Rough Idle
Common causes, likely diagnostic paths, OBD references, and repair-next steps for rough idle complaints.
Rough idle usually means combustion quality or airflow control is unstable at low RPM. The quickest separation is whether the engine is misfiring, running lean, or idling poorly because extra air is getting in or the throttle is not settling correctly.
Common Sounds or Signs
- Shaking or vibration while stopped
- Idle speed dips, hunts, or feels uneven
- Light stumble when shifting into gear
- Uneven exhaust note at idle
Quick Checks
- Scan for pending misfire, lean-condition, or high-idle codes before replacing parts
- Watch fuel trims and idle speed data at hot idle
- Inspect intake ducting, PCV plumbing, and vacuum hoses after the MAF
- Inspect spark plugs and ignition coils if the engine has a clear cylinder-drop feel
- Inspect the throttle body for carbon buildup or sticking
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
- Vacuum leak after the mass air flow sensor
- Dirty or sticking throttle body
- Worn spark plugs or weak ignition coils
- Contaminated or inaccurate MAF signal
- PCV leak or intake gasket leak
Likely Diagnostic Paths
- If idle speed hangs high or fuel trims climb lean at idle, inspect for vacuum leaks and throttle-body issues first.
- If the engine feels like one cylinder is dropping out, start with misfire codes and ignition or injector checks.
- If airflow and fuel-trim data look implausible together, verify the MAF signal before replacing parts.
Diagnostic Path
Pick the first inspection path.
Air Leak and Idle Control Path
Use fuel trims, smoke testing, and throttle behavior to separate vacuum leaks from throttle or MAF problems.
- Inspect intake ducting, PCV hoses, and vacuum lines
- Compare fuel trims at idle and light throttle
- Check throttle body condition before quoting replacement
Misfire Path
When rough idle feels like a cylinder drop, prove spark, injector, or compression before estimating parts.
- Check P0300-family codes and misfire counters
- Inspect spark plugs, coils, and injector connector fit
- Use compression testing when the same 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.
Throttle Body Replacement Cost
A strong next path when idle control stays unstable after inspection, cleaning, and relearn checks.
Mass Air Flow Sensor Replacement Cost
Relevant when scan data and intake inspection point to biased airflow readings.
PCV Valve Replacement Cost
Useful when crankcase ventilation is pulling in unmetered air and upsetting idle quality.
Intake Manifold Gasket Replacement Cost
A practical next path when smoke testing or bank-side leak checks point to an intake sealing problem.
Spark Plug Replacement Cost
Worth pricing when rough idle is tied to worn plugs, weak spark, or overdue tune-up history.
Explore Related Systems
Use when multiple systems overlap.