Rough Idle
A rough idle means the engine is shaking, stumbling, or running unevenly when the car is sitting still. The most common fixes are cleaning or repairing air and fuel metering problems, such as a dirty throttle body, vacuum leak, or worn spark plugs, usually costing about $100 to $450 at an independent shop.
- Severity
- Moderate
- Can I drive?
- Drive with caution
- DIY?
- Possible with some experience
- Repair cost range
- $100 – $500
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Why it happens
At idle, the engine should run smoothly with a steady RPM. Rough idle happens when one or more cylinders are not getting the right mix of air, fuel, or spark, or when extra air is entering the engine through a leak. It can also happen if a sensor is sending bad data to the computer or if the engine has a misfire. The problem may be mild at first, but it often gets worse as the cause gets worse.
Common symptoms
- Engine shakes at stoplights
- RPMs hunt up and down
- Check engine light on
- Stalls when coming to a stop
- Rough or uneven exhaust note
- Vibration in the steering wheel or seat
Causes, fixes and costs
Rough Idle
General guide for most cars. Your car may differ.
A rough idle can turn into a stall, poor acceleration, or a no-start if the underlying problem gets worse. Stop driving if the check engine light is flashing, the engine stalls repeatedly, or the car runs so rough that it shakes hard enough to feel unsafe.
Most likely causes
- 1
Vacuum leakHigh
A vacuum leak adds unmetered air and often causes a high, unstable, or rough idle. You may also notice hissing, lean codes, or a check engine light.
Quick check: Listen for hissing around intake hoses and the PCV hose; inspect for cracked, loose, or split vacuum lines and intake boots.
- 2
Dirty throttle body or idle air control problemHigh
Carbon buildup can make the throttle plate stick or reduce airflow control at idle, causing low, surging, or unstable idle speed. This is common if the idle improves slightly when you press the gas.
Quick check: Look for heavy black buildup in the throttle body and note whether the idle changes when the throttle is lightly opened.
- 3
Worn spark plugs or ignition coilMedium
A weak spark often causes a misfire that is most noticeable at idle, especially if the engine shakes more in gear or the check engine light flashes.
Quick check: Scan for misfire codes and inspect plug condition, coil boots, and signs of oil or moisture in the plug wells.
- 4
Fuel delivery problemMedium
Low fuel pressure or clogged injectors can make the engine stumble at idle and sometimes during acceleration. Hard starting, hesitation, or lean codes make this more likely.
Quick check: Listen for the fuel pump prime, note any long crank or hesitation, and have fuel pressure tested if the idle is rough under load or when cold.
Can you fix it yourself?
Possible with some experience
Difficulty: Moderate. Basic checks like inspecting hoses, cleaning the throttle body, and replacing spark plugs are possible for many owners with simple hand tools. If the problem needs fuel pressure testing, smoke testing, or diagnosis of sensors and misfires, a shop is usually the better choice.
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- Check for loose or cracked vacuum hoses and intake ducting.
- Scan for OBD2 codes and note whether the check engine light is flashing.
- Inspect and clean the throttle body if it is heavily carboned.
- Replace worn spark plugs if they are overdue.
- Check ignition coils and plug wells for oil or moisture.
- Clear codes and see whether the idle improves or the codes return.
Estimated repair cost
$100 – $180
Throttle body cleaning: labor plus cleaner; some cars need throttle relearn
- Vacuum leak repair$120 – $350
- Spark plugs$150 – $350
- Ignition coil replacement$180 – $450
- Fuel injector service or repair$150 – $500
Prices vary by engine layout, parts quality, and local labor rates.
Frequently asked questions
Can I drive with a rough idle?
Usually yes for a short time if the engine is not stalling and the check engine light is not flashing. Drive gently and get it checked soon, because a rough idle can point to a misfire or vacuum leak that may get worse.
Why is my car rough only when stopped?
That usually means the engine problem shows up most clearly at low RPM, such as a vacuum leak, dirty throttle body, weak spark, or injector issue. Many of these problems smooth out a bit once the engine speed rises.
Will a rough idle trigger a check engine light?
Often yes, especially if the computer sees a misfire or a fuel-trim problem. Common codes include misfire codes and lean mixture codes.
Can I fix rough idle myself?
Sometimes. Cleaning the throttle body, checking hoses, and replacing overdue spark plugs are reasonable DIY jobs for many drivers, but smoke testing, fuel-pressure diagnosis, and intermittent misfires are better left to a shop.
Will rough idle fail an inspection?
It can, especially if the check engine light is on or the engine is misfiring. If the car shakes badly at idle, fix it before inspection.
Related codes
Related problems
This guide covers most cars and is general advice, not an inspection. Prices are typical US independent-shop ranges and vary by region, car and shop. If your brakes, steering or a warning light feel unsafe, stop driving and contact a mechanic.