Vibration at idle
Where the noise comes from
the engine bay
Do not put it off: check within days
Every car trembles a little at idle, a diesel especially. This is about something else — a shake you feel in the steering wheel, the seat and the rear-view mirror, one that was not there before. Its most common cause is the engine mounts: the rubber blocks that hold the engine in the body and damp its movement. Over the years the rubber hardens and cracks, and the vibration the mounts used to absorb goes into the body.
The second group of causes is the engine itself running unevenly. Misfires (when one cylinder keeps failing to fire — usually because of the spark plugs or coils), unmetered air leaking through a cracked hose or an intake gasket, a dirty throttle body or injectors all make the idle unsteady. Telling these apart from the mounts is not hard: with them the needle on the rev counter trembles or wanders and the check-engine light often comes on, while with worn mounts the engine runs smoothly — it is the body that shakes.
Possible causes
| Cause | How likely | Danger |
|---|---|---|
| Engine mounts | Most often — especially on cars older than 8–10 years | shop this week |
| Misfires: spark plugs, coils | Common — if the revs tremble and the check light is on | shop this week |
| An air leak through a hose or intake gasket | Common — if the revs wander, sometimes with a hiss | shop this week |
| A dirty throttle body or injectors | Common past a hundred thousand kilometres | safe to drive |
Can you keep driving
In most cases you can drive: vibration at idle is not the symptom that leaves a car on the hard shoulder. With worn mounts you have a week or two to calmly book a garage; dragging it out for months is unwise — broken mounts add knocks when moving off and shifting and speed up wear on the parts around them.
A separate story is an engine that is clearly misfiring: shaking in jerks, the check light flashing or lit, power down. You can drive with that too, but gently and not far: unburnt fuel from misfires finishes burning in the exhaust and overheats the catalytic converter, and replacing it is expensive. In that case the diagnosis is needed within days, not whenever.
What to check yourself
- The mount test: engage a gear (D on an automatic) and hold the brake. Vibration from worn mounts usually grows noticeably in this position.
- Watch the rev counter: the needle steady while the car shakes argues for the mounts; a needle that trembles or wanders means the engine is running unevenly.
- Switch the air conditioning on and see whether the shake changes: under the extra load weak mounts and an uneven idle show up more clearly, and that detail is useful to the mechanic.
- Without removing anything, look over the thin rubber hoses under the bonnet: cracks and hoses that have slipped off are a frequent place for air leaks.
- Recall when the spark plugs were changed: mileage well past the interval makes them the first suspect for misfires. If the check light is on, start with a computer diagnosis: the fault codes narrow the search.
Narrow the cause down with questions
Answer a few questions and the diagnostic tree will shorten the list of causes for your case.
How the app helps
The Pro-Stuk app asks the same questions — does the shake grow with a gear engaged, do the revs hold steady, is the check light on — and turns the answers into likely causes with percentages. The report gives a plain conclusion: drive on calmly, book a visit this week, or show the car to a mechanic within days.
Common questions
Why does it shake only at idle while the car drives smoothly?
At idle the revs are at their lowest and the engine rocks on its mounts at a frequency the body passes readily into the cabin. As the revs rise the movements become smaller and are felt less. That is why worn mounts give themselves away at the traffic lights rather than on the motorway.
What is a misfire?
It is when the mixture in one of the cylinders fails to ignite from time to time — most often because of worn spark plugs or a faulty ignition coil. At that moment the engine loses part of its power and shudders, while the unburnt fuel finishes burning in the exhaust and overheats the catalytic converter.
Will cleaning the throttle body help?
It will if that is the cause: deposits get in the way of metering air accurately at idle, and the revs become uneven. But cleaning is not a universal cure: with worn mounts or misfires the vibration will still be there afterwards.
The app for iPhone and Android
The questionnaire, the sound recording and the full report with probabilities are all in the Pro-Stuk app.
Installed from a store, the app updates itself.