Rough Engine Idle – Common Causes, With Possible Solutions

Rough Engine Idle - Common Causes, With Possible Solutions
Rough Engine Idle - Common Causes, With Possible Solutions

If your vehicle has a rough engine idle, possibly rising and falling; or having difficulty maintaining a steady rate of rotation, you have a problem.

Consequently, rough engine idle, is not a normal operating condition.

As a result, there can be a variety of reasons for a, rough engine idle.

It is best to try to diagnose and fix a rough engine idle; before it gets worse and before it gets expensive!

Also, the downside of a rough idle can be, reduced fuel economy, poor performance, starting issues; or potential major engine problems in the near future.

Rough engine idle is a common problem, of which the exact cause can be difficult to diagnose; as several factors may come into play.

Also, how your engine idles is a good indicator, of its overall health.

The reasons for a rough idle can vary, with some being; cheap easy fixes and some requiring more complex repair procedures.

Start With Checking For Any Trouble Codes; That Might Relate To Rough Engine Idle

So, today’s modern vehicles have sophisticated engine computers that monitor the operation of; sensors, actuators, fuel, combustion and exhaust systems. As a result, the vehicle engine computer can detect; small deviating operating parameters and store related trouble codes as necessary.

Check Engine Light
Check Engine Light

You can then use a code reader, to identify the specific problem, and then; determine if that malfunction is what is causing, the rough idle problem. The code may indicate the component, circuit or system involved in the fault. As a result, This gives you an additional advantage.

What Are The Conditions, When A Rough Engine Idle Occurs:

  • Does it happen on cold starts, after the vehicle has been parked for hours?
  • Does it happen, when you restart a warmed-up vehicle?
  • When does it happen? Does it happen all the time?
  • Are there any, odd noises?
  • Do you see any, smoke coming out?

When looking for solutions for a rough engine idle; make yourself a checklist to narrow down the possibilities.

While a rough idling engine may seem to be a simple inconvenience; it often indicates a deeper problem within the engine. The vehicle should be inspected and repaired as soon as possible. Because, small problems have a way of turning into expensive repairs.

A long list of systems, components and electronics, can cause a rough idle. That makes diagnosing the root cause a challenge, especially if you aren’t sure where to look.

Ultimately, everything from the ignition and fuel injection systems, to valves and pistons, can cause a rough engine idle.

A List Of Common Areas To Look At, For Rough Engine Idle Problems:

The #1 Cause Of Rough Engine Idle, Is A Vacuum Leak

Vacuum Leaks

Vacuum Leaks, Rough Engine Idle
Vacuum Leaks

Anything that upsets the air/fuel balance, will cause a rough engine idle. Usually the most common cause.

Also ReadVacuum Leak Detection – The Safe Way to Find Leaks

Carbon Buildup, On Electronic Throttle Body Or Idle Air Control Valve

Electronic Throttle Body, Rough Engine Idle
Electronic Throttle Body

Late model fuel injected engines have electronic throttle bodies; that no longer use an idle air control valve. Carbon buildup in the throttle body, can decrease the airflow. The computer doesn’t know, the carbon buildup is there. All it knows is that, the engine isn’t running right, based on it’s calculated opening. So, if you have an older vehicle, you may have a dirty idle air control valve. Consequently, some idle air bypass or idle air control valves can be cleaned.

Dirty Mass Airflow (MAF) Sensor

Mass Airflow (MAF) Sensor, Rough Engine Idle
Mass Airflow (MAF) Sensor

The mass airflow (MAF) sensor, is located right after the air filter and is responsible; for telling the computer exactly how much air is entering the engine. Over time, paper fibers from the air filter and crankcase vapors; can build up on the hot wire or plate and bake on. That baked on crud acts as an insulator, causing the computer to get incorrect readings. You can clean the (MAF) sensor yourself, with an aerosol can of (MAF) sensor cleaner.

Also ReadMass Air Flow Sensor (MAF) – Measuring, Volume, Density, Temperature

Fuel Injectors

Fuel Injectors, Rough Engine Idle
Fuel Injectors

Sometimes dirty fuel injectors, can be the root cause, of a rough engine idle. Fuel injectors disperse fuel into your vehicle’s engine; at a precise angle and quantity, to ensure optimal performance. That being said, dirty fuel injectors are also a major contributor, to poor gas mileage. Using an injector cleaner fuel additive, is a simple way; to prevent this problem and keep your engine, running smoothly and efficiently.

Also ReadFuel Injector – Deposits Can Build Up And Clog The Fuel Injector

Carbon Buildup, On Direct Injector Engines

Direct Injection, Rough Engine Idle
Direct Injection

In direct injection engines, the fuel injector delivers fuel right into the cylinder; so the valves never get fuel spray. After shutdown, crankcase vapors rise to the top of the engine and settle on the intake valves; where they condense and harden. The result is, carbon buildup that can cause, a rough engine idle. Fuel injector cleaner will NOT remove this build up; because the cleaner never sees the backside of the valves. You must inject a cleaner through the intake using a procedure known as, air induction cleaning.

Engine Valves

Carboned Up Engine Valves, Rough Engine Idle
Carboned Up Engine Valves

Exhaust and Intake Valves, can be a big problem, if they are carboned up. This can cause lower compression, as the valve might stick open.

Also ReadSticking Valves From Carbon Deposits – What Should You Do

Spark Plugs And Wires

Spark Plugs
Spark Plugs

A rough engine idle, can be caused by, spark plugs or wires. Spark plugs use the electrical current; received from ignition coils to ignite the air/fuel mixture, within the combustion chamber. A plug that is damaged or installed incorrectly, can result in fuel being burned at an inconsistent rate. If the damage is bad enough, you may also notice your engine running rough. As a result, replacing your spark plugs on time and using the right plugs and installation techniques; are critical to the smooth operation of your engine. Check your plugs and wires, to see what condition they’re in.

Also ReadSpark Plug Color – Evidence Of What Is Happening Inside The Engine

(EGR) Valves

Carbon Buildup On (EGR) Valve
Carbon Buildup On (EGR) Valve

The (EGR) valve is a mechanical device and just like valves and the throttle body; it can develop carbon buildup that prevents it from closing fully. When that happens, the (EGR) valve allows exhaust flow at idle; which causes a rough engine idle condition. Most often, you can use a throttle body cleaner, to remove the carbon.

Also ReadExhaust Gas Recirculation (EGR) Valve – What You Need To Know

Fuel Pressure

Fuel Pressure Gauge
Fuel Pressure Gauge

If the fuel pump is failing and not delivering the proper amount of pressure or volume; it simply won’t deliver the amount of fuel the computer expects to see. You must attach a fuel pressure gauge, to actually measure fuel pressure. Consequently, if your pump doesn’t deliver the right pressure or volume, replace it.

Also ReadFuel Pressure Regulators – Function And Failure Symptoms

Also ReadElectric Fuel Pumps – How Do They Work – How Can They Fail

Engine Coolant Temperature (ECT) Sensor

Engine Coolant Temperature (ECT) Sensor
Engine Coolant Temperature (ECT) Sensor

The computer needs to know the engine and air temperature, to calculate the right air/fuel mixture. So, if it’s getting the wrong readings; it will deliver the wrong amount of fuel, for a given amount of air. Engine coolant and air temperature sensors, usually don’t fail completely. Instead, they give false readings. So, if your engine is hard to start on cold mornings; requiring you to depress the gas pedal, that’s a symptom of; a failing engine coolant temperature (ECT) sensor.

Also Read(ECT) – Engine Coolant Temperature Sensor-Function, Failure And Testing

Oxygen (O2) Sensors

Oxygen (O2) Sensors
Oxygen (O2) Sensors

Your oxygen sensor is part of your vehicle’s emissions system. It protrudes into the exhaust system, continuously monitoring the oxygen content of the exhaust. It sends this information to the engine computer, which uses it to; maintain the correct air-fuel balance for efficient and clean combustion.

The oxygen (O2) sensor can be adversely affected by, the high temperature environment it works in. As a result, it can become, covered with carbon deposits or simply wear out. When this happens, it sends incorrect information to the engine computer. The engine can run too rich or too lean. A too-rich mixture will result in, poor fuel economy. A too-lean mixture, will cause a rough engine idle.

Also Read(O2) Oxygen Sensor – What They Do – How They Fail – Rich-Lean Testing

(PCV) Valves

(PCV) Valves
(PCV) Valves

The (PCV) valve is responsible for metering a set amount of airflow; from the crankcase to the intake where it is burned. The (PCV) valve contains a safety plunger, to prevent a backfire from entering the crankcase. So the plunger is an obstruction to airflow. Over time, oil vapor and carbon, can build up on the plunger, reducing airflow. That reduction in airflow, can cause a rough engine idle. In addition, a cracked (PCV) hose, can also cause a rough engine idle.

Also ReadPCV Valve – What Does It Do ? – Signs Of A Bad PCV Valve

Air Filters

Air Filter
Air Filter

Most engine air filters use a folded paper element; which can become clogged if not changed at the appropriate interval. So, it is just as important for your engine to get enough air; as it is for it to have enough fuel. Finally, a clogged filter will reduce the flow of air into the engine, causing a rough engine idle.

Leaking Head Gaskets

Leaking Head Gaskets
Leaking Head Gaskets

Coolant in your oil, oil in your coolant, missing coolant, coolant overflowing out of reservoir, overheating; are all signs of a head gasket leak and they can easily cause a rough engine idle.

Also ReadCylinder Head Gasket Leaks – Know The Signs And Symptoms

Rough Engine Idle On Carbureted Engines


Older vehicles utilize a carburetor, rather than a fuel injector. Black exhaust smoke is a common indicator of a problem with the carburetor. A carbureted system that is running well, shouldn’t produce excessive amounts of black smoke. So, look out for this, as a sign that something is wrong. Using a carburetor cleaner, is a simple step to help dissolve these carbon deposits and keep them clean; in order to prevent or decrease a current rough engine idle.


So, an engine which is operating properly, should run smooth without any excess noise. When it begins to idle ‘rough,’ there are a number of possible causes. Consequently, how your engine idles, is a good indicator it its overall health.

And, as we always say, Is a wire loose? Did a vacuum line fall off ? Do not look for a complicated solution, to a simple problem. Because, it is common to have a problem in one area, that impacts three or four others.

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