Read the guide: Vibration frequency explained
How to test your car at idle
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Step 1: Drive until the engine is fully warm. A cold engine idles faster and can hide the problem.
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Step 2: Park on level ground outdoors, set the parking brake, shift to Park (or neutral with a manual) and turn the AC off.
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Step 3: Read the idle speed on the tachometer, for example 750 RPM, and enter it with your number of cylinders.
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Step 4: Place the phone flat, screen up, on the center console or the seat. Tap Start (on iPhone, tap Allow), then keep your hands off the phone for 30 seconds.
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Step 5: Measure again with the AC on. Then, with an automatic, measure in Drive with the driver's foot firmly on the brake.
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Step 6: Compare the three results. A big jump in Drive points to worn engine or transmission mounts.
What your result means
Your result lands on one of four levels. This page uses the machines and appliances scale, which is based on RMS velocity in millimeters per second (mm/s).
| Level | Range (RMS velocity) | What to do |
|---|---|---|
| Smooth | Under 0.71 mm/s under 0.028 in/s | Your engine idles smoothly. A faint buzz in the seat is normal. |
| Normal | 0.71 to 1.8 mm/s 0.028 to 0.071 in/s | A normal idle for most cars, especially with the AC on or in Drive. Save the result so you can compare later. |
| Rough, check it | 1.8 to 4.5 mm/s 0.071 to 0.177 in/s | More than a healthy idle. Look for a misfire (check engine light, a rhythmic stumble) and worn engine mounts, especially if it's worse in Drive. |
| Too much vibration | 4.5 mm/s or more 0.177 in/s or more | A strong idle shake. Have it checked soon. If the check engine light is flashing, avoid driving: a severe misfire can damage the catalytic converter. |
Reading your car vibration at idle result
Car vibration at idle comes from the engine’s own rhythm, and the frequency tells you which part of that rhythm is strongest. The tool works out two numbers from what you entered:
- Engine rotation: RPM ÷ 60. At 750 RPM that’s 12.5 Hz.
- Firing frequency (four-stroke): RPM ÷ 60 × cylinders ÷ 2.
| Engine at 750 RPM | Rotation | Firing frequency | Can a phone see the firing? |
|---|---|---|---|
| 3 cylinders | 12.5 Hz | 18.75 Hz | Yes |
| 4 cylinders | 12.5 Hz | 25 Hz | Yes, close to the limit |
| 5 cylinders | 12.5 Hz | 31.25 Hz | No |
| 6 cylinders | 12.5 Hz | 37.5 Hz | No |
| 8 cylinders | 12.5 Hz | 50 Hz | No |
A phone measures up to about 30 Hz. For a five-cylinder engine, a V6 or a V8, the tool flags that the firing frequency is out of reach, so rely on the strength and on the Park versus Drive comparison. A pulse that fast can also show up as a false lower frequency. At 60 samples a second, a V8’s 50 Hz firing can look like 10 Hz.
One more pattern is worth knowing. Each cylinder fires once every two turns of the engine. So when one cylinder misfires, the stumble repeats at half the rotation speed: about 6 Hz at 750 RPM. A clear peak there often means a misfire.
Common causes and fixes
Worn engine or transmission mounts
Rubber mounts hold the engine and soak up its shake. When they crack or sag, the shake comes straight into the cabin. Signs: much worse in Drive than in Park, and a clunk when you shift or pull away. A simple engine mount vibration test: in Park with the hood open, a helper stands to the side, not in front, and watches the engine while the driver briefly revs it. A big rock or jump suggests a worn mount.
Misfire
A misfire feels like a rhythmic stumble, often with the check engine light on. A scan tool shows codes starting with P030, and the last digit often names the cylinder. Many auto parts stores read codes for free. Common fixes are spark plugs, ignition coils and fuel injectors.
Low or uneven idle
A dirty throttle body, a cracked vacuum hose or a failing sensor can make the idle low or uneven. Cleaning the throttle body and checking hoses are common first steps.
Extra load
The AC compressor, alternator and cooling fan all load the engine. A small increase with the AC on is normal. A big one points back to the mounts or idle control.
See a mechanic soon if the check engine light is flashing, if the shake comes with overheating or a burning smell, or if you hear a clunk every time you shift.
What this tool can’t measure
- Firing frequency on V6 and V8 engines, or anything above about 30 Hz. You’ll see “The main vibration may be faster than your phone can measure.”
- Which mount is worn or which cylinder misfires. That takes a visual check or a scan tool.
- Readings from different spots. Seat cushions soften the shake, so always put the phone in the same place when you compare.
- What’s normal for your model. Diesels and small engines shake more by design. Compare with your own earlier results or the same model.
- A certified rating. The bands come from machine vibration guidance (ISO 10816-1), so for a car they are a rough guide. Treat every reading as a good guide, not a certified measurement.
Frequently asked questions
Is some vibration at idle normal?
Yes. Every piston engine makes some vibration, and a light buzz in the seat or steering wheel is normal. It should feel steady and even, not lumpy. Diesels and engines with fewer cylinders tend to shake more.
Why does my car shake more in Drive than in Park?
In Drive, the engine pushes against the transmission while the brakes hold the car still, so the mounts carry more twist. A slight increase is normal. A big jump is a classic sign of worn engine or transmission mounts.
How do I tell engine mounts from a misfire?
A misfire feels like a rhythmic stumble and often turns on the check engine light. Worn mounts feel like a steady shake that gets much worse in Drive or when you pull away, often with a clunk. The frequency helps too: a peak near half the engine rotation speed hints at a misfire.
Why does my car shake when the AC is on?
The AC compressor adds load, so the engine works harder at idle. A small change is normal, and many cars raise the idle a little to cope. A big shake with the AC on points to weak mounts or an idle control problem.
Can I test a hybrid or a car with auto start-stop?
Only while the engine is actually running. Turn start-stop off if your car allows it. A hybrid's engine often shuts off when you stop, so there may be nothing to measure at idle.
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