# Why is my car idling rough? **Verdict:** Drive to a shop this week - **Most common cause:** Worn spark plugs or a failing ignition coil - **Urgency:** This week - **Typical fix:** $15-$700 - **Causes to check:** 6 ## Quick answer A rough idle, where the engine shakes, stumbles or the RPM bounces while stopped, is most often caused by worn spark plugs or a failing ignition coil. Vacuum leaks and a dirty throttle body are the next most common causes. A flashing check engine light with a rough idle means a serious misfire and needs prompt attention. ## What is happening? At idle the engine runs on the least air and fuel it can, so small faults show up there first. Each cylinder needs a strong spark, the right amount of fuel, and air the computer has measured. When one cylinder fires weakly or not at all, the crankshaft gets an uneven push and you feel a rhythmic shake or stumble. When extra air sneaks in through a vacuum leak, or too little gets past a dirty throttle, the RPM dips, hunts or surges instead of holding steady. It matters because many rough idles are misfires, and a misfiring cylinder sends unburned fuel into the exhaust. That fuel burns in the catalytic converter, which can overheat and fail, and it can thin the oil on the cylinder walls. A flashing check engine light means the misfire is bad enough to do that damage now. Not every rough idle is the engine, though: worn mounts can make a healthy engine feel rough. ## What causes it? The 6 most likely reasons 1. **Worn spark plugs or a failing ignition coil** (most common), $150-$600. Worn plugs or a weak coil cause one cylinder to fire weakly or not at all, which you feel as a rhythmic shake at idle. It is most common on engines past their plug change interval and often sets a cylinder-specific code such as P0301. Damp weather can make a cracked coil or boot worse. How to confirm: The check engine light is on or flashing, and a code reader shows P0300 or a cylinder-specific code like P0301 to P0306. Plugs past their service interval (often 60,000 to 100,000 miles) are a strong suspect. Fix: Replace spark plugs, and the bad coil if needed. 2. **Vacuum leak** (common), $100-$500. A cracked vacuum hose, loose intake boot or leaking intake gasket lets in extra air the engine did not measure, leaning out the mixture so it runs unevenly and the idle may be high or hunting. You may hear a hiss with the hood open. Rubber hoses harden with age and heat, so leaks are common on older engines. How to confirm: You hear a hiss from the engine bay with the engine running, and the scan tool shows P0171 or P0174. The idle often smooths out when you raise the RPM. Fix: Replace the cracked hose or gasket. 3. **Dirty throttle body or idle air control fault** (common), $75-$300. Carbon buildup around the throttle plate restricts the small amount of air the engine needs at idle, so RPM dips or hunts when you stop or turn on the AC. It builds up faster on engines with lots of short trips or idling. On older cars, a separate idle air control valve can stick and cause the same problem. How to confirm: Idle is low or surges, especially with the AC on, and may set P0505, P0506 or P0507. You may see black carbon around the throttle plate after removing the intake hose. Fix: Clean the throttle body and relearn idle. 4. **Dirty or failing mass air flow sensor** (less common), $15-$400. A contaminated MAF sensor misreads how much air enters the engine, so the computer adds the wrong amount of fuel. The idle can be rough or surging, and the car may also hesitate on acceleration. Dirt from a torn air filter or excess oil from an oiled aftermarket filter are common culprits. Careful cleaning with MAF cleaner sometimes restores it. How to confirm: Rough idle together with hesitation, and codes like P0101 or P0171. Cleaning the sensor with MAF cleaner sometimes fixes it right away. Fix: Clean or replace the MAF sensor. 5. **Stuck-open EVAP purge valve** (less common), $120-$300. A purge valve that sticks open feeds fuel vapor from the charcoal canister into the intake at idle, which upsets the mixture. The rough idle is often worst right after refueling and may come with a hard start. Topping off the tank past the first click can flood the canister and contribute. It may set code P0496. How to confirm: The rough idle or hard start happens mostly after filling the tank, and EVAP codes like P0441 or P0496 may be stored. Fix: Replace the purge valve. 6. **Worn engine mounts** (less common), $200-$700. The engine may idle fine, but worn or collapsed rubber mounts pass its normal vibration into the cabin, so the car feels rough, especially in Drive at a stop. The shake often eases in Neutral. A clunk when shifting or accelerating is a common sign. Mounts wear with age and heat and are not a misfire. How to confirm: Vibration is worse in Drive than in Park, and you may see the engine rock or hear a clunk when shifting into gear. There are no codes. Fix: Replace the worn mounts. ## How to narrow it down at home 1. Read the codes with an OBD-II scanner (many parts stores do it free). Misfire, lean, or idle codes point straight to the likely area. 2. With the engine idling and the hood up, listen for hissing from hoses and look for cracked or disconnected rubber lines. 3. Compare the shake in Park and in Drive with your foot on the brake. Much worse in Drive with no codes often points to motor mounts. ## Is it safe to drive? > **Drive to a shop this week.** A mild rough idle is usually safe to drive for a short time while you book a repair. If the check engine light is flashing, drive gently to a shop, avoid hard acceleration, and do not go far, because a severe misfire can overheat and damage the catalytic converter. ## When does it become urgent? - The check engine light is flashing: drive gently to a safe place and limit further driving. - The engine stalls at stops or when you turn on the AC. - You smell raw fuel or see black smoke from the exhaust. - The temperature gauge rises or the idle is rough along with overheating. - Loud clunks come from the engine bay when shifting into Drive or Reverse. ## What will the repair cost? | Cause | Fix | Typical cost | | --- | --- | --- | | Worn spark plugs or a failing ignition coil | Replace spark plugs, and the bad coil if needed | $150-$600 | | Vacuum leak | Replace the cracked hose or gasket | $100-$500 | | Dirty throttle body or idle air control fault | Clean the throttle body and relearn idle | $75-$300 | | Dirty or failing mass air flow sensor | Clean or replace the MAF sensor | $15-$400 | | Stuck-open EVAP purge valve | Replace the purge valve | $120-$300 | | Worn engine mounts | Replace the worn mounts | $200-$700 | Detailed breakdowns: [spark plug replacement cost](/cost/spark-plug-replacement), [ignition coil replacement cost](/cost/ignition-coil-replacement), [mass air flow sensor replacement cost](/cost/mass-air-flow-sensor-replacement), [evap purge valve replacement cost](/cost/evap-purge-valve-replacement). ## What to tell your mechanic Tell the shop whether the shake is steady or comes and goes, whether it happens cold, hot, with the AC on, or after refueling, whether the check engine light was on or flashing, and when the plugs were last changed. They will read codes and misfire counters, check fuel trims for a vacuum leak, inspect plugs and coils, and look at the throttle body and mounts. ## How to prevent it - Replace spark plugs at the interval in your owner's manual, often 60,000 to 100,000 miles for iridium plugs. - Change the engine air filter on schedule and keep the air box sealed. - Clean the throttle body when idle starts to dip, using a cleaner safe for your throttle type. - Stop fueling at the first pump click instead of topping off, which protects the EVAP system. - Have a check engine light read promptly so a small misfire is fixed before it hurts the converter. ## Which models get the most engine complaints? | Model | Engine complaints to NHTSA | All complaints | | --- | --- | --- | | [Ford Escape](/vehicles/ford-escape) | 3,905 | 10,274 | | [Chevrolet Silverado](/vehicles/chevrolet-silverado) | 2,167 | 7,658 | | [Kia Soul](/vehicles/kia-soul) | 2,079 | 3,189 | | [Hyundai Sonata](/vehicles/hyundai-sonata) | 2,059 | 4,126 | | [Honda Pilot](/vehicles/honda-pilot) | 1,989 | 5,776 | | [Ford Edge](/vehicles/ford-edge) | 1,944 | 5,306 | | [Hyundai Tucson](/vehicles/hyundai-tucson) | 1,932 | 5,363 | | [Kia Sorento](/vehicles/kia-sorento) | 1,899 | 4,128 | | [GMC Sierra](/vehicles/gmc-sierra) | 1,872 | 5,041 | | [Hyundai Santa Fe](/vehicles/hyundai-santa-fe) | 1,828 | 3,779 | Owner complaints filed with NHTSA in the engine category for 2015-2026 models of 63 popular cars. Counts follow sales as well as reliability, so compare each with the model's total. ## Trouble codes that often go with it - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0301](/codes/p0301): Cylinder 1 Misfire Detected - [P0171](/codes/p0171): System Too Lean (Bank 1) - [P0174](/codes/p0174): System Too Lean (Bank 2) - [P0101](/codes/p0101): Mass or Volume Air Flow "A" Circuit Range/Performance - [P0505](/codes/p0505): Idle Air Control System - [P0506](/codes/p0506): Idle Air Control System RPM Lower Than Expected - [P0507](/codes/p0507): Idle Air Control System RPM Higher Than Expected - [P0441](/codes/p0441): Evaporative Emission System Incorrect Purge Flow - [P0496](/codes/p0496): Evaporative Emission System High Purge Flow ## Frequently asked questions ### Can a rough idle damage my engine? Yes, if it is caused by a misfire and left alone. Unburned fuel from a misfiring cylinder can overheat the catalytic converter and wash oil off the cylinder walls. A rough idle from worn motor mounts does no engine harm, but the mounts themselves should still be replaced before they fail completely. ### Why does my car idle rough when the AC is on? Because the AC compressor adds load, and a dirty throttle body or weak idle control cannot add enough air to compensate. Cleaning the throttle body often fixes it. A failing AC compressor clutch or worn engine mounts can also make the idle feel worse with the AC running. ### Can a bad battery cause a rough idle? Rarely. Low system voltage can make some cars idle unevenly, especially right after a battery was disconnected and the idle has not relearned. Plugs, coils and vacuum leaks are far more likely causes. A quick charging test at a parts store rules out the battery and alternator. ### Is it safe to drive with a rough idle? A mild rough idle is usually safe to drive for a short time while you book a diagnosis. If the check engine light is flashing, the car stalls, or it loses power, limit driving and get it checked right away, since a severe misfire can damage the catalytic converter quickly. ### How much does it cost to fix a rough idle? It depends on the cause: about 75 to 300 dollars to clean a throttle body and relearn idle, 100 to 500 dollars for a vacuum leak, and 150 to 600 dollars for spark plugs and a coil at an independent shop in 2026. Engine mounts run about 200 to 700 dollars. ### Why does my car idle rough when cold? A rough cold idle is often a weak coil or worn plug that misfires until the engine warms, a vacuum leak from a hose that seals once it expands, or a carbon-fouled throttle. Some engines also run a fast, slightly uneven idle for the first minute by design. Persistent shaking is worth checking. ### Why does my car idle rough after getting gas? A rough idle right after refueling usually points to an EVAP purge valve stuck open, which pulls fuel vapor into the intake. Topping off past the first pump click can make it worse. It often sets a check engine light. Bad fuel is a less common cause. ### Can a dirty air filter cause a rough idle? A badly clogged air filter more often causes sluggish acceleration than a rough idle, since idle needs little air. It can contribute on some engines, and a torn filter lets dirt reach the MAF sensor, which does cause rough running. Filters are cheap to check and replace yourself. ### Can I fix a rough idle myself? Often, yes. Replacing spark plugs, swapping a suspect coil to another cylinder, cleaning the throttle body and MAF sensor, and replacing the air filter are all common DIY jobs with basic tools. Read the codes first with an inexpensive OBD-II scanner so you work on the right part. ### Why does my RPM go up and down at idle? A hunting or surging idle usually means the computer is fighting an air problem, such as a vacuum leak, a dirty throttle body or a sticking idle air control valve. A faulty coolant temperature sensor can also confuse idle control. Watching fuel trims on a scan tool helps a shop narrow it down. ## Sources - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) - [Car Care Council: Car care guide](https://www.carcare.org/car-care-guide/) ## Other car problems - [Why is blue smoke coming from my exhaust?](https://diagxcar.com/symptoms/blue-smoke-from-exhaust) - [Why does my car smell like something is burning?](https://diagxcar.com/symptoms/burning-smell-from-car) - [Why does my car crank but not start?](https://diagxcar.com/symptoms/car-cranks-but-wont-start) - [Why does my car hesitate when I accelerate?](https://diagxcar.com/symptoms/car-hesitates-when-accelerating) - [Why does my car jerk when I accelerate?](https://diagxcar.com/symptoms/car-jerks-when-accelerating) - [Why is my car leaking oil?](https://diagxcar.com/symptoms/car-leaking-oil) - [Why does my car lose power when accelerating?](https://diagxcar.com/symptoms/car-loses-power-when-accelerating) - [Why does my car shake at idle?](https://diagxcar.com/symptoms/car-shakes-at-idle) - [Why are my brakes squeaking?](https://diagxcar.com/symptoms/squeaky-brakes) - [Why does my car make a humming noise while driving?](https://diagxcar.com/symptoms/humming-noise-while-driving) ## Repair costs - [Spark plug replacement cost](https://diagxcar.com/cost/spark-plug-replacement): $120-$450 - [Ignition coil replacement cost](https://diagxcar.com/cost/ignition-coil-replacement): $150-$400 - [Mass air flow sensor replacement cost](https://diagxcar.com/cost/mass-air-flow-sensor-replacement): $150-$450 - [EVAP purge valve replacement cost](https://diagxcar.com/cost/evap-purge-valve-replacement): $120-$350 - [Crankshaft position sensor replacement cost](https://diagxcar.com/cost/crankshaft-position-sensor-replacement) - [EGR valve replacement cost](https://diagxcar.com/cost/egr-valve-replacement) - [Fuel injector replacement cost](https://diagxcar.com/cost/fuel-injector-replacement) - [Head gasket replacement cost](https://diagxcar.com/cost/head-gasket-replacement) - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement) - [Timing belt replacement cost](https://diagxcar.com/cost/timing-belt-replacement) - [Timing chain replacement cost](https://diagxcar.com/cost/timing-chain-replacement) - [Valve cover gasket replacement cost](https://diagxcar.com/cost/valve-cover-gasket-replacement) ## How long parts last - [How often should you change your spark plugs?](https://diagxcar.com/how-often/change-spark-plugs): 30,000-100,000 miles - [How often should you replace your air filter?](https://diagxcar.com/how-often/replace-air-filter): Every 15,000-30,000 miles - [How long do oxygen sensors last?](https://diagxcar.com/how-long/oxygen-sensor) - [How long does a timing belt last?](https://diagxcar.com/how-long/timing-belt) ## Trouble codes that lead here - [P0010 "A" Camshaft Position Actuator Circuit (Bank 1)](https://diagxcar.com/codes/p0010) - [P0011 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)](https://diagxcar.com/codes/p0011) - [P0012 "A" Camshaft Position - Timing Over-Retarded (Bank 1)](https://diagxcar.com/codes/p0012) - [P0013 "B" Camshaft Position Actuator Circuit (Bank 1)](https://diagxcar.com/codes/p0013) - [P0014 "B" Camshaft Position - Timing Over-Advanced or System Performance (Bank 1)](https://diagxcar.com/codes/p0014) - [P0015 "B" Camshaft Position - Timing Over-Retarded (Bank 1)](https://diagxcar.com/codes/p0015) - [P0016 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor A)](https://diagxcar.com/codes/p0016) - [P0017 Crankshaft Position - Camshaft Position Correlation (Bank 1 Sensor B)](https://diagxcar.com/codes/p0017) - [P0018 Crankshaft Position - Camshaft Position Correlation (Bank 2 Sensor A)](https://diagxcar.com/codes/p0018) - [P0019 Crankshaft Position - Camshaft Position Correlation (Bank 2 Sensor B)](https://diagxcar.com/codes/p0019) - [P0021 "A" Camshaft Position - Timing Over-Advanced or System Performance (Bank 2)](https://diagxcar.com/codes/p0021) - [P0101 Mass or Volume Air Flow "A" Circuit Range/Performance](https://diagxcar.com/codes/p0101) ## Warning lights linked to this - [Check engine light](https://diagxcar.com/warning-lights/check-engine) ## Terms explained - [What is EGR valve?](https://diagxcar.com/glossary/egr-valve) - [What is Misfire?](https://diagxcar.com/glossary/misfire) --- Source page: https://diagxcar.com/symptoms/rough-idle Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.