Troubleshooting / Why Does My Engine Stall at Idle?

• Why does my engine stall?
• Why is my engine running rough?
• Why won't my engine crank over?
• Why does my engine have low power output?
• Why does my engine hesitate?
• How an engine works
• How to check compression
• How to retrieve trouble codes
• How to test engine vacuum
• How to test fuel delivery system
• Engine cranks but wont start
• Engine cranks excessively
• Engine stalls
• Engine vibration
• Engine has excessive smoke
• Rapid ticking sounds when engine is cranked
• Engine has lean mixture codes
• Engine idles high
• Engine idles low
• How to test a fuel injector
• Engine is making excessive noises
• How to tune up your engine
• How to jump start your engine
• How to change engine oil and filter
• How does a MAF mass air flow sensor work?
• How to test an oxygen sensor

Why Does My Engine Stall at Idle?

Introduction

At idle your engine produces the least amount of emissions than in any other time when it is operating. Engine idle is necessary for proper vehicle operation; it allows the engine to operate at the lowest RPM level possible without your foot on the gas pedal.  Before we start we need to know one of two things is the engine running poorly causing the engine to stall at idle or is the engine running fine and it's just the idle condition that is the problem. If your engine is running rough please visit, why does my engine run rough? If your engine cranks over but won't start visit why won't my engine start? If your engine won't crank over visit why won't my engine crank over? If your engine is running ok but it stalls at idle you are in the right place. Below we have created a guide to aid the diagnoses and repair procedure for most common engine stalling at idle problems.

READ COMPLETELY BEFORE BEGINNING!

Car Repair Guide

• Step 1 - Anytime you have a problem with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, Air Bag) inspect all fuses using a test light and check the under hood power distribution center and under dash fuse panels. If all fuses test ok continue to the next step. How do I check my car fuses?
   
  
• Step 2 - To check for problems with electronically controlled components such as an engine, transmission, ABS brake, or SRS (supplemental restraint system, Air Bag) and the fuses test ok a trouble code scan is needed to identify any system trouble. Use a simple scanner tool to retrieve trouble codes and check if they relate to the specific problem, like an ignition coil failure code. If a trouble code is present but does not pertain to the immediate problem like an EVAP code ignore it until a later time after the engine is running properly. The reason we repair non-related codes after the engine is running properly is because sometime false codes can be triggered by a rough running engine. Once the engine is running properly the code present might cycle and turn itself off. You might say "if the engine isn't running right shouldn't it have a check engine light and a trouble code?" Sometimes conditions occur that will not be detected by the computer, example: if the intake or exhaust valve operation fails the computer cannot detect the failure because the problem is not sensor related, so the engine doesn't run smooth and the computer thinks everything is ok with no codes. If the trouble code retrieved relates to a cylinder misfire like an injector driver or ignition coil failure first repair these problems and re-test system. If no trouble codes are present proceed to the next step. How do I scan my service engine soon light?
   
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  Check Engine Light Scan for Trouble Codes Video

• Step 3 - Check for broken or dilapidated vacuum hoses or air intake boot on and around the engine, your car's engine is designed to run on a system that can hold vacuum. Vacuum hoses are typically connected to the engine intake manifold and will supply engine vacuum to various accessories like power brakes. Some cars are designed with a larger vacuum transfer hose that connects the intake manifold to the IAC (idle air control) motor. If broken or dilapidated these vacuum lines can cause the engine to lose vacuum which will allow the engine to run rough and stall. Inspect all engine and accessory vacuum lines look for missing, torn or dilapidated lines and replace as needed. Also have a helper rest their foot on the gas pedal just enough to keep the engine running and double check the engine when it is running to listen for any whistling noise coming from the engine that is not usually present. Follow the noise and inspect vacuum lines in that area, also when the engine is running it will pull inward a broken or weak piece of the hose to create a larger vacuum leak. Check the integrity of all vacuum hoses at each end of the hose, typically this is where a vacuum hose fails. If all vacuum hoses check "ok" proceed to the next step.
  
  
  Failed Air Intake Boot
   

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• Step 4 - The engine throttle body is designed to control the air intake into the engine; along with the throttle body there is an IAC motor which controls engine speed by regulating idle air volume. When your foot is off the gas pedal the car's computer adjusts this motor to allow more or less air to the engine to control engine idle speed. When this system develops coking (a light tar condition from mass amounts of air being processed) it will cause a poor engine idle quality and cause stalling. A throttle body and IAC motor service is required. (Note any car that is designed with a magnetic non-motor operated IAC like Lexus is subject to carbon and should be cleaned about every 40,000 miles to avoid stalling.)
   
• Step 5 - Some car's are designed with an air intake boot, this boot is used to transfer engine intake air from the MAF (mass air flow) sensor to the engines throttle body which leads to inside the engine. When an air intake tube fails it allows non-metered air into the engine throwing off the engine management system and causing the engine to stall at idle. Inspect the air intake boot by removing it and flexing the tube while looking at the folds in the flex joints, this is usually an easy task to perform. There should be no rips or tears in the tube, if so replace the tube with a new unit and recheck system.  If the air intake tube checks "ok" proceed to the next step.
   
• Step 6 - An IAC (idle air control) motor is designed to adjust the engine idle RPM speed by opening and closing an air bypass passage inside the throttle body. An IAC motor can fail one of two ways, either the motor short circuits and stops working or the motor will develop high resistance and cause the IAC control motor to react slowly, either failure will cause the engine to stall at idle. When a trouble code scan is performed it sometimes won't always detect a failed or weak IAC motor. To check the IAC motor remove the unit, with the wires connected turn the key to the "on" position without starting the engine, the IAC should move in or out. If the IAC motor does nothing it has probably failed, replace it with a new unit and recheck system. Note: while the IAC motor is removed clean (use aerosol carburetor cleaner) the passages the IAC uses to control idle air speed. If the IAC valve checks "ok" proceed to the next step.
   
• Step 7 - Most cars are designed with an EGR valve (exhaust gas recirculation) that allows exhaust gas to enter the engine intake manifold at cruising sped. This EGR valve is used to help control the engine's production of poisonous NOX (NO2) gasses by lowering the engine's combustion chamber temperature. The EGR valve is designed to only open when the engine is under power and not at idle. This valve is subject to carbon from the exhaust system and can cause the valve to stick open and allow the exhaust gasses into the engine simulating a vacuum leak. To check for this condition, remove the EGR valve, at the mounting flange of the valve there will be 2 main ports an intake and an outlet port. Air should not be able to flow from the inlet to the outlet, if air does flow though the valve it is stuck or jammed with carbon, clean or replace as needed.
   
  
• Step 8 - If your car is equipped with an automatic transmission the torque converter can cause the engine to stall. The fluid coupler known as the torque converter is constructed with clutch shoes that lock up to connect the engine at cruising speeds. If the torque converter malfunctions it can cause the clutch shoes not to release. This will cause the engine to stall at idle because the torque converter will not release, forcing the engine to stall. This condition will usually start to occur after long a drive on the freeway and get worse from there. If your car shutters as it is coming to a stop and then stalls you could have this problem. If you are coming to a stop and you think this is your problem try putting the car into neutral gear and see if it still stalls. If the engine does not stall when in neutral the torque converter has failed and needs replacing. Unfortunately the only way to replace the torque converter is to remove the transmission. (Always flush the transmission fluid and replace the filter anytime major work is performed)

(Note: cars with automatic transmissions are equipped with a torque converter that when it fails will cause the engine to stall, but only when you put the transmission into forward or reverse gear.)

WARNING! Always have the vehicle under inspection on level ground, in park with the emergency brake on. Always wear protective eyewear, gloves and necessary clothing before inspection or work begins. Never crank an engine over when anyone is near the battery or engine. Always have an operational fire extinguisher close by, obey all first aid instructions in the event of an accident. Never stand in front or behind a vehicle when cranked over or running. When engine is cranked over keep hands and clothing free from rotating components.

Troubleshooting / Why Does My Engine Stall at Idle?

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Additional Resources

• Why does my engine stall at idle?
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• Why does my engine crank over but not start?
• Why won't my engine crank over?
• How do I test the engine ignition system?
• Why does my engine over heat?

Check Manufacturer Specific Engine Stalls at Idle Questions and Answers

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