Does your car bounce or shake when you put it in idle? Does its engine rev inconsistently or idle below 600 RPM? Do you hear shaking or skipping sounds? It sounds like your vehicle has a case of rough idling. This problem can lead to engine damage that’s costly to fix.
There are several possible reasons behind the rough idle in your vehicle. Knowing the common causes of rough idling makes it easier to resolve the issue and restore your vehicle.
Here are the most common reasons for rough idling in your car:
Spark plugs ignite the fuel in the cylinder by generating an electric spark between the center and grounding electrodes. A bad car spark plug degrades the consistency and efficiency of fuel combustion. It can cause the air-fuel mixture to burn incompletely, causing rough idling.
You can often identify a bad spark plug by the part’s color.
A black spark plug is covered in engine oil that has leaked into the cylinder. The oil can prevent the spark plug from igniting properly. It also usually produces a burning smell. In contrast, a white spark plug burned because it ran too hot for various reasons, such as the engine running lean or the spark plug being loose.
Sometimes, you can clean a dirty spark plug to restore its performance. However, a damaged spark plug will require replacement.
Fuel injectors spray fuel into the combustion chamber. While fuel usually contains cleaning additives, carbon and dirt gradually accumulate on the injector’s tip. A dirty fuel injector can get stuck, interrupting the engine’s fuel supply and causing issues like rough idling.
You can check the fuel injector for issues by pressing a screwdriver beside the injector and listening for ticking sounds. Fuel injectors tick when they spray fuel into the cylinder, and the ticking follows the rhythm set by the spark plug and pistons. If the injector ticks erratically or doesn’t make a sound, you might need to clean or replace it.
Older car models lack a fuel injection system. Instead, they rely on a carburetor to mix air and fuel in the correct ratio for efficient combustion. The carburetor also delivers the air-fuel mixture to the engine.
As the vehicle and its carburetor put on the years, they become more prone to developing issues. A bad carburetor can produce a lean air-fuel mixture, leading to rough idling.
Fortunately, you can determine if the carburetor caused the engine to have a rough idle. Turn off the engine and remove the air cleaner while keeping the float bowl full. Watch the four main barrels of the carburetor when someone steps on the gas pedal.
A working carburetor’s barrels should release a small jet of fuel that briefly continues after releasing the gas. If the fuel jets don’t appear, you probably have a problem with your booster pump.
Engine compression refers to the pistons and valves compressing the air-fuel mixture in engine cylinders for ignition. If the mixture isn’t compressed enough, it might not ignite properly, causing problems like engine misfires, oil leaks, and rough idling.
Older, high-mileage (100,000 miles or more) cars are more prone to have low engine compression in a cylinder. Their camshaft lobes and valves have worn out, reducing the engine’s effectiveness at compressing air-fuel mixture.
You can usually notice the effects of low compression when you let your vehicle idle. If your car has a rough idle, check its engine for signs of bad parts that cause low compression.
The vacuum system acts like a massive air pump that regulates the movement of air, fuel, and exhaust gases in the engine. If something goes wrong with the system, it might pump excessive amounts of air into the engine. Adding too much air creates a lean mixture that can lead to the engine idling roughly.
Rough idling can also happen because of a leak in the vacuum system. Parts like a vacuum hose or O-ring can get damaged, wear out, or fail, causing air to leak out of cracked segments. The ensuing vacuum leaks are another possible cause behind rough idling because they reduce vacuum pressure in the engine.
The air filter screens the air flowing into the engine. It stops dirt, debris, and other contaminants that can damage the engine from entering the compartment.
As the air filter captures more contaminants, it becomes dirtier. A clogged filter reduces the air that the engine receives, leading to rich running that deposits soot on the spark plugs. The dirt can prevent spark plugs from generating enough spark to ignite the fuel, and the incomplete combustion process results in rough idling.
While you can clean and reuse some air filter designs, others require the installation of a new filter to eliminate rough idling.
Oxygen sensors (O2 sensors) measure the oxygen level in the exhaust gases expelled from the engine. They send their readings to the powertrain control module (PCM), which uses their data to adjust factors like the air-fuel mixture ratio to improve combustion, control emission levels, and enhance fuel economy.
A bad O2 sensor might send inaccurate readings to the PCM that indicate the previous air-fuel mixture didn’t have enough oxygen. The PCM will try to compensate for this perceived error by adding more air, inadvertently creating a lean mixture that can cause rough idling.
The positive crankcase ventilation (PCV) recycles exhaust gases in the crankcase by diverting some of the gases to the intake manifold to augment the engine’s air intake. A PCV valve controls how much exhaust leaves the crankcase. It opens when the engine needs a boost.
Like any other part, the PCV valve eventually wears out and can abruptly get damaged. If a worn-out or damaged valve gets stuck in the open position, it will keep releasing exhaust gases into the intake manifold. The air-fuel mixture becomes lean, leading to issues such as rough idling.
Whatever causes rough idling in your car, identify and repair it as soon as possible. It’s much easier and more affordable to fix the underlying issue behind a rough idle than to repair or replace the entire engine.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic. The accuracy and timeliness of the information may change from the time of publication.