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If you turn your car on and it’s not running smoothly, there could be various reasons that it’s idling rough. The issue could be something small or large. But either way, it’s best to diagnose the issue as soon as possible. Here are a few reasons why your car’s engine not idling smoothly.

What is rough idling?

A view of a tachometer in a car.
A view of a tachometer in a car. | Wodicka/ullstein bild via Getty Images

When your car’s engine RPMs (revolutions per minute) rise and fall or struggle to maintain a steady rate, that’s known as rough idling. According to Repair Pal, “A smooth idle depends on just the right mixture of air and fuel… there can be many places where that combination is getting unbalanced.”

The first step is to identify when the car idles poorly. Does it happen when the car is cold? Does it happen after the car is warmed up? Does it happen all of the time? It’s a good idea to note when the car idles rough so that you can notify the mechanic working on your car. This information will help them narrow down the issue.

Common causes for rough idling

A mechanic checks an injector on an engine.
A mechanic checks an injector on an engine. | Gregory Rec/Portland Press Herald via Getty Images

In case you’re trying to diagnose the rough idling issue yourself, here are a few common causes:

  • Dirty fuel injectors: Your car’s fuel injectors spray fuel into the engine at the right moment. However, those injectors operate at high temperatures and pressures. Over time, they can get dirty and clogged up from the combustion process, which leads to less fuel going through the injectors and poor idling.
  • Solution: Removing the injectors and having them cleaned can eliminate the rough idle issue.
  • Bad spark plugs: If you haven’t changed the spark plugs on your car’s engine recently, then they could be bad. Over time, the spark plugs can be fouled with carbon deposits or oil from the combustion process. These carbon deposits will reduce the amount of power produced by the plugs when they ignite, which can lead to a rough idle.
  • Solution: Changing your car’s spark plugs and wires can aid with bringing the car’s idle back to normal. Make sure to gap the plugs to the factory specification to ensure they are working correctly.
  • Dirty air filter: Your car’s air filter is meant to filter out the dirt in the air so that it doesn’t enter the engine. Over time, this filter can get dirty, which restricts the airflow to the engine. As you can guess, this restricted airflow can cause poor engine idling.
  • Solution: Changing your car’s air filter is simple and typically takes a few minutes. By changing the air filter, the air coming into the engine won’t be restricted, and the idle may return to its normal level.

A qualified mechanic should be able to identify the issue easily

People working on cars that possibly have the most expensive 10-year maintenance.
Mechanic garage | Nathan Laine via Getty Images

If your car is suffering from rough engine idling, then it could be one of the aforementioned three issues. However, it can also be caused by other damaged or dirty parts like the PCV valve, a vacuum leak, or too many miles on the engine.

Either way, a qualified mechanic should be able to pinpoint the issue easily and get your car running normally in less than a day.


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