Rough idle
Idle hunts, shakes, or dips — especially when cold. Find the most likely causes and the quickest checks.
MED RISK
Common symptoms
- Idle hunting up/down
- Engine shakes at idle
- Stalls when coming to a stop
- Worse when cold or with AC on
Cost expectations
DIY: £0–£80 (cleaning + plugs). Garage: £80–£300 depending on diagnosis and parts.
Note: prices vary by car and region. Use this as a planning range.
Most likely causes (ranked)
- Vacuum / intake leak — Unmetered air makes the ECU chase the target idle.
- Dirty throttle body / idle control — Idle airflow becomes inconsistent.
- Ignition weakness (plugs/coils) — Minor misfires show up most at idle.
- Fuel delivery issue (injector leak/pressure) — Rich/lean swings cause hunting.
- MAF/MAP sensor drift — Bad load signal makes idle control unstable.
Quick checks (fast, low-cost)
- Listen for hissing; inspect intake hoses, PCV lines, and clamps.
- Check fuel trims (short/long). Big positive trims often mean air leak.
- Clean throttle body (if safe/allowed) and reset adaptions if your car requires it.
- Swap coils between cylinders if you also have a misfire code.
- Smoke test the intake if symptoms persist.
Recommended tools
These tools make diagnosis faster and stop you buying random parts.
Related fixes
If your symptoms don’t match perfectly, check these next.
What to do next
- Fix obvious vacuum leaks first (cheap wins).
- If trims are normal, prioritize ignition/fuel checks.
- If you have codes, resolve them before chasing rare causes.
AutoSolveHub helps you narrow possibilities. Always confirm critical repairs with proper testing and safe procedures.