Your Trusted Auto Repair Shop for 40+ Years
Shop Talk Logo

Why Does Your Car Idle Rough?

New cars are relatively quiet, have a smooth ride, and all the gadgets and gizmos seem to function very well. Fast forward a few years and thousands of miles and that once new car may be a bit noisier, have a rougher ride, and you may have to get creative to get certain functions to work. 

Just because your car is aging, doesn’t mean it’s time for a new one. A little TLC will go a long way to get your car back to that like-new feeling again. Older cars may experience rough idling or vibrations that can be eradicated just by simple maintenance. 

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 idling include:

Worn Out Spark Plugs

Worn Out Spark Plugs: Car is Idling RoughCombustion needs air, fuel, and spark. Spark plugs, spark wires, and the ignition coils help produce the voltage to ignite the air/fuel mixture in each of the cylinders. Over time, spark plugs collect oil and carbon deposits that lessen the power created during ignition. Replace spark plugs every 30,000 – 50,000 miles for optimal operation and to prevent damage to electrodes that may cause damage to the ignition system.

Vacuum Leak

Under the hood are many hoses that snake around the engine. These hoses are meant to create a vacuum seal for the air and fuel controlled by the throttle to regulate air flow and engine speed and vacuum within the intake manifold. After a few years and exposure to extreme temperatures, the rubber hoses wear down and develop leaks. When too much air is added to the air/fuel mixture it can cause misfires, rough idle, and an increase of RPMs. Have a technician inspect your vehicle’s hoses regularly for cracks, breaks, or dryness and replace every 50,000 miles.

Clogged 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. 

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. 

What To Do If Your Car is Idling Rough

As you drive, your vehicle is taking air into the engine. However, to prevent dirt, debris, and other contaminants from damaging delicate components in your engine, it’s equipped with an air filter. As time goes on, the air filter becomes clogged and may do more harm than good because its not allowing the proper amount of air flow needed to mix with fuel for combustion. A shortage of air can cause rough idling and poor fuel economy. Air filters should be replaced every 15,000 miles, however, if your vehicle suddenly idles poorly, start by replacing your air filters. If this doesn’t fix the problem, visit your local automotive service center for a thorough inspection. 

Rough idling is an unusual behavior for a vehicle. Routine maintenance can prevent major damages to your engine and can help ensure components continue operating optimally. When your older car is acting not-so-new and starts vibrating, shaking, or idles poorly, visit your local Sun Auto Service. 

In addition to checking the components above, our ASE Certified Technicians will also check some other common issues that can cause rough idling including:

  • Failed fuel pump
  • Leaks from the head gasket
  • Idle air control valve
  • Mas Air Flow (MAF) sensor
  • Coolant sensor
  • Evaporative emissions control system
  • Throttle valve
  • Throttle position sensor