Rough Idle (Engine Shakes at a Stop)
ModerateQuick answer
A rough idle means at least one cylinder isn't pulling its weight at low RPM — where there's no momentum to hide it. The usual suspects in order: vacuum leaks, worn spark plugs, a dirty MAF or throttle body, a tired coil, or a stuck PCV/EGR valve. Most of the list is cheap.
Idle is the engine's hardest balancing act: minimum fuel, minimum air, no flywheel momentum to smooth over weak combustion events. Problems invisible at 2,500 RPM — a small vacuum leak, one lazy cylinder, slightly wrong idle airflow — all surface as shake, surge, or stumble at a stoplight.
The pattern carries information. Rough ONLY at idle, smooth driving = think air metering (vacuum leak, throttle body, PCV). Rough everywhere but worst at idle = think a specific cylinder (plug, coil, injector) and expect misfire codes. Surging up-down rhythmically = idle control or a leak the computer keeps fighting. Worse with AC on = the system has no headroom left for extra load.
Most likely causes
Ranked from most likely and cheapest to least likely and most expensive.
- 1.
Vacuum leak
The classic: unmetered air leans the idle mixture. Big positive fuel trims at idle confirm it.
Related code: P0171 Step-by-step guide: How to Find a Vacuum Leak (Four Methods, Cheapest First)
- 2.
Worn spark plugs (full set)
Past-interval plugs make every cylinder slightly weak — idle shows it first.
Step-by-step guide: How to Change Spark Plugs (and Read the Old Ones Like a Mechanic)
- 3.
Dirty MAF sensor or carboned throttle body
Bad airflow data or disturbed idle air — both are cleaning jobs, not parts.
About this part: Mass Air Flow (MAF) Sensor Step-by-step guide: How to Clean a MAF Sensor — the $10 Fix for Lean Codes and Hesitation
- 4.
Failing ignition coil on one cylinder
The shake gets a rhythm to it; expect a cylinder-specific misfire code.
Related code: P0301 About this part: Ignition Coil
- 5.
Stuck PCV valve or EGR valve
Both dump uninvited gases into the idle mixture — engineered vacuum leaks when stuck.
About this part: PCV Valve
- 6.
Dirty or leaking fuel injector
One cylinder over- or under-fed shakes exactly like an ignition miss.
About this part: Fuel Injector
What to check first
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1 Scan first, even with no light
Pending misfire codes and fuel trim data turn a vague shake into a named cylinder or a confirmed lean condition. Five minutes with any scanner reshapes the whole hunt.
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2 Read the fuel trims at idle
Long-term trim above ~+10% at idle that improves with RPM is the vacuum-leak signature — go straight to the leak hunt. Trims normal? The air system is likely innocent; think cylinders.
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3 Do the cheap cleanings
MAF cleaner on the sensor, throttle body cleaner on the plate and bore (idle relearn after, where applicable). Twenty dollars and an hour resolves an enormous share of idle complaints.
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4 Chase the cylinder if trims are clean
Plugs past interval? Replace the set. Misfire code present? Coil-swap test on that cylinder. The standard misfire ladder finishes the job from here.
Frequently asked questions
- Why is it rough only with the AC on?
- The compressor adds load at exactly the moment the engine has least to give. A healthy idle system compensates invisibly; a marginal one (dirty throttle body, weak idle control, small leak) gets exposed. Fix the margin, not the AC.
- Rough idle but no check engine light — how?
- The computer tolerates a lot before setting codes, and single-digit-percent roughness lives below the misfire monitor's thresholds. Pending codes and live data see what the light doesn't — scan anyway.
- Can a rough idle damage anything?
- Mostly it's a comfort and economy tax, but persistent misfires send unburned fuel to the catalytic converter, and that bill is real. A rough idle that's worsening, or any flashing light, moves this from annoyance to assignment.