Symptoms
- Engine shakes or vibrates noticeably at stops
- RPM needle bounces instead of holding steady
- May stall at idle in more severe cases
- Possible check engine light
Likely causes
- CommonWorn or fouled spark plugsWeak spark causes inconsistent combustion, especially noticeable at low RPM.
- CommonVacuum leakUnmetered air entering the intake throws off the air/fuel mixture.
- CommonDirty throttle body or idle control valveCarbon buildup can disrupt the small amount of air needed to hold a steady idle.
- PossibleDirty or failing fuel injector(s)A clogged injector delivers uneven fuel to one cylinder.
- PossibleWorn ignition coilA weak or failing coil causes intermittent misfire on one cylinder.
What to do
- Get the check engine light scanned for misfire or lean-condition codes, even if it's not currently on.
- Listen and feel for whether it's a steady rough idle or an occasional stumble, which helps narrow the cause.
- Inspect vacuum hoses for cracks, looseness, or a hissing sound near the intake.
- Have spark plugs and ignition coils inspected, especially if they're overdue for replacement.
- Have the throttle body cleaned if it's dirty, using a throttle-body-safe cleaner.
- Have a shop run a smoke test to find hard-to-spot vacuum leaks if the above doesn't fix it.
Tools and parts
- OBD-II code reader
Spark plugsSearch: “spark plugs for [year make model]”
Throttle body cleanerSearch: “throttle body cleaner”
When to call a pro
A shop with a smoke machine can find small vacuum leaks that are very hard to locate by ear or sight alone.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.