Car Stalling Issues: Common Causes and Solutions

Troubleshooting Car Stalling Issues: A Complete Guide

A car can stall for various reasons, which can generally be categorized into issues related to the fuel system, computer sensors, or the ignition system, or a combination of both. Before diagnosing the problem, it helps to identify when the stalling happens. For example, if the engine stalls more when it’s cold, the cause is more likely to be fuel related, than an ignition problem. I’ll walk you through some of the most common causes of car stalling issues.

Fuel-related stalling causes

• Low fuel pressure— Low fuel pressure from a worn-out fuel pump starves the engine of fuel, causing it to lope, run rough, sputter, and stall.

A clogged fuel filter can cause stalling during hard acceleration or at higher speeds

• Low fuel volume— A worn-out fuel pump can output fuel at the correct pressure but not pump enough volume. This would show up as stalling at higher speeds or hard acceleration when the engine needs more fuel volume. In some cases, low fuel volume can cause a no-start or start and then stall when staring a cold engine. That’s because a cold engine needs a rich fuel mixture.

• Clogged fuel filter— Most late-model cars don’t have replaceable fuel filters like older vehicles. The fuel filter is in the tank. However, if it gets clogged, it can starve the engine at high speeds and hard acceleration.

• Fully or partially clogged fuel injectors— A single clogged fuel injector on a 4-cylinder engine can rob the engineof enough power that it will lope, sputter, and stall. One way to check

Computer or sensor-related issues that cause car stalling problems

• Faulty engine coolant temperature sensor (ECT)— The computer needs accurate engine temperature data in order to properly calculate the amount of fuel. If the sensor values are way off, the engine may get too much or too little fuel. A faulty ECT usually shows up as a no-start or hard start when the engine is cold.

• Faulty upstream oxygen sensor— The computer relies on the O2 sensor data to determine how well it’s doing when calculating the air/fuel ratio. If the sensor is slow, contaminated, or faulty, the computer can’t do its job, and the result is often car stalling problems.

Air induction problems that are related to stalling

• Carbon buildup in the throttle body— Carbon buildup can reduce the amount of air entering the system at idle speeds. If your stalling problem occurs mostly at idle, checking the throttle body for carbon buildup should be at the top of your list

• Idle air control [bypass] valve (IAC)— On early fuel-injected

Types of IAC valves used in early fuel-injected engines.

engines, the throttle plate is fully closed at idle. The IAC bypasses air around the closed throttle plate to supply enough air to keep the engine running when your foot is off the gas pedal. If the IAC isn’t working properly or the passages are clogged with carbon, you’ll experience a rough idle and stalling, more when the engine is cold than when it’s at operating temperature. Late-model engines don’t use an IAC because they have an electronic throttle body.

• Unmetered air— Unmetered air is just a fancy term for a vacuum leak. It can be caused by a cracked or broken vacuum line or by a torn or cracked air intake duct that runs from the air filter box to the throttle body.

Any air entering the engine that isn’t detected by the computer will cause performance issues, including a high idle speed, rough idle, or hesitation in acceleration. An air leak is most likely to cause stalling when the engine is cold.

A tear in air intake hose allows unmetered air into the engine

How to diagnose an air issue

Check the Idle air bypass valve and throttle body

If your vehicle has an IABP, check it for carbon buildup. Remove the IABP and spray the ports with throttle body cleaner to remove carbon deposits. Wipe off all deposits with a clean rag and dry with compressed air.

Next, open the throttle plate and check for carbon buildup in the throat of the throttle body. Carbon buildup can prevent the throttle plate from closing completely. In an engine equipped with an IABP, a partially open throttle plate will throw off the incoming air/fuel balance, causing car stalling. Carbon buildup in the throttle body usually doesn’t cause a problem on a cold start, but can easily cause stalling once the engine warms up and the PCM/ECM reduces idle speed and air/fuel mixtures to an “engine warm” setting. At that point, the carbon buildup may crack the throttle plate open enough that the IABP can no longer compensate for the extra air. Throttle body cleaning can correct this problem.

WARNING: The Throttle body cleaning procedure is different for electronic throttle bodies than for cable operated throttle bodies. Spraying excess throttle body cleaner into an electronic throttle body can damage the electronics, costing you several hundred dollars for a new throttle body.

For more information on cleaning an electronic throttle body, see this post.

Check for a vacuum or air duct leak

Examine the rubber/plastic air duct that runs from the air filter box to the throttle body. Any crack or hole in that duct will cause unmetered air to enter the engine, throwing off the air/fuel mixture and causing a rough idle, stumble, hesitation, poor gas mileage, and even car stalling. If you find a tear in the air duct, replace the entire duct. Do NOT try to glue, seal, or repair the duct. The repair will NOT last. Replacement air ducts are available from the dealer and dormanproducts.com.

If the air duct checks out, look for a disconnected vacuum hose. If you find one, reconnect it. Next, check for an intake gasket leak by spraying carb cleaner around the intake manifold gasket. A leak will cause the spray to get sucked in quickly. If you find a leak, get it repaired.

©, 2018 Rick Muscoplat