What Causes Rough Idle?

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Those who have owned a vehicle with an internal combustion engine for more than a few years are likely familiar with rough idle. It can be a frustrating and confusing engine problem to run into. However, rough idling isn’t an issue in itself. Instead, a rough idle typically indicates there is an underlying problem with the vehicle or engine.

A shaky, unstable idle is a common symptom of dozens and dozens of potential engine problems. The sheer number of underlying issues can make diagnosing and repairing the vehicle a challenge. There are a number of causes that are most commonly associated with a rough idle, though. In this article, we discuss rough idle, common causes, diagnostics, & more.

What is Rough Idling?

Rough idle is a general term for an internal combustion engine that isn’t idling as smoothly as it should. When a vehicle is in “idle” mode it means the engine is not under load or in motion. Most modern engines should idle around 600 to 1,000rpm (with the majority falling closer to 600-800rpm). Rough idling refers to an engine that is unstable, shaky, and/or excessively vibrating at idle speed.

However, there are numerous types of rough idle. It may simply be excess vibrations or a slight choppiness to the idle. In some cases, the RPM’s may be bouncing up and down or the engine feels like it’s about to stall. It may only idle poorly in park or drive specifically. Ultimately, there are differences in rough idling but they all mean the same thing. A rough idle just means something with the idle is incorrect.

Common Causes of Rough Idle

Again, there are dozens and dozens of potential problems that can lead to an improper idle. The list would be lengthy if we included every single one. As such, we split the common causes of rough idle into categories including:

  • Ignition system
  • Fueling system
  • Carbon build-up
  • Sensors & Electrical
  • Engine & transmission mounts
  • Vacuum leaks
  • Low compression

If you would rather consume this content via a video, check out our How to Diagnose and Fix A Rough Idle Car video below:

1) Ignition System

The ignition system primarily includes the spark plugs, ignition coils, and ignition wiring. Let’s leave the wiring aside for now as we’ve included sensors & electrical causes in another section. Anyway, spark plugs and ignition coils are two of the most common causes of rough idle.

Spark plugs create the spark that ignites the air-fuel mixture within the combustion chamber. It’s the ignition coils that generate the high voltage required to fire the spark plugs. Without a proper spark, the air-fuel mixture will not burn completely or won’t burn at all. This leads to misfires which in turn cause a rough idle.

Generally, faulty spark plugs or ignition coils will show engine misfire codes like P0300, P0301, P0302, etc. Using a code reader can help narrow down which cylinder(s) is misfiring. However, if spark plugs or ignition coils are the cause of your rough idle and they’re due for replacement then it’s likely best to replace all of them.

Common Causes of Rough Idle - Spark Plugs

2) Fuel System Issues

The fuel system is another common cause of a shaky, unstable, and poor idle. A few of the most common fueling system problems that lead to rough idle include:

  • Fuel injector
  • Fuel pump
  • Clogged fuel filter

An engine relies on the proper mixture of air & fuel to achieve a complete and efficient combustion process. If any of those pieces are missing or incorrect then you’ll likely experience rough idle along with other symptoms and issues. We talked about the ignition system last and the same concept applies to the fueling system.

Injectors may become stuck, clogged, or leak. A stuck or clogged fuel injector won’t flow enough fuel which leads to a lean air-fuel ratio (AFR) and a poor idle. Leaking injectors flow too much fuel which causes rich AFR and also a rough idle.

Faulty fuel pumps or a clogged fuel filter will also lead to a lack of fuel flow and lean AFR’s. Injectors, fuel pumps, and the fuel filter are the most common fueling related issues. However, a number of other fuel system problems may also lead to a bad idle.

3) Carbon Build-Up

With carbon build-up we’re specifically talking about gasoline direct injection (GDI) engines. The use of GDI has become more common the last couple decades as it offers better fuel economy, lower emissions, and more power compared to multi-point injection (MPI). It’s the best of all worlds so who could disagree with direct injection, right?

However, direct injection does have one drawback – carbon build-up. All engines produce some degree of oil blowby which makes its way back into the intake tract. It then sticks to intake valves and ports. With traditional MPI, fuel is sprayed into the intake valves and detergents in the fuel clean off any deposits.

That isn’t the case with GDI, though. Fuel is sprayed directly into the cylinders, so the deposits harden and form carbon build-up. As the carbon deposits continue building they begin restricting air-flow into the cylinders, which can in turn cause misfires, rough idle, and a number of other symptoms.

4) Sensor & Electrical Failures

Next on the list of common causes of rough idle are problems with sensors and electrical issues. This is where the list of potential problems that can cause a poor idle get pretty lengthy:

  • Mass air-flow (MAF) sensor
  • Manifold absolute pressure (MAP) sensor
  • Throttle position sensor (TPS)
  • Crankshaft position sensor
  • Camshaft position sensor
  • Oxygen (O2) sensors
  • Ignition wiring
  • ECU issues

Sensors are critical components of modern vehicles and engines. The engines computer – also known as the Powertrain Control Module (PCM) or Engine Control Unit (ECU) – relies on dozens of sensors. It uses the signals from these sensors to control ignition timing, fuel flow, air flow, and just about everything between.

All it takes is one faulty sensor to throw things out of balance and cause a rough idle among other symptoms. For example, a faulty MAF sensor may lead the PCM to believe there’s too much or too little air flow. O2 sensor issues can cause issues with air-fuel ratios. The list goes on.

Electrical components are also critical for proper engine operation. Faulty sensor wiring or ignition wiring can cause the same issues. Much less common – but still possible – are faults with the ECU itself.

5) Engine & Transmission Mounts

Engine and transmission mounts aren’t directly related to engine operation. They’re simply designed to keep the engine & transmission secure and absorb any shocks and vibrations. However, the mounts are often made of rubber and are prone to degrading with age and mileage.

As they wear down they’re no longer able to absorb bumps and vibrations as well as they once were. Symptoms of old, worn engine or transmission mounts are more noticeable while driving. You’ll likely notice a bumpier, harsher ride and hear clunking or banging sounds.

That said, internal combustion engines also produce some vibrations – even while idling. This is especially true with the use of ancillary components like the drive belt and cooling fan. Worn engine or transmission mounts won’t absorb vibrations as well. This can lead to the feeling of a rough idle. Although, it’s usually a modestly poor idle so if your rough idle is really bad then it’s unlikely to be the mounts.

6) Vacuum Leaks

Vacuum in an ICE is the difference in air pressure between the intake manifold and the outside atmosphere. The difference in air pressure creates suction (vacuum) which helps pull air into the engine. Vacuum also controls systems like brake boosters, PCV valves, EGR valves, and much more depending on your specific engine and model.

Ultimately, vacuum helps control engine rpm, air flow through the engine, and a number of ancillary components. Proper vacuum is important to smooth engine operation and performance. Any vacuum leaks or inconsistencies can lead to a host of symptoms and problems, including rough idle. Some potential vacuum leaks and issues include:

  • Damaged vacuum lines
  • Cracked intake manifold
  • Faulty EGR or PCV valves
  • Leaking gaskets or seals

Since vacuum controls so much there are a lot of possible areas for vacuum leaks or problems. The vacuum lines are a good starting point and the lines tend to become brittle and begin cracking with age. A cracked intake manifold or manifold gasket failure can also lead to improper vacuum.

Faulty valves such as the exhaust gas recirculation (EGR) and positive crankcase ventilation (PCV) valves can also cause rough idle. An engine also relies on airtight seals to ensure proper airflow, so things like the throttle body seal can also cause vacuum and rough idle issues.

Rough Idle Caused by Low Compression
Compression tester for car fixing usage

7) Low Cylinder Compression

Low compression is one of the least common causes of rough idling compared to most other issues in this article. However, low cylinder compression is something to consider – especially on older, higher mileage vehicles. Internal combustion engines aren’t designed to last forever, and the rotating assembly takes a lot of abuse during its life.

Cylinder walls are prone to scratching and wear, piston rings wear down, valves may no longer seat properly, among other possible causes of low compression. If the cylinder isn’t airtight then you have low compression, which can lead to rough idle and other frustrating symptoms.

Unfortunately, low compression typically indicates a serious internal engine problem that is likely expensive to fix. It’s uncommon on newer engines but design flaws and fluky issues do happen. Otherwise, if your engine has 200,000+ miles then low compression is a very possible culprit of a rough idle.

How to Diagnose Rough Idle Issues

The list of potential issues that lead to rough idling is pretty close to endless. Just about any ignition component, air, fueling, sensors, leaks, etc. can lead to a rough engine idle. With so many possibilities how the heck do you diagnose and fix a rough idle? Fortunately, modern engine computers are very capable and make our lives much easier.

A modern ECU/PCM is able to store fault codes and data about potential engine problems. Often, plugging a code reader into the OBDII port will provide you with a diagnostic trouble code (DTC) – if any are stored in the cars computer.

As such, a code reader is an essential piece of equipment when it comes to diagnosing the underlying issue that’s causing a shaky idle. Let’s jump in and discuss some easy steps to diagnose a rough engine idle.

Steps to Diagnose Rough Idling

  1. Plug code scanner into OBDII port to identify potential diagnostic trouble codes
    • If a code is present it will likely help point you in the right direction. For example, if you see DTC P0171 – system too lean (bank 1), that points to a likely fuel delivery issue or O2 sensor problem.
  2. Start with the basics once a specific code is identified.
    • Using the previous example – check fuel lines, look for any possible fuel leaks, noticeably damaged O2 sensors or wiring, etc. Don’t jump straight into the expensive or complicated things.
  3. Note and confirm the conditions that cause the rough idle. Is it only in park or drive? Is the engine also stuttering or showing other symptoms while driving?
    • Issues like a vacuum leak may only cause symptoms at idle. Once vacuum builds with higher RPM the engine may operate seamlessly. Consider other issues that may only cause a shaky idle and no other issues while driving.
  4. If nothing obvious is out of place and/or you do not have any stored DTC then it’s time for more extensive testing or having a qualified repair shop (with more sophisticated equipment) diagnose the issues.
    • For example, a smoke test is an effective way to identify vacuum leaks. Compression and leak-down testing can help identify deeper internal engine problems.

Ultimately, it’s hard to put together a foolproof list of the steps to diagnose rough idling. There are so many possible causes and the exact procedures depend on symptoms, engine fault codes, and other factors.

Don’t let that distract from the main point, though. A rough idle is usually a symptom of another underlying problem, and modern vehicles are very good at identifying issues and storing codes. Use a code scanner and start with the basics. If nothing is pointing you in the right direction then it’s time for more extensive testing or a visit to a qualified auto repair shop.

Rough Idle FAQ

What causes rough idle when ac is on?

Common causes of a rough idle when the AC is on are a faulty idle air control (IAC) valve or a problematic cooling fan. The IAC valve controls the amount of air required for a steady idle, so any faults can lead to an unstable, rough idle. On the other hand, the cooling fan typically runs faster with the AC on, so a faulty cooling fan may lead to a poor idle.

Can a bad serpentine belt cause rough idle?

Yes, a bad serpentine (drive) belt can cause an unstable, rough idle. This belt powers a number of ancillary components like the alternator, AC compressor, power steering pump, and more. Issues with the serpentine belt can affect a number of systems which in turn may lead to a rough idle among other symptoms.

Can a leaking valve cover gasket cause rough idle?

It’s unlikely a leaking valve cover gasket will directly cause an engine to idle roughly. However, oil from a valve cover gasket leak can drip into the ignition coil and spark plug holes. As oil builds up on ignition coils or spark plugs then it can lead to engine misfires and a rough idle.

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