Mechanics

Stalls

Easy step by step guide on how to repair an automotive engine that stalls either while driving or when at idle, this information pertains to most vehicles.

Difficulty Scale: 5 of 10

Begin with the vehicle on level ground engine off (cold) with the emergency brake set.

Stalls While Driving - (Stalls at Idle Below)

Step 1 - The computer system designed for a vehicle's engine can detect problems in the management system by illuminating the "check engine" warning lamp, this step pertains to engine stalling while driving and at idle. Learn more


Check Engine Light MIL

Step 2 - A crankshaft angle sensor is used to give feedback information to the engine computer (PCM) when this sensor shorts the engine stalls, which is not always picked up by the computer. Learn more


Removing Failed Crankshaft Sensor CAS or CKS

Step 3 - A fuel pump is designed to supply the fuel injection system with an adequate amount of fuel volume and pressure while the engine is running, when this pump fails the engine will stall. Learn more


Failed Fuel Pump

Step 4 - Monitoring the fuel injector pulse can help narrow the problem down, if injector pulse stops, the engine will stall. The following test will need to be modified using longer test wires and locating the light portion of the test inside the vehicle to be observed while driving. Learn more


Monitoring Fuel Injection System

Step 5 - The computer system uses a steady supply of voltage to operate, if the main system relay or ignition switch fails the engine will stall, this problem could be intermittent, once the electrical connections cool down the system with operate again until heated. Intermittent stalling can be a frustrating problem to find, but if a test light is located inside the passenger compartment with the leads connected to a suspected failed circuit such as the ignition switch it can be observed by the driver, if the engine stalls and the light is still illuminated, the circuit is okay, continue testing. Learn more


Test Computer System Relay

Step 6 - A system fuse is designed to protect an electrical circuit is case of a short circuit, but when a fuse ages it can become loose causing an unstable supply of electricity. If a fuse fails the engine will stall, if the problem is intermitted test the fuses with the engine running and employ the "wiggle test" which means wiggle all main fuses and relays to see if the engine stalls then repair the connection. Learn more


Wiggle Test

Step 7 - A catalytic converter is designed as part of the vehicle's emission system which can break apart internally effectively plugging the exhaust system causing the engine to stall. To check for this condition start with the engine cold, take a rubber mallet and gently strike the catalytic converter, if it sounds like rocks inside, remove to inspect for internal damage. Learn more


Broken Catalytic Converter

Step 8 - Good battery connections are mandatory for the electrical system to stay healthy and operate correctly, if the battery cables are loose or corroded it can cause the engine to stall. Learn more


Battery Terminal Corrosion

Step 9 - There are a variety of sensors that provide feedback data for the engine management computer PCM (powertrain control module) and when they short circuit can cause the computer to shut down, sensors most prone to failure deal with extreme heat like an oxygen or coolant sensor. Learn more


Failed Oxygen Sensor

Step 10 - The main computer system or PCM (Powertrain control module) controls fuel, ignition, transmission and emission systems and when it fails will cause the engine to stall. (Note: When replacing a computer re-programming is necessary before installation.) Learn more


Computer - Powertrain Control Module (PCM)

Step 11 - Engine oil weight is important on today's close tolerance engines, if the incorrect weight oil has been installed the engine will hold open the valves by not allowing the lifters to bleed down which causes the engine to stall.


Wrong Weight Oil

 

Stalls at Idle

Step 1 - Vacuum hoses are meant to transfer engine vacuum to various accessories such as a brake power booster, when these hoses break or crack and fall off it will cause the engine to lose vacuum and stall at idle. Learn more


Broken Vacuum Line

Step 2 - An air intake boot is used to transfer air from the mass air flow sensor to the throttle actuator, if this boot becomes torn or disconnected it will cause the engine to stall at idle. Learn more


Air Intake Boot

Step 3 - On an older engine, an idle air control motor is responsible for setting the idle speed of the engine, when this item fails it will cause the engine to stall at idle. Learn more


Idle Air Control Valve

Step 4 - Coking is a condition that naturally occurs in the intake system of over time. This little tar like substance impedes air flow which disrupts the integrity of the computer system and can cause the engine to stall, and may have hard starting as well. Learn more


Coking Condition

Step 5 - Older engine's are designed with an EGR valve (exhaust gas recirculation) that allows exhaust gas to enter the intake manifold at cruising speeds. When this valve sticks open, it allows exhaust gases to enter the intake system effectively causing a large vacuum leak which will cause the engine to stall at idle. Learn more


EGR Valve

Step 6 - Automatic transmissions are equipped with a torque converter that can fail which will cause the engine to stall by not allowing the engine to become disengaged from the engine at a stop, this condition will usually start to occur after long a drive on a freeway, the car will shutter as it stalls coming to a stop. Learn more


Torque Converter Cutaway

Helpful Information

A shorted sensor doesn't always set a check engine light or MIL (malfunction indicator light). The computer is programmed to trigger a "service engine soon light" when the sensor has failed for a predetermined amount of time, say one minute, it only takes an intermitted shorting sensor one second to stop the computer system (stall engine). To troubleshoot this problem, temporally disconnect major sensors one at a time and retest ie: oxygen, coolant and MAF. (This will cause a service engine soon light, codes will need to be cleared after testing). When troubleshooting a stalling problem additional codes maybe present such as an EVAP code which should be ignored until the more major problems are repaired. Not all electrical problems can be detected by the computer.

Many electronic components are used simultaneously while using electrical contacts to operate various systems which can make diagnosing stalling problems tedious. An electrical circuit that is starting to fail will do so more when the circuit is warm or hot. Signs of heated or melted connections have high resistance and could cause a shorted connection.


Corroded Connection

The majority of the engine stalling problems will be electronic based, while some problems are obvious such as a loose connector, others can be difficult to detect such as an internally shorted wiring harness, though rare, it does happen.


Shorted Wiring Harness

A poor running engine is more apt to stall until the issue is resolved. Learn more

AUTHOR


Written by
Co-Founder and CEO of 2CarPros.com
35 years in the automotive repair field, ASE Master Technician, Advanced Electrical and Mechanical Theory.


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Article first published (Updated 2015-02-28)