What to Look For: A rough, inconsistent start, bouncy idle and a lack of power when accelerating.
Wrong Idle Speed - Most cars have a proper idle speed, usually between 600 and 1000 RPMs. Idle car speed can change due to wear and tear. Fortunately, an adequate tune-up can restore the correct idle speed.
What to Look For: Idle RPMs will usually drop below 600 RPM or whatever is typical for your vehicle. The idle speed feels slower than usual.
Faulty Fuel Pump - The fuel pump sends gasoline from the tank to the injection system. Over time, the pump can fail due to wear and tear or become clogged. As a result, the engine doesn’t receive enough fuel.
What to Look For: A bad fuel pump can cause difficulty when starting the vehicle, a rough idle, and stalling.
Clogged Fuel Filter - The fuel filter filters the contaminants from gasoline. Over time it can become clogged, which slows the flow of fuel. In turn, the engine doesn’t receive enough fuel.
What to Look For: In addition to rough idle, random misfires, difficulty starting, and a car that won’t start are common symptoms.
Faulty Engine Temperature Sensor - Cars require different fuel mixtures depending on the engine’s temperature. A cold-start requires a richer mixture, for example. When the temperature sensor fails, the fuel injection system thinks the car is warmer than it is, which can result in the wrong mixture being used.
What to Look For: A lousy temperature sensor results in rough idle during cold starts. Once warm, the problem may be less noticeable.
Faulty Airflow Sensor - Like the temperature sensor, the airflow sensor helps your car know how much fuel to use, based on the amount of air in the engine. When this sensor fails, the vehicle’s computer can’t correctly calibrate the right mixture of fuel to air.
What to Look For: Rough idle is one of the first symptoms to appear, followed by sputtering on acceleration or even stalling, as the problem worsens.
Vacuum Leak - Your car sucks in a lot of air, and it’s able to do that thanks to a vacuum in the intake manifold. A leak in this system harms performance, as your vehicle can’t regulate the right mix of air to fuel.
What to Look For: Rough idle, slow acceleration, sputtering or a lack of power on acceleration are all symptoms of a leak in the vacuum system.
Faulty EGR Valve - The EGR valve is responsible for recirculating exhaust fumes back into the combustion system. Over time, this valve can become stuck open or closed, and performance issues, including rough idle, are the most common systems.
What to Look For: In addition to rough idle, reduced fuel economy, knocking noises, and slow acceleration may also be present.
Failing Spark Plugs - The spark plugs create the spark that ignites the fuel in the engine. Faulty spark plugs, therefore, struggle to ignite the fuel, causing rough idle and difficulty starting.
What to Look For: Your car may struggle to turn over and will idle rough once it does.