Car Running or Idling Rough: Causes & Fixes
Reviewed by the Wrenchit.ai editorial team · Last reviewed · Editorial standards
Causes ranked by likelihood
| Cause | Likelihood |
|---|---|
| Worn spark plugs — Most plugs need replacement at 60–100k miles (iridium can go further). Cheap, easy DIY on most cars. | Most likely |
| Failed ignition coil — Causes a single-cylinder misfire (P0301–P0308). Swap with another coil — if the misfire moves, replace. | Possible |
| Vacuum leak — Hissing sound, lean codes (P0171/P0174), high idle. Often a torn intake boot or cracked PCV hose. | Possible |
| Dirty throttle body or idle air control valve — Carbon buildup on the throttle plate. Clean with throttle body cleaner — $10 fix on older cars. | Less likely |
| Dirty or failed fuel injector — Causes consistent misfire on one cylinder that doesn't move when you swap coils. | Less likely |
How to narrow it down
- Note when it happens. Record speed, temperature, and whether braking, turning or accelerating triggers it.
- Scan for codes. Plug in an OBD-II scanner — a stored code narrows the cause quickly.
- Check: Dirty throttle body or idle air control valve. Carbon buildup on the throttle plate. Clean with throttle body cleaner — $10 fix on older cars.
- Check: Worn spark plugs. Most plugs need replacement at 60–100k miles (iridium can go further). Cheap, easy DIY on most cars.
- Check: Vacuum leak. Hissing sound, lean codes (P0171/P0174), high idle. Often a torn intake boot or cracked PCV hose.
- Check: Failed ignition coil. Causes a single-cylinder misfire (P0301–P0308). Swap with another coil — if the misfire moves, replace.
- Check: Dirty or failed fuel injector. Causes consistent misfire on one cylinder that doesn't move when you swap coils.