When you start your vehicle, you expect it to fire up and run smoothly. That smooth operation indicates an engine that is in rhythm, running at a consistent RPM, and idling as it should.
If you start your vehicle and find yourself bouncing in your seat, experiencing an intermittent shudder or shake, or you see your RPM fluctuating, you’re dealing with a rough engine idle.
Simply put, idling happens to a vehicle when you start your engine and allow it to run without pressing the accelerator or changing gears. For a healthy vehicle, maintaining a steady rate of about 1,000 revolutions per minute means the engine is able to provide enough power to operate your car’s basic systems, but when your car is experiencing issues, your RPM will not be so steady.
Read on to learn a few of the reasons why your engine may have a rough idle, how to identify them and some simple fixes to try.
So, What Is Rough Idle?
It refers to an engine being ‘bouncy’ or unstable, usually making odd noises, while the vehicle is stationary and the engine is running. It will see unsteady RPM fluctuations or a struggling engine that fails to maintain a smooth, consistent speed.
The idle speed should feel consistent without skipping or slipping. In most of today’s cars, an idle speed of 600 to 1000 RPMs is average. If your car is idling rough, though, it won’t feel smooth. The RPMs will jump up and down, for example, or they’ll fall below 600 RPM (or whatever is typical for your vehicle).
Rough idle is easy to detect when you start your vehicle, and it may be dependent on the engine temperature when you start your car. A cold-start rough idle, for example, may be caused for different reasons than a vehicle that idles rough only when the engine’s hot.
In other words, take note of when your vehicle shakes during idle, as well as any sounds. This can help your mechanic identify the problem.
Why Does My Car Idle Rough?
An engine needs the proper air and fuel ratios to obtain the smoothest combustion process. A well-functioning engine that is receiving the optimal mixture of air and fuel is able to produce a smooth ride as well as generate the power that is required to operate critical systems such as the power steering, air conditioning, electrical system, and the cooling system.
A car that is idling rough produces a sluggish feeling and strange vibrations. This is often due to a failed component or one that is malfunctioning and needs attention before the engine quits all together.
In addition to poor fuel efficiency, poor performance, trouble starting, or too high or too low RPMs, rough idling may be a precursor to severe engine problems in the future. Vibrations and rough idling should be taken as a warning sign to get your vehicle to an auto shop before you wind up with a broken-down vehicle or costly repairs.
Common Causes of Rough Idle
Now we have established what rough idle is, let’s delve into the most common causes.
#1. Fuel system issues.
The first and most common cause of rough idle is issues with your fuel system. These can include a clogged fuel filter, dirty fuel injectors, or even a malfunctioning fuel pump. These can disrupt the proper fuel-air mixture ratio, leading to rough idling.
The first possibility is a dirty fuel injector. The fuel injector is frequently exposed to high temperatures and pressure, resulting in carbon buildup in the pintle or nozzle.
The cylinder fed by a stuck or clogged injector engine won’t receive the right amount of fuel. This can cause anything from a high-frequency shutter or shake to a mildly rough idle.
A clogged fuel filter can also create the same problem. Over time, the fuel filter can get contaminated, which can slow down the flow of fuel.
A faulty fuel pump is another probable cause of vibration at idle. This part is responsible for sending the right amount of fuel pressure and volume to the engine so that the vehicle can run properly. Typically, when a fuel pump fails so that it produces air (cavitation) and the fuel system is returnless, air bubbles will have to pass through the injectors and can cause misfires and rough idle.
A faulty fuel pump can also lead to problems like hard starting, stalling, hesitation, and misfiring if left unaddressed.
#2. Faulty Spark plugs, spark plug wires, and ignition coils.
A rough idling engine can be caused by spark plugs or spark plug 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 while driving. Pay attention to any jerks or stutters, particularly while accelerating under a load, as a symptom.
How to fix it? Get a good mechanic to inspect each spark plug, ignition coil, and other parts of your ignition system. Ideally, they’ll replace all the spark plugs in your car if required.
#3. Dirty/Faulty Fuel Injectors.
Modern vehicles are engineered to achieve superior fuel efficiency. One of the many ways this is accomplished is by using high-pressure fuel injectors that dispense the proper amount of fuel into the engine’s cylinder at the precise time.
Fuel injectors operate in a hostile environment where they are exposed to constant fuel pressure and extreme temperatures. Over time, the nozzles become blocked from the build-up of carbon left behind from the combustion process.
Blocked fuel injectors affect the spray patterns or prevent the necessary amount of fuel to pass through to complete the combustion process. A disruption in the combustion process causes poor vehicle performance and rough idling. Replace fuel injectors between 50,000 to 100,000 miles. Pro tip: better fuel will prolong the life of fuel injectors.
How to fix it? Cleaning or replacing the fuel filter and adding a fuel injector cleaning treatment to the fuel tank should take care of clogged fuel injectors. If the rough idle persists, it’s time to take your car to an auto repair shop.
#4. Vacuum Leak.
Most vehicles have a maze of hoses throughout which create a vacuum for both fuel and air. In older engines with a carburetor, this vacuum pulls the fuel into the engine. Most newer vehicles, however, have a throttle to regulate engine speed and airflow through the system.
This also creates a vacuum within the intake manifold. These hoses can wear out over time and a leak may develop. If too much air gets mixed in with the fuel, it will cause your engine to misfire. This will result in a rough idle, usually at a higher RPM.
How to fix it? Look for a hissing sound while your engine is running to spot a vacuum leak. Replacing the damaged vacuum hose should be an easy fix.
#5. Faulty Sensors and Valves
Modern vehicles rely on a variety of sensors and valves to run properly. Your temperature sensor, for example, alters the fuel/air mixture based on the needs of your engine when cold or hot. Your airflow sensor controls the mixture based on the amount of air in the engine.
Your EGR valve recirculates exhaust fumes back into the combustion system and your PCV valve takes air and fuel from the crankcase and routes it back through the intake manifold. In short, if any of your vehicle’s sensors or valves are failing or stuck, it could cause your engine to run rough.
#6. Motor Mounts
A car that idles rough but drives smooth may be due to broken motor mounts. The purpose of motor mounts is in the name! These cylindrical components are what keep your engine in place. Broken or weak mounts are unable to support the engine properly, creating a vibration at idle.
Though there is no schedule for replacing motor mounts, their average lifespan is between 5-10 years. Have your technician inspect them regularly to look for dry rot or breakage from age.
#7. Incorrect Idle Speed
Car engines are usually made to idle at a standard speed of 1000RPM. If your car has a rough idle, it may be because your idle speed setting is incorrect. This setting can be easily reset using a butterknife or similar tool.
But keep in mind, the idle speed should be set in stone – if it is inconsistent or changes at random intervals, you may have a bigger issue on your hands.
Some other causes of a rough idle
The items listed above are the most common causes of a rough idle. But there are many other elements under the hood that can cause this problem. Your mechanic will systematically check the relevant components of your engine and its emissions system to isolate the cause of your rough idle. These include:
- Head gasket leak
- Fuel pump failure
- MAF (Mass Air Flow) sensor
- ECT (Engine Coolant Temperature) sensor
- IAC (Idle Air Control) valve
- TPS (Throttle Position Sensor) sensor
- Throttle valve
- EEC (Evaporative Emissions Control) system
Diagnosing rough idle
You will usually be able to immediately identify if your vehicle is experiencing rough idling. While some cases are less severe than others, a rough idle is usually recognisable by a shaking and bouncing sensation in the vehicle. Odd sounds, as well as inconsistent RPM counts, may also be signs your car is rough idling. (above 1,000).
One of the main ways is to observe and listen for unusual knocking, tapping or misfiring noises. You can also observe the engine’s behaviour, including RPM fluctuations, vibrations, and hesitation.
Other ways to identify rough idle can include –
- Dashboard warning lights for specific issues
- Inspect spark plugs
- Regularly examine the air intake system
- Check for vacuum leaks in hoses
- Perform compression tests
- Consult service manuals or professionals
By following these steps and conducting a thorough and regular inspection of your vehicle, you can identify the cause of rough idle and take appropriate steps to solve the issues. If diagnosis becomes challenging or if you’re uncertain, consulting a qualified technician is advised.