Engine Stalls At Idle
Mechanic-first idle stall workflow for airflow, throttle body, vacuum leak, and fuel trim checks before parts replacement.
An engine that stalls at idle needs the idle air, fuel trim, and load-control story separated before a throttle body, sensor, or fuel part is quoted.
Common Sounds or Signs
- Idle drops and engine dies at stops
- Stall when shifting into drive or turning on A/C
- Rough idle, surge, or low idle speed
- Restart may be immediate or require throttle input
Quick Checks
- Scan for lean, throttle, MAF, idle, and misfire codes
- Inspect intake boots, vacuum hoses, and PCV leaks
- Check throttle body deposits and idle relearn needs
- Review fuel trims at idle versus raised RPM
- Check charging voltage if idle stalls with electrical load
Inspection Priority
- Check for vacuum leaks and unmetered air first
- Compare fuel trims at idle and cruise
- Inspect throttle body condition before replacing it
Common Causes
- Dirty throttle body or idle airflow issue
- Vacuum leak or PCV fault
- MAF signal skewing fuel control
- Weak fuel delivery or misfire at idle
Diagnostic Path
Pick the first inspection path.
Air and Throttle Path
Use when trims, idle speed, or throttle deposits point to airflow control.
- Inspect throttle plate and bore
- Smoke test intake and vacuum hoses
- Check idle relearn requirements after cleaning
Fuel Trim and Misfire Path
Use when the stall comes with lean data, rough running, or load sensitivity.
- Compare short and long fuel trims
- Inspect MAF data and intake sealing
- Check misfire counters and fuel pressure if needed
Next Best Step
Inspect before expanding the quote.
Inspect ignition coils
Check coil boots, carbon tracking, and whether the miss follows a swap.
Check spark plugs
Inspect gap, fouling, wear, oil, coolant, and plug-well condition.
Verify injector operation
Move to injector balance, pulse, or leak-down checks if the misfire stays.
Check compression if needed
Use compression or leak-down testing when spark and fuel checks do not move the fault.
Related Inspection
Recommended Next Repair Paths
Compare repair paths before replacing parts.
Throttle Body Service
Use when deposits, idle relearn, or throttle airflow evidence supports it.
Vacuum Leak Diagnosis
Strong path when idle trims are lean or smoke testing finds unmetered air.
Fuel Trim Diagnosis
Use before replacing sensors when scan data is the main clue.
Explore Related Systems
Use when multiple systems overlap.