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Why Your Engine Is Stalling, and How to Stop It

Mia Bevacqua
March 14, 2018

engine stalling

If your engine keeps stalling, it can ruin your day. It can also be dangerous — no one wants to be stuck in the middle of an intersection. But will it lead to a big repair bill? That depends on the cause. Let’s find out what it could be.

What your engine needs to run

Your engine uses a mixture of fuel and air, and needs four things to run:

  • Intake: The ability to draw the air-fuel mixture into the cylinder
  • Compression: The ability for the piston to compress the air-fuel mixture
  • Combustion: The spark to ignite the air-fuel mixture
  • Exhaust: The ability to remove the byproducts of combustion from the cylinder

Together, these four items are the basis for the term “four-stroke engine,” which is the most common type of engine used in cars for the past century. 

If your car is stalling, it typically means you have a problem with your air-fuel ratio. This is especially true if the vehicle cuts out at low speeds or right after it’s started. Stalling that happens at high speeds is more commonly caused by a problem with the ignition or charging system. If your car is stalling, it's a good idea to get your car to a mechanic for diagnosis.

Reasons your vehicle might stall

Stalling is typically attributed to one or more of the following:

  • Air delivery problems
  • Fuel delivery problems
  • Sensor issues
  • Vacuum leaks
  • Ignition problems
  • Emissions equipment issues
  • Charging system problems 
  • Engine problems
  • Control module issues

Let’s take a closer look at how these affect everyday gasoline engines. Diesel, turbocharged and supercharged engines may differ.

Get it diagnosed by a professional
 

Air delivery problems 

Throttle body problems: A dirty or restricted mechanical throttle body can inhibit airflow, causing the engine to stall. Cars made since the mid-2000s have an electronic throttle body that’s controlled by the car’s computer. A problem with an electronic throttle body or its circuitry can stall the engine.

Solution: With a mechanical throttle body, cleaning may be all that’s needed to fix the problem. With an electronic throttle body, the circuit needs to be checked and, if it’s good but your problem still exists, you will need a new throttle body or actuator.

Idle air control valve issues: The idle air control (IAC) valve controls the engine idle speed by regulating airflow around the throttle plate. If you have a tachometer in your dash, you can see the IAC valve at work. When you turn on the air conditioning, the idle speed may raise from 700 RPM to 1,000 RPM to help the engine deal with the extra load. That is your IAC valve working to keep your car from stalling. A dirty or faulty IAC valve can prevent the engine from getting the air it needs, causing it to stall. 

Solution: Often, cleaning the IAC valve passages in the throttle body will fix the problem. If your car is running well at cruising and highway speeds, but is idling poorly and eventually stalling, this is your best place to start. Most IAC valves are located within the throttle body. 

As mentioned above, the IAC allows airflow around the throttle plates, so this solution requires the IAC valve to be removed from the throttle body for proper cleaning. Simply cleaning the throttle body itself will not clean the IAC valve and passageways. If the IAC valve does not respond to certain changes, like the air conditioner being turned on, it will need to be replaced.

Fuel delivery problems

Poor fuel pressure can stem from several sources, the most common being the fuel pump. A weak fuel pump can starve the engine for fuel, causing it to stall. Other causes of low fuel pressure include a restricted fuel filter or fuel line, or a faulty fuel pressure regulator. Typically, a bad fuel injector or two will cause a misfire, but will not cause the engine to stall.

Solution: Because of the risks associated with gasoline, it’s best to let a professional handle these repairs. If you have low fuel pressure, you’ll typically need a new fuel pump.

Sensor issues

There are two basic engine fuel management systems in use today: speed density and mass air flow.

In a speed density system, the car’s computer determines fuel injector control based on input from the throttle position sensor (TPS), manifold absolute pressure (MAP) sensor, coolant temperature sensor (CTS) and intake air temperature sensor (IAT). A problem with any of these sensors, or their circuits, can cause stalling.

In a mass air flow system, the computer determines fuel injector control based on input from the mass air flow (MAF) sensor. A problem with the MAF sensor or its circuit can cause stalling or prevent the car from starting. These are the most common sensor-related causes for engine stalling, but there are other sensors that can have the same effect. 

Keep in mind that all your sensors provide information to the computer. A sensor can not directly do anything but provide that information. If the computer decides that sensor information requires something to change, the computer uses an actuator — such as the IAC valve or fuel injectors — to make the change. 

Fortunately, most sensor problems will trigger the check engine light and store a diagnostic trouble code, which can help point you in the right direction to repair the problem.

Solution: If you have an OBD-II code scanner you might be able to figure out which sensor is causing the problem. But most of these repairs should be left to a professional mechanic.

Vacuum leaks

A vacuum leak allows unwanted air to enter the engine. This causes the engine to run lean, which means it’s getting too much air and not enough fuel. If it’s a large leak, it can cause the engine to stall — though most leaks are smaller and result in other symptoms. Common vacuum leak sources include hoses and lines, the positive crankcase ventilation (PCV) valve and the intake manifold.

Solution: A technician can locate the source of the vacuum leak and repair as needed.

Ignition problems

Almost all vehicles made within the last decade or so have a coil-on-plug ignition system. With this design, there is a high-voltage ignition coil that sits atop each spark plug and fires it when it receives a signal voltage from the ignition timing device or computer.

The car’s computer makes decisions regarding spark control based on input from the crankshaft position sensor, camshaft position sensor and knock sensor. A problem with an individual coil or spark plug may not cause your engine to stall. But a problem with the crankshaft or camshaft position sensor or ignition switch will.

Solution: Determine the cause of the ignition problem. In many cases, the remedy will be either replacing the crankshaft or camshaft position sensor or ignition switch. These tests are best left to your mechanic.

» MORE: Get an accurate estimate for your car repair

Emissions equipment issues

Cars made since about 1974 have emissions equipment that can fail and cause stalling. Some common offenders include the exhaust gas recirculation (EGR) valve and catalytic converter. 

If the EGR valve is stuck open, it will allow too much exhaust gas to enter the engine. This can result in surging and stalling. A clogged catalytic converter can put too much backpressure on the engine, causing it to stall.

There are many other components that can have the same effect. Fortunately, the car’s computer monitors emissions system operation. If it detects a problem with one or more components, it will turn on the check engine light and store a trouble code. 

Solution: If you have an OBD-II code scanner, you can try to pinpoint the problem area. Catalytic converter problems will need to be handled by a mechanic. If your EGR valve is stuck, you might be able to access it and fix it, depending on your make and model of car. Some require disassembly and special tools, though — in this case, a mechanic is likely your best option.

Charging system problems

Battery issues: A bad battery, or loose or damaged battery cables, can cause a vehicle to stall without warning.

Solution: In some cases, the battery cables may just need to be tightened or replaced. If the problem is with the battery itself, it will need to be replaced. These problems can be handled by most folks — but be careful of the risk of electrical shock.

Faulty alternator: The alternator charges the battery and supplies power to the entire vehicle when the engine is running. An alternator that is overcharging can cause vehicle electronics to go haywire. If the alternator is undercharging, the battery won’t get charged and onboard electronics won’t have proper voltage. Either scenario can result in stalling.

Solution: If the alternator is bad, you’ll need a new one. This project can involve a lot of disassembly and is often best left to a professional.

Engine problems

An engine with low compression may not produce enough power to keep running. The engine may stall right after it’s started, or when a load is put on it.

Solution: In most cases, the engine will need to be rebuilt or replaced. This is an expensive repair, and may not be worth it, depending on the age and value of your vehicle. Talk with your mechanic, do the math and decide if you would be better off getting a new car.

Control module issues

The engine control module (ECM) — in essence, the engine’s computer — monitors and regulates all aspects of engine performance. A problem with the computer or its circuit can cause the engine to stall.

Solution: This job is typically best left to the pros. The technician will check that the computer’s circuit is working, and then check for any technical service bulletins. If there aren’t any, the ECM will need to be replaced. This can sometimes be pricey.

 

Mia Bevacqua

About the Author

Mia Bevacqua is an automotive expert with ASE Master, L1, L2 and L3 Advanced Level Specialist certification. With 13-plus years of experience in the field, she applies her skills toward writing, consulting and automotive software engineering.

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