Rough Idle — Causes & How to Fix It

What's happening

At idle, the engine is producing just enough power to run accessories — a small change in airflow, fuel, or ignition has a big effect. Rough idle (shaking, unstable RPM, occasional near-stalls) usually traces to a vacuum leak, a bad spark plug or coil, a clogged injector, or a dirty/failing idle air control system.

You might also notice

• Engine shaking visible from outside
• Tachometer needle wandering at idle
• Occasional stall when stopping
• Vibration in the steering wheel at lights

Likely causes (most common first)

1. Vacuum leak (cracked PCV hose, split intake boot, leaking intake manifold gasket)
2. Worn or fouled spark plug
3. Failing ignition coil
4. Dirty/failed mass airflow (MAF) sensor
5. Carbon-clogged throttle body or idle air control valve
6. Clogged or dead fuel injector
7. Low or contaminated fuel

What to check first

1. Listen for hissing at idle near the intake — vacuum leak telltale
2. Read codes (P0171, P0300-series, P0507 are common)
3. Inspect the PCV hose and intake boot for cracks
4. Pull and inspect spark plugs at the recommended interval

Common OBD2 codes for this symptom

Don't have the code yet? Look up your code or read it with AXLY.pro.

Can I keep driving?

Yes, but a rough idle that gets worse can leave you stranded. Diagnose within a couple weeks — most fixes are well under $100.

Confirm with the actual code

Symptom-based diagnosis narrows the field — reading the actual stored code finishes the job. AXLY.pro is a free iPhone app that pairs with any Bluetooth OBD2 adapter and reads every stored DTC.