here’s what gunks up the EGR valve:

• Unwanted Guests: Carbon builds up in the valve over time from the recycled exhaust. More carbon means less flow through the valve and more problems.
• Dirty Engine Oil: When nasty engine oil gets inside the EGR system, it leaves gunk on the valve that messes with its job.
• Incomplete Combustion: When fuel doesn’t burn properly, more carbon gets made. That carbon can clog up the valve.
• Moisture and Rust: Humid climates and road salt equal rust in the valve. Rust bits then get stuck and jam things up.

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. Mass Air Flow Sensor Anomalies Misreading Intake Air Flows at Low Engine Revolutions

The MAF (mass air flow) sensor is an essential part for your car’s engine. It measures how much air is going into the engine by using a heated wire.

Air flows over the wire and cools it down. Then the sensor sees how much electricity it takes to keep the wire hot. That amount matches how much air is flowing in.

The MAF sensor sits between the air filter and throttle body. There, it gets coated in dust, dirt, and oil over time. This buildup confuses the sensor so it can’t read right.

When the MAF sensor fails, it really messes things up. The engine might stall out completely, especially when slowing down or idling. Bad readings throw off the fuel mix so combustion suffers. That leads to misfires and stalls.

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 Fail To Provide Sufficient Spark For Ignition

Fouled spark plugs cut the power to the engine. This stops the car when you hit the brakes.

The plugs need to ignite the fuel-air mix in the cylinder. When they fail, unburned fuel builds up instead. The engine misses and loses power. It could die completely at a stoplight.

How do spark plugs become fouled?

Spark plugs become fouled due to the following reasons:

• Carbon – Fuel additives and oil leave carbon on the electrodes over time. The carbon blocks the spark.
• Oil – Engine oil leaks into the cylinder. The oil coats the plug and fouls it. This situation can arise due to worn piston rings, valve seals, or a malfunctioning PCV (Positive Crankcase Ventilation) system.
• Rich mixture – Too much fuel and not enough air. The excess fuel doesn’t burn fully. It leaves carbon on the plugs. A bad oxygen sensor, bad fuel injectors and clogged air filter cause this.
• Heat – Plugs work best within a temperature range. Too hot and the metal and ceramic of spark plug wear out fast. This fouls the plugs. Engine overheating is usually the cause.

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 Inhibiting Smooth Airflow Transition to Idle Mode

The throttle body is responsible for controlling the airflow into the engine. Over time, gunk builds up inside it. All that nasty buildup keeps the throttle valve from opening and closing smoothly.

Without enough air getting to the engine, your car starts sputtering at stops or refuses to start at all.

How to check?

First peek inside to see if anything busted or came loose. Check if thick chunks of gunk are clogging things up in there while you’re at it. Try wiggling the throttle valve around too. It should glide like butter. If not, something’s up.

Chances are gunk and grime still sneaked their way in. Use a throttle body cleaner and a soft brush or rag to gently clean the throttle body and remove any buildup.

Once you remove the gunk, your throttle body can work its magic again. Your engine will breathe easy and you can hit the road without a care.

11. Dirty Engine Air Filter Restricting Combustion Air Supply for Idle Operation

When it comes to keeping our cars running smoothly, most folks don’t think much about the engine air filter.

The air filter keeps gunk and grime from getting into the engine. We’re talking dirt, dust, pollen – all sorts of nasty stuff floating around outside. The filter grabs those bits before they can cause trouble.

It’s the gatekeeper – protecting the engine and letting clean air flow through. That airflow is mighty important for the engine to run properly.

But just like any filter, it can get clogged over time. All that trapped gunk starts blocking the flow. Suddenly, there’s not enough clean air making it to the engine.

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 at deceleration.

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.

12. Exhaust Backpressure from Clogged Catalytic Converter Upsetting Combustion Stability

Inside the catalytic converter is a honeycomb-like structure coated with precious metal catalysts such as platinum, palladium and rhodium.

As hot exhaust gases pass through the converter, the catalysts facilitate chemical reactions that convert hydrocarbons and carbon monoxide into water vapor and carbon dioxide. It also converts nitrogen oxides into nitrogen and oxygen. This helps reduce the emissions coming out of the tailpipe.

The proper functioning of the catalytic converter is essential for keeping pollution levels down and meeting emissions regulations.

A clogged or damaged catalytic converter cannot properly treat emissions, allowing more pollutants to pass through the exhaust system.

How a Clogged Catalytic Converter Causes Engine Stalling?

When you lift your foot off the gas to slow down or stop, the engine needs extra vacuum in the intake manifold to keep running smooth. This vacuum helps suck in the air/fuel mix so the engine doesn’t stall out when you ease up on the gas pedal.

But if the exhaust system gets clogged or restricted, the engine can’t properly create that vacuum it needs.

Also, when the catalytic converter gets blocked up, exhaust gases build up and allow unburnt fuel vapors to pile up in the cylinders and fuel system.

So, when you come to a stop with all those extra fuel vapors in there, it can be too much for the available air coming into the engine. It basically chokes on the overly rich air/fuel blend and stalls out.

How Does a Clogged Catalytic Converter Affect Engine Performance?

A clogged catalytic converter restricts the flow of exhaust gases through the exhaust system. This causes a build up of backpressure in the system.

The effects of this increased backpressure on engine performance include:

• Loss of engine power: As backpressure builds up, it restricts the engine’s ability to efficiently expel spent exhaust gases. This increases pumping losses inside the engine, resulting in less usable power sent to the drivetrain. Acceleration will suffer.
• Increased fuel consumption: The engine has to work harder to overcome the restricted exhaust flow. This can cause an increase in fuel consumption.
• Overheating: Excessive backpressure causes heat to build up in the engine’s cylinders and exhaust system components, leading to overheating issues.

How can I test it?

To test a bad catalytic converter, you can use a laser thermometer. Follow these steps:

• Start the engine for a few minutes.
• Keeping the engine running, pop up the hood and point out the thermometer at a point where exhaust gases enter the catalytic converter.
• Now, under the vehicle, point out the thermometer at a point where gases exit the catalytic converter.
• If there is a significant difference between both temperatures, it means that exhaust gases are being trapped in the catalytic converter, and are not exiting properly.

How to fix?

Because the catalytic converter is part of the vehicle’s emissions system, swapping it out requires some precision. This makes it a job better left to professional mechanics. Proper installation of the new converter is critical, as is making sure there are no exhaust leaks.

The average catalytic converter replacement cost is between $1,000 to $2,500, including parts and labor. The catalytic converter itself ranges from $500 for a basic unit to over $2,000 for a high-end unit.

13. Lack Of Engine Compression

Simply put, compression refers to the pressure created in the engine’s cylinders when the pistons move upward and compress the air/fuel mixture before ignition.

This compression is vital for combustion and generating power. The higher the compression, the more power engine can produce.

How does it cause stalling when slowing down?

When you take your foot off the gas to slow down or stop your vehicle, the engine speed and airflow decline.

As a result, the cylinders become more reliant on compression to continue running smoothly. Compression generates the air/fuel mixing needed for ignition when the cylinders aren’t pumping in as much air.

With weak compression, that mixing isn’t present. The ability of the mixture to burn lessens and it gets harder to ignite.

Misfires and partial combustions begin to take place, leading to power reduction and stuttering. While idling and at low RPMs, things get even worse. There simply isn’t adequate compression for consistent, trustworthy burning.

How does it occur?

Your engine can suffer from compression loss due to the following reasons:

• Worn piston rings are usually the culprit. The rings create a tight seal between the piston and cylinder wall to keep the combustion trapped. When they get worn down over time, they can’t maintain that good seal anymore. That lets the compression slip out. It also causes more oil to burn off as blue smoke from the exhaust.
• The engine cylinder wall itself can develop problems too. Getting overheated a lot can warp and groove the surface. Driving without enough lubrication or with dirty oil can scuff it up as well. Anything that makes the wall less smooth will disrupt the ring seal.
• The intake and exhaust valves play a role too. If they sink down into their seats over time, they won’t close up tight anymore. That lets compression leak by. The valve seats can get pitted or worn down too, with the same effect.
• More serious issues are cracks. Cracks in the cylinder head or engine block will provide an easy path for the compression to escape. Overheating is a common cause of cracks, but freezing, defects, or tuning problems can do it too.
• The head gasket that sits between the head and block is important as well. It seals the combustion chamber and keeps everything contained. It also routes the coolant and oil. If this gasket blows, you’ll notice compression loss along with coolant and oil leaks. You might even see white smoke with a sweet smell from the exhaust.

To learn more about head gasket failure and leaking of valves, you can read my guide on car won’t start after overheating.

How to test it?

Follow these steps to perform a compression test to detect loss of compression in engine:

1. First, disconnect the electric connectors of the fuel pump, fuel injectors and ignition coils/distributor.
2. Remove all spark plugs. Use a suitable spark plug socket to remove spark plugs.
3. For the first cylinder, screw the compression gauge in the hole where the spark plug goes.
4. Turn on the ignition and depress the accelerator pedal fully to keep the throttle plate open.
5. Crank the engine up to 5 times or do it until the needle on the compression gauge is peaked and doesn’t climb anymore. If you have a push start/stop button, press brake pedal and push the button multiple times.
6. Note down the pressure reading for each cylinder on a piece of paper. If you don’t know how cylinders are numbered in the engine, you can read this guide.
7. If one of the pressure readings is significantly lower, it can indicate a problem with that single cylinder. 
8. If you only have a low reading on one engine cylinder or the cylinders aren’t adjacent to each other, the issue is with the valve seal or piston ring. 
9. If adjacent cylinders have low-pressure readings, the issue might be with the head gasket.
10. When all your pressure ratings inside the engine cylinders are below 100 PSI for gasoline engines or below 275 for diesel, you may have a valve timing issue, or piston rings are worn-out.
11. To determine worn-out piston rings, a wet compression test is performed. The wet compression test involves adding a small amount of oil through a spark plug hole to see if the compression reading improves, which can help determine whether the low compression is due to piston rings or valves.
12. If the compression reading improves, it suggests that the low compression reading was due to worn-out piston rings. When oil is added to the cylinder, it acts as a lubricant and seals off the gaps between the piston rings and cylinder walls, leading to improved compression readings.

Final Thoughts

In summary, it is inconvenient when your car stalls upon stopping. This is often caused by minor mechanical issues that can be identified and addressed.

Common culprits include malfunctioning idle air control valves, clogged fuel filters, faulty fuel pumps, bad ignition coils or spark plugs, the idle stop-start feature, vacuum leaks, or alternator/battery problems.

Transmission issues like a faulty torque converter or solenoid can also cause stalling. Checking readily accessible systems like engines sensors using OBD readers can quickly point to the root cause.

With proper diagnosis, stalling issues upon stopping can typically be resolved without major repairs. The first step should be scanning for diagnostic trouble codes to guide the investigation.

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