11 Common and 9 Less Common Causes of Rough Idling
When you’re stopped at a traffic light, you might notice your car making a slight crackling noise, bubbling, or rocking off the ground. If you look at your tachometer, you might see the needle moving up and down periodically, even if you’re just idling in your driveway.
Sure, it usually runs weird when it’s cold, but it’s a 20-year-old car, so that’s normal, right? What we’ve described above is called “rough idling.”
What is rough idle? “Idling” means the car is stopped and the engine is running. This is the point where the engine is revving hard enough to power a generator, and maybe a heater and/or air conditioning, but can’t make any progress at all because the transmission is “not in gear.”
A “rough” idle is the same as a normal idle speed, but it’s irregular and not as smooth as you’re used to on modern smooth engines.
If your car is idling smoothly, but you feel a roughness, then you have a rough idle. This is usually caused by changes in the amount of air going into the engine, regularity of the ignition, vacuum lines, and many other factors. No, I’m not talking about how your car’s engine speed increases when you turn on the air conditioning.
Symptoms
1. Shaking – When idling, your car may feel like it’s vibrating or shaking more than usual. This can be unpleasant, especially if you’re waiting at a stoplight or in a parking lot.
2. Stalling – Your car may stall or nearly stall while idling because the engine speed is lower than normal. This is not only frustrating, but can also be dangerous if it happens in the middle of a traffic jam.
3. Starting Problems – If your car has trouble starting or takes longer than normal to start, an idling problem could be the cause.
4. Check Engine Light – As with any other problem, if your car is idling roughly, the check engine light may come on. This light is a useful indicator that something is wrong and you should use a code reader to find out the cause.
Common causes of rough idling
1) Vacuum leak
When you leave your car parked for long periods of time or drive it for long periods of time, the rubber hoses in the engine compartment break down. Because of this, they no longer have a perfect seal like they did in the factory, and some of the pressure created by the vacuum lines escapes through these cracks.
This means that the engine vacuum system becomes unbalanced and the sensors may fail. If you have a vacuum leak, the engine computer will shut off power here and there to protect the engine from uncontrolled detonation (misfire).
2) Faulty Spark Plugs
If one of your spark plugs doesn’t fire in the correct sequence or doesn’t produce a spark at all, it can cause your idle to fluctuate when stopped. This is because “spark” is one of the most important combustion principles needed to keep your engine running.
For example, if three of the four spark plugs in a four-cylinder engine fire in the correct rhythm, you’ll hear three “pops” and then it goes quiet (if you slow the engine down enough to hear a distinct pop). You’ll definitely feel this when you’re standing at a stop sign or when you start your car for the first time in the morning.
If your car is idling abnormally, you should check your spark plugs and replace them if they are dirty or otherwise damaged.
3) Dirty Injectors
If your fuel injectors are dirty, they can cause a rough idle or your car won’t start at all. You may also notice that your gas mileage drops and you’re spending significantly more money on pumps.
A good way to test if an injector is dirty, broken, or not working as it should is to take a long screwdriver and press it against the block right next to the injector. You can then press your ear against the end of the screwdriver just like a doctor’s stethoscope and listen to the sound.
If you hear nothing or an irregular rhythm like “click, click, *nothing*, click,” then you have a fuel injector problem.
At this point, you’ll need to clean or replace the fuel injectors. This could be the cause of the rough idle.
4) Carburetor Issues
Carburettors are a never-ending tunnel of confusion, but there are four simple tests you can do to determine if your “carburetor” is working properly.
1. Start the engine, put the parking brake on, keep the transmission in neutral, remove the air filter and look at the carburetor. If the main jet is wet or dripping fuel, the float level is probably too high, making the mixture too rich. Adjust this to make the carburetor run a little cleaner.
2. Start the engine, put the parking brake on, keep the transmission in neutral and check the idle system. Slowly press the accelerator pedal until the RPMs reach about 3000 and observe how the engine behaves in this RPM range. Does the engine run smoothly or unevenly in this area? If it is the latter, you may need to adjust the idle.
3. Keep your float bowl full and your air cleaner removed, but turn your engine off. Now, look down into the four main barrels of your carburetor as you push on the throttle (you can have someone press the accelerator in the car for you if you would like). When you press the throttle, does a small squirt of fuel come out for a bit from all four barrels and continue for a little after you have released the accelerator? If not, you have a problem with your accelerator-pump system.
4. Another issue could be with your main metering system. With your engine up and running at 2000 RPM (give or take), cover the main “air horn” with some flat object like a piece of cardboard. Now, pay attention to whether the engine increases in RPM at all. It should since the engine will think it needs more fuel to accommodate the decrease in air. If it doesn’t, you’ll want to take your car to a specialist to work on the main metering system.
5) Air Filter
If your air filter is dirty, particulates will start to become present in the air going into your engine during the combustion cycle. Either that, or the grime in your air filter will not allow the air required to complete the combustion cycle at the engineered level. This could result in a misfire and otherwise “rough” idle.
If you’re experiencing this issue in the field, get out a bottle of water, poke a hole in the top of its cap to create a high-pressure opening (like a hose). Remove your air filter and use the bottle to spray out any fine particles you can, then dry out your air filter or give it time so it’s not spewing water in with your air.
This should at least get you home or to the next destination where you can purchase an air filter replacement.
6) Faulty PCV Valve
“PCV” stands for Positive Crank Ventilation, and if you notice excessive smoke coming out of your exhaust or you’re burning far too much oil or just more than normal, you might have a bad PCV valve. Check out this article we wrote to help find if your PCV valve is faulty.
7) EGR Valve
An Exhaust Gas Recirculation Valve (EGR) is another emissions-focused invention meant to burn off any excess nitrogen-filled particles that would be otherwise harmful to our planet. This valve opens and closes to allow exhaust gas to be recirculated into the combustion chamber in order to get burned again, saving fuel and burning off gasses completely.
8) Oxygen Sensor
Your oxygen sensor (or O2 sensor) is a finicky little thing usually connected to your exhaust system. If you notice a check engine light coming on (either constant or intermittently) or your car has a rough idle, then you could also have a faulty O2 sensor. Rough idling, bad fuel economy, and a check engine light are all common symptoms of an oxygen sensor problem.
9) Head Gasket Leak
If your head gasket leaks, it can be a very expensive and stressful experience. This is because it’s a pain to get access to your head gasket and the seal it makes is a critical component of successful combustion and the longevity of your engine.
Make sure if you see white smoke coming out of your exhaust after the car has warmed up, you may have a head gasket leak or a blown head gasket. Also, if your coolant is leaking, looks milky, bubbly, or you constantly have to top it off, you probably have a leaky head gasket.
10) Bad Fuel Pump
If your fuel pump is faulty or otherwise not functioning in tip top shape, you could have varying amounts of fuel being jetted into your combustion chamber during normal idle. This could cause one bang followed by a smaller bang, which you’ll feel through the floorboard as a popping/sputtering sound. Check out this article on all the possible causes and fixes for a bad fuel pump.
11) Problems With Piston Rings
Piston rings help seal the combustion chamber and control the flow of oil. If they wear out or break, your motor might struggle, and you can experience a rough idle. Worn piston rings can cause oil to seep into the combustion chamber, leading to poor combustion and a rough idle.
Additional Rough Idle Causes
If the above suggestions don’t fix the problem, try these out. There has to be something wrong here:
1. Idle speed needs adjusting
2. Loose or faulty distributor cap
3. Dirty fuel filter
4. MAF (Mass Air Flow) sensor
5. ECT (Engine Coolant Temperature) sensor
6. IAC (Idle Air Control) valve
7. TPS (Throttle Position Sensor) sensor
8. Throttle valve
9. EEC (Evaporative Emissions Control) system
