Dan Ferrell writes about DIY car maintenance and repair. He has certifications in Automation and Control Technology and Technical Writing.
How to Determine What's Causing Rough Idling in Your Car
Engine rough idle problems arise because car engines are demanding. Components in the fuel, ignition, emission and other systems should work correctly. It only takes a minor problem like a fouled spark plug to cause your engine to vibrate.
Idle problems may originate in a worn-out component, a failed part, or a blown gasket. The number of potential sources of a rough idle can make your diagnostic a little difficult. Still, you have at your disposal several tools to help you troubleshoot and fix the problem sooner.
Even more, on modern vehicles the car computer monitors many sensors and actuators and usually can detect small deviating operating parameters and store related trouble codes as necessary. The code may indicate the component, circuit or system involved in the fault. So this gives you an additional advantage.
In this guide, you'll find many of the components that, when going bad, will usually cause engine rough idle problems. The components are ordered from the most frequent to the least, either because they usually receive little attention or because they are bound to fail within a number of miles of operation.
To make the most out of this list, pay attention to the 'Other symptoms' at the bottom of each component description. And compare these symptoms and conditions to the ones you've noticed in your car. This will help you zero in on your diagnostic faster.
For example, when your engine idles rough, you may notice the problem only shows up before the engine warms up; then idle smooths out. This may indicate a bad ECT sensor as described in Other Symptoms in that section.
So pay attention to all the symptoms and conditions when given in each component listed to make your diagnostic easier.
Do You Have a Driveability Problem?
1. Spark Plugs
2. Air Filter
3. EGR Valve
4. MAF Sensor
5. ECT Sensor
WARNING Flashing CEL
6. PCV Valve
7. Oxygen Sensor
8. IAC Valve
9. TPS Sensor
Five Tips for Finding Hard to Find Rough Idle Sources
10. Throttle Valve
11. Fuel Pump
13. Vacuum Leak
14. Head Gasket Leak
Do You Have a Driveability Problem?
Before starting hunting for the source of a rough idle, you can do this simple method to confirm that you have a driveability problem. It'll work in most cases.
1. Set your transmission to Park (automatic) or Neutral (manual), and apply the parking brake. Start the engine and let it idle.
2. Now, listen to the sound coming out of the tailpipe. If you hear a clear popping sound, most likely you have a performance problem; if not, check the engine mounts. A worn-out, loose or damaged engine mount can cause the engine to vibrate.
1. Spark Plugs
A faulty spark plug may cause several engine performance problems, including an engine rough idle. The same conditions under which the spark plug operates, along with poor engine maintenance, may foul the tip with ash, oil, or carbon deposits.
On top of this, miles of service cause the plug electrodes to wear out, widening the gap. This causes an increase in the voltage needed for the spark to jump between the electrodes; but this condition may burn the plug tip and other ignition components, resulting in a rough idle and possibly other engine performance problems.
- Remove the spark plug to inspect it. Check the condition of the plug tip. Look for signs of contamination like oil (which may indicate worn rings, a scored cylinder or leaking valves), ash (poor fuel or oil leak), carbon (weak ignition, rich mixture), damage (perhaps from preignition or use of a wrong plug for your model).
- Also, check the gap between the electrodes using a wire feeler gauge and regap the plugs. You can find the correct gap for your plugs in your car owner's manual or vehicle repair manual. If you don't have this manual, you can buy an inexpensive aftermarket manual online or with your local auto parts store. Your vehicle repair manual also has the service schedule for your spark plugs.
- Spark plug wires represent another potential problem in the system. Check spark plug wires resistance using a digital multimeter, and replace them according to the maintenance schedule as described in your vehicle repair manual. Some manufacturers suggest replacing the spark plugs every 2 or 5 years or more, depending on the type of plugs your engine is using. Also, check other ignition components like rotor, distributor cap, and ignition coil(s) for wear or damage.
Other Symptoms: you may notice misfires, an increase in fuel consumption, loss of power and hard starting.
Read More from AxleAddict
2. Air Filter
The air filter traps dirt, dust particles and other matter out of the air going into the intake manifold.
Most filters use paper as the filter element and after a few months of work, the elements clogs, reducing air flow rate. If you haven't changed the filter in more than two years, you'll notice a rough idle.
Most car manufacturers suggest replacing the air filter every year. Check your vehicle repair manual or car owner's manual.
When installing a new filter, thoroughly clean the filter housing. Carefully close the air box and properly re-attach any ducts you had to disconnect to prevent unmetered and unfiltered air into the intake manifold; otherwise, you'll cause more driveability problems.
Other Symptoms: A clogged air filter may also cause an increase in fuel consumption, stalling, in extreme cases, and other driveability problems.
3. EGR Valve
The exhaust gas recirculation (EGR) valve allows a certain amount of exhaust gases into the intake manifold to help lower engine temperature and harmful emissions.
When the valve fails, it may get stuck close or open, or even leak. It is when the valve remains open when engine idle suffers. Sometimes, the engine may stall at idle because of the lean of the air-fuel mixture.
It is possible for you to test the EGR valve at home, using a handheld vacuum pump.
Other Symptoms: An EGR stuck open may include a rough idle, rough performance at low engine speeds, stalling at idle, increase in fuel consumption, fuel odors, and possibly a check engine light on.
4. MAF Sensor
The mass airflow (MAF) sensor can also cause a rough idle. This sensor measures the amount of air flowing into the engine using a sensing element.
A dirty sensing element is the most common fault, but the sensor itself can develop other faults.
In most cases, a malfunctioning MAF sensor will cause the computer to store a trouble code. So scan your computer for trouble codes.
Also, you may test the sensor at home using a digital multimeter. Consult the vehicle repair manual for your particular make and model.
Other Symptoms: power loss, surging at idle, hesitation, stalling during acceleration and other driveability problems.
5. ECT Sensor
A bad engine coolant temperature (ECT) sensor can also cause engine performance problems.
ECT operation has an effect on the fuel system. Your car computer uses this sensor's signal to lean or enrich the fuel mixture, depending on coolant temperature.
An electrical open in the sensor circuit may cause the computer to lean the fuel mixture trying to compensate for conflicting signals from other sensors.
On some modern vehicles, a malfunctioning ECT sensor can upset ignition timing.
You can test your engine ECT using this guide.
Other Symptoms: With a bad ECT, you may notice a continuous lean or rich mixture, an increase in emissions and fuel consumption, and engine stalling when cold; operation may smooth out once engine temperature increases.
6. PCV Valve
The PCV valve allows your engine to move blow-by gases out of the crankcase by reintroducing them into the intake manifold to get reburn.
Over time, the valve may fill with dirt and sludge, blocking blow-by gases, or simply the valve may stop working.
Car manufacturers suggest servicing the valve at regular intervals to prevent sludge from blocking the valve and gradually destroying the engine.
However, it is when the valve leaks that you may notice a constant lean air-fuel mixture with a consequent rough idle.
Other Symptoms: misfires, hard starts, and increase in oil consumption.
8. IAC Valve
Besides regulating idle speed, the car computer uses the idle air control (IAC) valve to supply air to an engine with fuel injection system.
It's not uncommon for buildup to block IAC valve passages over time, upsetting engine idle. Also, the valve itself may fail and cause similar symptoms.
On most vehicle models with an IAC sensor, the valve is accessible and you can remove it to inspect air passages underneath.
Consult your vehicle repair manual to test the valve using a digital multimeter. And watch the video below so that you have an idea of how to clean valve ports and passages.
Other Symptoms: You may notice a constant idle speed, either high or low.
7. Oxygen Sensor
The oxygen sensor measures the amount of oxygen in the exhaust gases.
The sensor sends this measurement to the car computer as a voltage signal. Then, the computer uses this information to increase or decrease the amount of fuel delivered to the engine.
Over time, the oxygen sensor (O2s) wears out or may become soiled with carbon buildup, causing the engine to idle rough.
Usually, a failing O2 sensor will cause the computer to store a trouble code, but you still need to confirm that your oxygen sensor needs replacement.
You may be able to test the oxygen sensor at home using a digital multimeter.
Other Symptoms: A bad oxygen sensor will reflect primarily in the fuel injection system or the computerized carburetor system of your car, resulting in a rich or lean fuel mixture, increase in fuel consumption or misfires.
9. TPS Sensor
The throttle position sensor (TPS) reports the opening angle of the throttle plate to the computer. For this, the TPS uses a voltage signal it sends to the computer.
This simple circuit can also cause problems. If the TPS variable resistor wears out, or the circuit shorts out, it can cause the computer to enrich the fuel mixture; however, if the sensor's circuit opens, it'll cause the computer to lean the mixture with a consequent rough idle.
Other Symptoms: A bad TPS may also cause stumbling, hesitation, surging, stalling and a hard starting.
5 Tips for Finding Hard-to-Find Rough Idle Sources
These tips give you some extra help when the source of the rough idles seems hard to diagnose or after finding the source of the problem.
- For a better diagnostic, scan your computer for possible trouble codes, even if you don't see the light coming on. You may have some pending codes that can steer you in the right direction.
- If you, or your shop, recently did some work on your engine and soon after you noticed a rough idle, make sure you didn't leave something unplugged, a loose vacuum hose, or didn't properly reassemble the air cleaning assembly. Even a small air leak can cause engine performance problems.
- Idle speed is the speed at which the crankshaft rotates when the throttle plate or valve is closed or at rest. On modern vehicles, the computer controls idle speed, although a bad or failing sensor or mechanical malfunction can upset it. On older car models, though, adjusting idle speed is part of the tune-up procedure. Refer to your car repair manual for the procedure and engine rpm specifications at idle.
- Ignition timing (spark timing) means how early or late spark fires in relation to the position of the pistons in the cylinders. An engine slightly out of time can idle roughly since the spark is not in sync with the cylinders. Too much timing advance or retardation causes serious driveability problems. Many problems may upset timing. However, the most common is an overstretched timing belt or chain, or a bad tensioner after miles of operation. Check your vehicle repair manual for the service schedule, and how to check the timing belt or chain. And be warned that on interference engines, a broken belt or chain can cause severe engine damage as well.
- You may have replaced a bad component that was actually causing your engine to idle rough or erratically. But after installing the part, things didn't improve. However, some engines, especially modern ones, seem fussy about the type of parts you install. A poor quality component may not work as expected, and you may think that you haven't found the source of the problem. So, save yourself some money and trouble. When replacing ignition components or some other electrical or electronic parts, try to use OE or buy from a known quality brand.
10. Throttle Valve
The throttle valve in the throttle body or carburetor connects to the accelerator pedal mechanically or, in some modern vehicles, electronically.
This valve helps control engine power, speed and airflow.
Over time, carbon buildup may grow around the engine valve and the horn—the walls inside the throttle were the valve seats—restricting or obstructing the operation of the valve and causing an engine rough idle.
You can check the throttle valve by removing the air cleaner intake on engines with carburetors or by disconnecting the air duct off the throttle body on modern vehicles.
If the throttle walls or throttle plate is covered with buildup, spray carburetor cleaner on a shop rag and thoroughly clean the components.
Other Symptoms: Poor engine performance when cold or as it reaches operating temperature, hard starting, poor acceleration, rich fuel mixture at idle and stalling.
11. Fuel Pump
Although you won't see this often, an engine rough idle can be a sign of problems with the fuel pump or fuel pressure regulator.
A worn-out fuel pump can fail to supply fuel at the adequate rate or with the correct volume to keep up with engine demand; also, the fuel pressure regulator can fail to hold pressure, causing too little fuel to reach the engine.
Other components in the fuel system can also cause your engine to idle rough. For example, a dirty (clogged) fuel filter or an obstructed or bad fuel injector.
On some models, the car computer can detect problems in the system and store trouble codes in memory that can guide you to the source of the problem. So scan your car computer, especially if the check engine light (CEL) has come on.
You can use the repair manual for your car to quickly check the fuel pump and pressure regulator using a fuel pressure gauge.
Other Symptoms: Noisy fuel pump, hard starting, power loss at highway speeds, engine stalling after a few minutes of operation.
The evaporative emissions control (EEC, sometimes EVAP or ECS) is another system in your vehicle that can cause rough idle problems.
The EEC system routes fuel vapors inside the fuel tank into the engine for burning to prevent harmful emissions.
Components in the system may consist of a purge solenoid, filler cap, relief valve, canister, purge valve, filter and hoses, depending on your particular vehicle make and model.
System faults may cause the computer to store a trouble code. So scan your car computer, if necessary, and consult your repair manual to troubleshoot the system.
Other Symptoms: A fault in one or more components in the system may cause the engine to idle poorly, stall, leak fuel and increase pressure inside the fuel tank.
13. Vacuum Leak
Vacuum leaks can cause your engine to idle rough.
The leak may happen for a variety of reasons: You were working on your engine and inadvertently pulled loose a vacuum hose, or you forget to reconnect a hose; also, a hose may become brittle after miles of operation and crack, or a gasket may blow.
So what can you do to locate the source of the leak amid many hoses, especially on Asian models? Here's a simple method:
First: check that all the vacuum hoses are properly connected at each end.
Second: trace each hose with your fingers to feel for possible damage.
Third: Set the parking brakes, start the engine and let it idle.
Then, using a length of rubber hose, you can put one end of the hose against your ear and use the other end to probe vacuum hoses around the engine. Don't forget the hose between the charcoal canister purge valve and the intake manifold. Consult your vehicle repair manual to locate it, if necessary.
Also, check the gaskets around the intake manifold and throttle body. You'll hear a noticeable hissing sound once you get close to the leak source. Make sure to keep away from moving parts.
An alternative method is to use a solution of soapy water: Spray along the length of each vacuum hose, at connectors and fittings, and around the gasket of the intake manifold and throttle body. If you notice bubbles after spraying at a particular spot, suspect a leak.
Other Symptoms: Besides a rough idle, a vacuum leak may produce backfiring, hard starting, stumbling, high idle, hesitation, misfiring, poor power brake action, and poor acceleration.
14. Head Gasket Leak
Although not as common, you may be dealing with a blown head gasket, a leaking or burned valve, or even a warped intake manifold.
Usually, this type of problem shows up on a model with high mileage or a grossly neglected engine. In either case, an idle problem from a blown head gasket usually shows up as a high rough idle.
You can do a compression check using a vacuum gauge, especially if you can't locate the source of the rough idle among the most common causes. The test can not only diagnose a blown head gasket but uncover the condition of piston rings and valve seats.
Other Symptoms: Missing, oil leakage, blue smoke, and overheating.
Proper Maintenance Is Key
Finding the cause of engine rough idle problems can be tricky at times, but paying attention to other symptoms besides the rough idle (shaking, noises, power loss) can help you close in on the source of the problem.
And keep in mind that the most rough idle issues come from lack of proper maintenance. So check those components or systems you have neglected lately. And scan your computer for possible trouble codes, even if the check engine light is not on. But, whatever the cause, don't shrug off the problem, even if it seems to be a simple one, because it can result in a costly repair if you don't take care of it now.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
Question: What is wrong with the car if it keeps dying every time I pull up to a red light and stop or when it's in park? When it dies, it starts right back up.
Answer: Usually, the problem is in the fuel system. This could be a worn out pump, but check for a clogging fuel filter and a faulty fuel pressure regulator. This other post may help:
Question: Why do my rpms run low and the engine shake while at a stand still running?
Answer: Have you heard any misfires? Check for the source if necessary:
Pop the hood open and hear for any hissing sounds, check for a vacuum leak. You may need a piece of rubber hose to hear closely.
Also, there may be a problem with the idle air control (IAC) valve-solenoid. You may need your vehicle repair manual to troubleshoot - it's not that difficult. A common issue with the IAC is carbon buildup. You may need to remove it to get a look underneath.
Question: Would a faulty fuel temperature sensor cause a rough idle? When I press the accelerator, it quiets down. For reference, the sensor code is p1083.
Answer: It sounds more like a vacuum leak. That could explain why the idle quiets down when you press the accelerator. Other reasons for a p1083 include a MAF sensor, a bad fuel injector, or another fuel system problem that may cause low fuel pressure.
Question: I have rough idle after I disconnected the battery. What would be the most likely cause?
Answer: Double check and see if you didn't disconnect or left loose something else by accident, like a vacuum hose, cable or wire.
Question: Why would the ETS light be on in my S70 Volvo?
Answer: It's possible that there's a problem with the ETM (electronic throttle module) - if the problem is the TPS (throttle position sensor), you might want to consider a contactless TPS. Some people seem to have good experience with this. Also, some models, including the S70, had a recall for the ETM, but I don't have the information. You might want to check with your dealer first before making any repairs.
Question: I just changed my radiator and AC condenser. Now, when the car idles with the heater on, it begins to idle rougher. Would either of the two parts be faulty and cause this issue?
Answer: The heater is an extra load the engine has to handle. There is extra power the engine is not handling well, perhaps a problem with the AC. But make sure you’ve changed spark plugs, ignition wires, and general maintenance items.
Question: My car has a random miss at idle. It fouls plugs every 30000kms or so. What should I try?
Answer: Infrequent use of the vehicle, too many short trips, long periods of idle (as opposed to highway driving) can cause spark plugs to foul. Take highway drives for 15 to 20 minutes.
Adjust the gap to specs using a wire feeler gauge. Check your owner’s manual, repair manual or the decal under the hood for the gap specs.
Also, a leaking EGR valve, clogging fuel filter, worn fuel pump, a damaged vacuum hose, worn valves or guide, piston rings or cylinders can foul spark plugs.
Question: My 2001 Ford Laser just got a new timing belt and other parts. The mechanic said it has a cracked cylinder head. Funny thing is, it only made bubbles, no other stuff you mentioned like overheating and no coolant in the oil. It’s misfiring, idling weirdly, and the rev gauge goes up and down while in park. What could be wrong?
Answer: The fact that there were bubbles in the coolant could possibly come from a cracked cylinder head, as your mechanic said. Overheating is usually the most common cause behind a cracked cylinder head, but hot spots created by air in the cooling system, and pre-ignition can also lead to engine mechanical problems and head gasket damage. If you need a second opinion, take your vehicle to a reputable shop for a diagnostic.
Question: My 2015 Nissan Qashqai 1.5 dCi has fluctuating rpm and stalling only at the 1st start of the day. It runs fine when warm. No fault codes or lights. I had the EGR valve cleaned and fuel filter changed, and it's still the same. What could be the problem? Any ideas welcome.
Answer: You may want to check for a possible air leak located after the air flow meter, a problem with the glow plug(s), fuel pressure regulator or sensor.
Question: I have a carbureted engine that idles roughly, and stalls at stops. The car gets worse as it comes to operating temperature. No vacuum leaks. Ignition system is all good. I cleaned the carb and it appears pretty clean to me. Any other ideas where to look? Undriveable currently as it is. My 1989 car has no scan tool connector.
Answer: Several faults can cause this. Check the idle air control (IAC) motor; the EGR valve may be stuck open; a leak in the throttle body gasket or intake manifold may also lead to this issue. I believe your vehicle may have a code reader connector in the engine compartment. Check your vehicle repair manual or with your local auto parts store. If it does, you may be able to buy a relatively inexpensive reader and get the codes from the computer. Follow the instructions that come with the reader. This other two post may also help you:
Question: I have a 2002 Buick Rendezvous. I've put in so many new parts, but now it starts right up at a cold start. Then when it warms up, it idles really low and shuts off. It will start right back up then die at random low idle again. Any clue what my next move is? I have replaced most sensors for ignition and spark and almost everything fuel related was replaced. Over $1000 worth of new sensors and fuel pump and filter. Any help would be greatly appreciated.
Answer: It seems the problem is temperature related. There are a number of sensors that can cause the engine to stall once it reaches operating temperature. Usually, a faulty sensor that doesn't send the correct information to the computer may cause the engine to idle rough or stall: crankshaft and camshaft position sensor, ignition control module, fuel pump, possibly EGR valve stuck open, idle air control valve stuck open.
Check section two of this post. It may help:
Question: Mazda 6 2006 shakes now after having a timing chain, VVT cap or solenoid, and water pump replaced. Check engine light came on after replacements for the O2 sensor and then that was replaced. Check engine light cleared until I picked it up and came right back on. And I noticed the car was shaky, and it wasn't before replacement. Also, there’s a misfire on cylinder 1 and a blown spark plug. What can be causing all these malfunctions after having my Mazda's timing replaced?
Answer: The fact that the spark plug was blown, probably indicates the problem. Maybe the plug was loose or cross-threaded. Which led to the shaky engine. Probably, solving this issue will fix the rest of the problems. Check the plug threads on the engine. If they are in good condition, install a new plug, or change all the plugs, if necessary. Make sure they are torqued to specification to avoid this type of issue. Over-tightening the plugs can also lead to other problems. You can find the torque for your engine in the vehicle repair manual for your specific model. You can find a copy at most auto parts stores. Take it from there and see what happens. Also, scan for trouble codes and check components indicated by the code before replacing anything. The problem can be in the indicated system. This other post may help:
Question: My 1997 Toyota Camry is idling roughly and misfiring when in park. When driving, it seems like it's about to die and jerking when traveling. Would my Camry's problems more likely be a speed sensor or something else?
Answer: If there's a problem with a sensor, there might be a trouble code in the computer. Download possible codes even if the check engine light is not on. A pending code may be stored in memory.
Also, these other two posts may help:
Question: I replaced my Ford Mustang's MAF sensor, EGR and car still idles, stalls and A/C turns off. Do I have an electrical problem?
Answer: You should only replaced a component when you know it is faulty. Usually the problem is with the fuel or ignition system. A vacuum leak can also lead to stalling issues. Check for diagnostic trouble codes, even if your check engine light is not on. There could be a pending code. If the engine stalls only when turning the AC on, the charging system may be the problem. Check any suspect component or circuit before replacing it or making repairs. This other post may help. Particularly, check section three:
Question: My 2002 Nissan Stagea surges until she's warmed up. What could be the problem?
Answer: This other post may help:
Question: My car revs really high when I put it in neutral after driving for a bit. It's not the IAC, I bought a new one. I don't think it's a vacuum hose and I unplugged my MAF and it still does the same thing. P0192 is the error code. Could it be the fuel rail?
Answer: If your application uses a vacuum hose between the intake manifold and fuel rail pressure sensor, make sure the hose is not damages or loose, or leaking fuel. Then check for bad wires or connector for the fuel rail pressure sensor. This can also mean a fuel system component or electrical circuit fault.
Question: Ford Explorer has very rough idle. Recently had to have the crankshaft sensor replaced and 106 miles later the harmonic balancer fell off the truck. Could this possibly be a result of something that was done improperly during the replacement of the crankshaft sensor?
Answer: Most likely not. The crankshaft sensor on most of these models is located on the engine front cover. There's no need to remove anything else. When reinstalling the balancer, use blue LocTite thread locker on the threads of the bolt. Check the torque specs on the vehicle repair manual for your specific model and install.
Question: I have a 1996 Subaru Legacy wagon. After I get the car warmed, RPMs go up and down. Sometimes it dies and some days it won't do it. And while going down the highway, after a while, it will almost kill itself. What's going on?
Answer: There could be a vacuum leak from a vacuum hose, a gasket, or a component that works with vacuum (for example, EGR valve). However, there are different types of faults that can cause engine surge (engine speeding up and down while holding the accelerator steady): vacuum leak, worn fuel pump, troubles with a fuel pressure regulator, faulty PCV valve, EGR valve stuck, and faulty sensor(s) (in emission control systems, mainly). First, download diagnostic trouble codes, even if you don't see the check engine light on. If there are some systems in need of maintenance (spark plugs, wires, fuel filter, air filter) check those, you'll need those items either way. Check for good fuel pressure, if necessary.
Question: Would changing my mounts cause my car's rough idling?
Answer: Not unless the mounts are not properly mounted. Double check they are properly mounted and the bolts are tight. If you only changed the motor mounts check the transmission mounts, if necessary.
Question: 2012 Chevy Malibu has rough idle when stopped in Drive. Idles fine in park/neutral. No vibration while driving. I replaced the intake gasket and valve cover gasket. Shake got better but not by a lot ( both visibly/audibly showed need). It has new plugs, oxygen sensor, and had all mounts checked. Still rough in traffic and at stoplights in Drive. Ideas? So frustrated! Thank you. I know there is a bit of a leak in exhaust gasket.
Answer: Usually vacuum leaks cause a rough idle when shifting to Drive. This other post may help: