A shake that only shows up when you're stopped is the engine's way of saying it can't hold a smooth rhythm. At idle there's nothing - no wheels, no wind - to mask vibration, so even small misfires or air leaks feel huge. Here are the most likely causes.
If the shake is mild and the check engine light is steady (not flashing), you can drive normally for a few days while you diagnose. A flashing CEL means a live misfire is destroying your catalytic converter - pull over.
Multiple cylinders firing weakly is the #1 cause of an idle shake. Almost always spark plugs (60k+ miles) or one weak ignition coil.
One dead cylinder is unmistakable at idle - the engine has a noticeable beat to it. Swap that coil to another cylinder. If the misfire follows, you found it.
Cracked vacuum hoses, leaky intake gaskets, or a torn PCV diaphragm let unmetered air in. The engine runs lean and rough only at idle (the leak gets masked at higher RPM).
Carbon on the throttle plate creates uneven airflow. The idle hunts and shakes. Clean with throttle body cleaner ($7) and a rag.
No code, but the engine visibly rocks while in Drive. Cheaper mounts are $80, dealer mounts can be $400+. A torn mount lets normal engine vibration into the cabin.
| If you notice... | ...most likely cause |
|---|---|
| Worse in Drive than Park | Motor mount or transmission load - confirms a mount issue |
| Goes away above 1500 RPM | Vacuum leak (leak gets diluted as airflow increases) |
| Steady beat (thump-thump-thump) | One cylinder misfire - check coils first |
| Random shake with no rhythm | Multiple-cylinder misfire (P0300) - spark plugs are the usual cause |
| Only when AC is on | IAC valve or alternator can't hold idle under load |
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If your scan tool shows one of these alongside this symptom, that's your starting point. Click any code for the full diagnosis, common causes, and repair costs.
At idle, no other vibration sources (wheels, transmission, road noise) mask the engine's natural pulses. Once you're moving, even noticeable misfires get drowned out.
Yes. Gasoline that's been sitting more than 6 months loses its volatility. It burns unevenly and can mimic a misfire. Run the tank low and refill with fresh fuel.
If you have 60k+ miles on the plugs, replace them first - they're cheap and almost always the cause. If new plugs don't fix it, swap the coil from the dead cylinder to a working one and see if the misfire moves with it.
Only if the issue is the throttle body or IAC valve and you just cleaned it. The procedure resets the ECU's idle position - the steps vary by make. It won't fix a misfire or motor mount.
Usually no - a clogged cat causes power loss and hot exhaust, not idle shake. But a bad cat will often be the result of a long-ignored misfire, so always fix the misfire first.
$10 for a vacuum hose, $30-$80 for plugs and coils DIY, $80-$250 for a motor mount, up to $600 if you have a shop do plugs + coils on a V6/V8. Worst case (cat damage from neglected misfire): $1,500.