Engine Misfire at Idle
Common causes, likely diagnostic paths, OBD references, and repair-next steps for engine misfire at idle.
A misfire at idle usually means one cylinder is not burning the mixture cleanly at low RPM. The fastest separation is whether the fault follows ignition parts, stays with one injector or cylinder, or points to a shared air-fuel problem.
Common Sounds or Signs
- Sharp shake or stumble while stopped
- Uneven exhaust pulse at idle
- Light pop through the exhaust or intake
- Flashing check engine light if the misfire gets severe
Quick Checks
- Scan for P0300 through P0304 and note whether the fault is random or cylinder-specific
- Swap the suspect coil or spark plug if the code is tied to one cylinder
- Inspect injector connector fit and listen for injector operation on the affected cylinder
- Check fuel trims and inspect for vacuum leaks if multiple cylinders misfire at idle
- Run compression testing if the same cylinder keeps misfiring
Inspection Priority
- Inspect ignition components first when misfire evidence is present.
- Verify fuel trim behavior before replacing parts.
- Check for vacuum leaks when misfires are random or lean-related.
Common Causes
- Worn or fouled spark plug
- Weak ignition coil or coil boot
- Faulty or restricted fuel injector
- Vacuum leak near one intake runner or across the intake
- Low compression or a sealing problem in one cylinder
Likely Diagnostic Paths
- If one cylinder keeps showing up, start with plug and coil swap testing before moving to injector or compression checks.
- If multiple cylinders misfire at idle, shared ignition, airflow, or fuel-delivery problems move higher on the list.
- If the check engine light flashes, repair the active misfire quickly to reduce catalyst damage risk.
Diagnostic Path
Pick the first inspection path.
Ignition Proof Path
Start with plug condition and coil behavior when the misfire is cylinder-specific or load-sensitive.
- Review P0300-family codes and misfire counters
- Inspect plugs, boots, oil intrusion, and carbon tracking
- Swap coils only when access and cylinder pattern support it
Fuel and Air Path
If ignition does not prove the fault, move to injector, vacuum leak, fuel trim, and compression checks.
- Listen for injector operation and inspect connector fit
- Smoke test when trims point lean at idle
- Run compression testing when one cylinder stays weak
Common Next Steps
Quick checks before expanding the estimate.
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.
Spark Plug Replacement Cost
A strong next path when the misfire follows the plug or plug condition is clearly worn or fouled.
Ignition Coil Replacement Cost
Use this path when the misfire follows the coil or coil output is clearly weak.
Fuel Injector Replacement Cost
Relevant when the misfire stays with one cylinder after ignition checks and injector testing confirms a fuel fault.
Intake Manifold Gasket Replacement Cost
Useful when smoke testing or runner-area leak checks show idle misfire is being caused by unmetered air.
Explore Related Systems
Use when multiple systems overlap.