Stalls
General stalling workflow for idle stalls, decel stalls, hot stalls, starts-then-dies, and intermittent shutdowns.
A stalling complaint should be tied to the condition where it happens: hot restart, idle in gear, decel, bumps, fuel level, A/C load, or highway cruise. Testing should capture what drops out first: RPM signal, fuel pressure, spark, throttle/idle control, power feed, or communication.
Common Sounds or Signs
- Engine shuts off at stop lights
- Starts then dies
- Stalls when hot and restarts after cooling
- Stalls when turning on A/C or shifting into gear
- Stalls over bumps or during deceleration
Quick Checks
- Scan all modules and save freeze-frame data for stall-related codes
- Monitor RPM signal, fuel pressure, throttle angle, idle command, battery voltage, and MAF/MAP data during the stall
- Inspect battery terminals, engine grounds, PCM power relay, and harness areas disturbed by engine movement
- Check throttle body cleanliness, idle relearn needs, vacuum leaks, and purge valve leakage for idle stalls
- Use heat, vibration, and wiggle testing only after a safe baseline inspection
Inspection Priority
- Confirm the symptom, code, or inspection evidence before replacement.
- Check related systems when the failure pattern is not isolated.
Common Causes
- Crankshaft position sensor or RPM signal dropout
- Fuel pump pressure loss, relay issue, or restricted fuel delivery
- Dirty throttle body, idle control issue, or vacuum leak
- EVAP purge valve stuck open causing rich idle or starts-then-stalls
- Poor power, ground, relay, or intermittent connector fault
- MAF/MAP signal error or engine mechanical issue
Diagnostic Path
Pick the first inspection path.
Dropout Path
Find whether RPM, spark, fuel, PCM power, or communication disappears first.
- Watch cranking/running RPM signal
- Check fuel pressure during the stall
- Monitor ignition and injector command
Idle Air and Load Path
For stalls at stops or with A/C load, inspect airflow, throttle, purge, and vacuum leak behavior.
- Compare commanded and actual idle
- Check purge valve sealing at idle
- Smoke test intake and PCV leak paths
Next Best Step
Inspect before expanding the quote.
Smoke test EVAP system
Use smoke testing when the leak source is not obvious.
Verify purge sealing
Check purge command and sealing before replacing the valve.
Check vent operation
Command the vent valve and inspect canister-side blockage or contamination.
Review fuel trims
Compare trims at idle, 2500 RPM, and cruise before pricing sensors.
Related Inspection
Recommended Next Repair Paths
Compare repair paths before replacing parts.
Intermittent Stall Diagnosis
Use when the fault must be captured under heat, vibration, or load.
Fuel Pump Replacement
Use only after pressure, command, and circuit testing confirm fuel delivery loss.
Explore Related Systems
Use when multiple systems overlap.