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How to Troubleshoot a Car Idling at a High Speed

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In many ways, the idle speed of your engine is similar to the heart beat of the human body. Both are rhythmic, control the function of other supporting components and provide power for the vehicles we drive or our bodies. As we exercise and move quicker, our heart rate's increase – just like the engine's RPM does during acceleration. In order for both systems to work efficiently, they have to have a resting rate. In the body medical professionals refer to this as the resting pulse; automotive technicians call it engine idle RPM.

Like a high, resting heart rate, high engine idle RPM can be an indicator of trouble with other mechanical components or set the engine up for reduced efficiency during acceleration or deceleration. Like the human body, high engine idle RPM is typically impacted by the overall health of individual components including the throttle body, throttle linkage, idle control valve, EGR vacuum system and even the vehicle's respiratory system – air intake filters.

Diagnosing health problems is the responsibility of medical professionals; however, the process professional mechanics use to troubleshoot problems associated with high engine idle RPM are very similar.

  • First, the technician completes a series of tests to determine if some electrical or mechanical components are showing signs or wearing out or have broken.

  • Second, the technician complete visual inspections of these components and can then provide a diagnosis of what's causing the problem.

  • Finally, they are able to prescribe the right treatment or repairs to fix what's broken.

Noted below are a few of the most common reasons that your engine will experience high idle speed and how you can troubleshoot to determine the source of this problem; so that it can be repaired by a professional mechanic.

Understanding how engine idle speed works

Nearly every engine on the road today is able to operate thanks in part to the four-part combustion process. There are four unique phases of operation for a "four-stroke" engine which include:

  • The induction stroke: This is when fuel vapor is sucked into the combustion chamber as the piston is pushed in a downward angle

  • The compression stroke: This is when the piston rises upward and compresses the fuel vapor. This creates pressure inside the cylinder. At the top of this stroke (or just before it reaches the top) the spark plug ignites the fuel vapor and creates an explosion inside the combustion chamber.

  • The power stroke: As the explosion occurs, the force drives the piston downward and provides the "power" for the rest of the cycle to be completed.

  • The exhaust stroke: On the way back up the cylinder combustion chamber, the piston expels burnt exhaust through the exhaust valves and eventually through the back of your tail pipe.

In technical terms, the definition of idle speed is the rotational speed of an engine while it is uncoupled from the drivetrain and the throttle is not depressed. Translated into simple English – when the car is not in gear and the foot is off the gas pedal. The engine idle speed is measured in revolutions per minute (RPM) and typically range anywhere from 600 RPM to 1,100 RPM; depending on the engineering preference of the automotive manufacture.

Idle speed on a fuel injection system is controlled by the volume of air coming throttle body blades as well as the amount of fuel being streamed into the throttle body by a component known as an idle control valve, or in performance engines a barrel valve. As the idle control valve is adjusted, more fuel will enter the fuel injector manifold or less fuel will flow. This fuel then combines with the air being introduced by the throttle body blades and atomizes the fuel to be distributed to each intake valve in the cylinder head.

When the fuel volume is low the idle is slower. On the reverse, as more fuel is introduced, the idle speed increases. Therefore, when you have a high engine idle speed issue, the problem is typically caused by too much fuel volume flowing into the fuel injection manifold. On many cars and trucks, the engine idle speed will be increased by an electronic ignition and fuel control system when parasitic accessories (meaning components that are dependent on their operation by other components like belt driven pulleys for example that power AC systems and power steering systems) are in use.

If everything is working as it's supposed to, the engine idle speed should always remain the primary constant. However, over a period of time some mechanical components that monitor or control the flow of fuel into the fuel injector will become damaged and may contribute to increasing or decreasing engine idle speed.

Some of these components include:

  • Electrical fuses or the idle control valve itself
  • Vacuum leaks
  • Malfunctioning throttle blades or throttle control cable
  • ECM malfunction

Each of these areas has specific checks or inspections that should be completed to troubleshoot whether they are the source of the high engine idle speed. In the following sections, we'll describe the best methods for troubleshooting each component to help you locate the reason why the engine idle speed is high on a vehicle you are servicing.

Method 1 of 3: Troubleshooting issues with the idle control valve

The idle control valve, also commonly referred to as the idle air control valve, is designed to control and regulate the engine idle speed, increasing and decreasing it as necessary to keep up with operating conditions. On most vehicles, this component is attached to the intake manifold or the throttle body of the fuel injection system. It regulates the flow of liquid fuel and air (from the throttle blades) into the vehicles intake manifold.

On fuel injected vehicles made after the early 1990s, this component is monitored and in newer vehicles, controlled by the engine control module (ECM) which adjusts the idle speed according to multiple parameters including the engine's temperature, atmospheric conditions (density altitude) and electrical system input.

In most cases, there are two components that fail within the ICV that can cause the engine idle speed to be higher than it should be. These two areas include the following:

  • Faulty fuse or relay: If the fuse or relay that sends information to the ICV is damaged, or the electrical harness attached to the ICV is faulty (including being loose fitting) it can cause the ICV to trim down fuel volume without decreasing the air volume. This will increase the engine's idle RPM in most cases by as much as 500 RPM.

  • Mechanical damage to the ICV: If the idle control valve is mechanically damaged, it may also cause the engine idle speed to be increased. Usually the issue will be caused by more fuel and air being introduced to the fuel injection manifold which naturally increases the engine's idle speed.

To troubleshoot issues with the idle control valve; complete the following steps:

Step 1: Download any error codes stored in the ECM. In most cases a faulty idle control valve, either through electrical connections or the component itself will trigger an OBD-II error code that will be stored in the ECM.

This will also typically illuminate the Check Engine Light. In order to download the error codes, you'll need a digital scan tool.

Step 2: Inspect the fuse box for blown fuse. Referring to your vehicle service manual, locate the fuse or electrical relay that supplies power to the idle control valve. If this fuse is blown or if the area near that fuse is hot, it could indicate an electrical relay problem exists which should be repaired by a professional mechanic.

Step 3: Inspect the electrical connections to the ICV. Finally, check the electrical harness that connects to the idle control valve. In some cases, the connection will be corroded by excessive debris, dirt or engine grime.

If this happens, it can diminish the electrical signal to the idle control valve and cause increased engine idle speed issues.

If you complete each of these troubleshooting inspections and can't locate the source of your high engine idle speed, proceed to the next most common method documented below.

Method 2 of 3: Troubleshoot for a vacuum leak

For those who may not know, the rate of airflow through the typical internal combustion engine is vital in order to regulate and factor the amount of power it can generate. Manifold vacuum is present on every engine on earth that uses a throttle system to regulate the flow of fuel and the amount of power the engine makes. As a throttle is opened, the ambient air outside fills the intake manifold and are combined with liquid fuel. At this point the two create a vapor. However, it also increases the pressure inside the manifold (or fills the vacuum).

A system of vacuum lines and storage tanks helps to regulate the vacuum pressure inside the motor. On a fuel injected motor, a vacuum leak can cause the engine's RPM at idle to increase. In fact, it's possible to double the idle speed if the leak is significant enough. When a vacuum leak is detected, the O2 sensors on the engine will detect extra oxygen is entering the intake manifold. To adjust, it asks the idle control valve to supply more fuel in order to maintain the fuel curve. The result is that the engine's idle speed is increased.

Most mechanics typically look for a vacuum leak first when they have a customer that has a high engine idle speed issue. There are a few troubleshooting methods they use to pinpoint the source of the vacuum leak so that it can be diagnosed and repaired.

To troubleshoot a vacuum leak, complete the following steps:

Step 1: Pinpoint the vacuum sources on your vehicle. Depending on your vehicle, you may have as many as 10 different vacuum line connections that might be leaking.

The leak can come from a connection or the vacuum line itself. Before you try to diagnose any vacuum leak, make sure you have a diagram of all vacuum lines on your specific vehicle that you can check off as you inspect each one.

Step 2: Complete the vacuum leak check inspection. This method might seem a bit outdated, but it's arguably the best way to find a vacuum leak without having expensive diagnostic equipment. To complete this step, you'll need to have a full can of carb cleaner, a pair of safety glasses and safety gloves. Here is how it's done:

  • Make sure your engine is running

  • Spray the carb cleaner onto all vacuum line connections based on your engine's diagram. While you are spraying each vacuum line or connection you'll be looking and listening for the engine RPM to suddenly drop; and then increase. Carb cleaner is essentially a solvent, but it also can act as an accelerator fuel. When the vacuum line is leaking, it's sucking in outdoor air into the intake. When you introduce fuel to the equation, it will increase the RPM.

The important thing to remember during this test is not to spray the carb cleaner near any exhaust pipes or hot objects on the engine.

If you've completed the vacuum leak test and found a vacuum line that is broken, damaged or loose, repair it and see if the high idle speed issue is resolved. If not, proceed to check the final possible issue.

Method 3 of 3: Troubleshooting issues in the throttle cable or throttle body

The final mechanical component that might be causing an engine idle speed to be higher than it should is the throttle body or throttle control sensor. On most domestic and foreign cars, trucks, and SUVs, the throttle body is located on top of the engine, directly on top of the fuel injection manifold. When the throttle system is damaged, it can cause the idle to be higher or lower than it should be. It's also possible that some attachments like the air intake tube or air cleaner can be the culprit of similar symptoms.

In order to troubleshoot issues with the throttle system on your vehicle, complete the following steps:

Step 1: Determine all components that comprise your vehicle's throttle system. In general, most throttle systems include:

  • Throttle body
  • Throttle control module
  • Throttle actuator
  • Throttle cable
  • Air intake housing

Step 2: Examine each component physically to see if any electrical connection or wiring harnesses are loose or damaged. Most accelerated idle issues in this area will be caused by a wiring harness that is damaged or loose fitting.

Check the connection to determine if this is the case with your vehicle.

Step 3: Clean the throttle body. Although this is not always a troubleshooting step, completing this process will allow you to inspect the condition of the throttle blades (which can cause a high engine idle speed).

Follow the steps in this article for cleaning the throttle body.

After you've gone through all of the above methods, you should be able to pinpoint a mechanical issue that might be causing your engine to idle higher than it should. If you've gone through all of these steps, it could be caused by a malfunction inside your on board computer. Unfortunately, this is a step that should only be completed by a professional mechanic.

If you've read the instructions above and don't feel comfortable completing the troubleshooting inspection, or just would rather have a professional mechanic do this for you, contact one of our local professional mechanics to complete your high engine idle speed inspection.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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