Why Is My Car Idling High?

Updated on June 17, 2020
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

A car idling high may be caused by one or more common components.
A car idling high may be caused by one or more common components. | Source

Is your car idling high? Usually, you can find the culprit behind one of these components:

  • Faulty idle air control solenoid
  • Leaking intake manifold or vacuum hoses
  • Bad PCV valve or ECT sensor
  • Sticking throttle linkage

The following sections deal with the most common sources of high idle problems. This can save you diagnostic and repair time. And, on many cases, you'll be able to fix the problem yourself, using a few common tools you may already have in your toolbox.

Index
1. Faulty Idle Speed Control
2. Vacuum Leaks
3. Leaking Air Cleaner Assembly
4. Faulty PCV Valve
5. Bad ECT Sensor
6. Throttle Linkage Problems
Video: An Accelerator Cable Causes High Idle
7. Carburetor Problems
8. Dealing With a Car Idling High
Carbon buildup in the IAC passages can interfere with the valve operation.
Carbon buildup in the IAC passages can interfere with the valve operation. | Source

1. Faulty Idle Speed Control

Idle speed controls are small electric actuators a car’s computer uses to raise or lower idle speed according to current engine operating conditions. This is why you notice a higher idle speed during a cold start and a lower idle once the engine reaches operating temperature.

On fuel injected models, you can find an idle speed motor mounted on the throttle linkage; an idle air control (IAC) solenoid next to the throttle body. They allow air to bypass the throttle plate.

When an idle speed control motor fails, it may cause high idle regardless of engine operating conditions.

A common problem is carbon soot that builds up along the valve passages an inside the motor. This interferes with motor operation and air flow. You can check air control passages by removing the control motor and carefully removing buildup using carburetor cleaner.

If necessary, you can troubleshoot an IAC solenoid at home using a digital multimeter.

On some older vehicle models you may find a fast idle thermo valve. Instead of computer control, this device uses a thermowax plunger that expands and contracts with the increase and decrease in temperature. It does the same work as an IAC valve. Some problems with this valve may include misadjustment or leaks. Sometimes you may be able to adjust the valve. But a valve leaking wax should be replaced.

Depending on your particular model, there could be other idle speed controls you may need to look into and, if necessary, adjust. Consult your vehicle repair manual.

Vacuum leaks are a common source of high idle.
Vacuum leaks are a common source of high idle. | Source

2. Vacuum Leaks

Vacuum leaks are a common source of high idle speed issues.

A vacuum leak may happen because of a:

  • Disconnected vacuum hose
  • Cracked or damaged vacuum hose
  • Leaking intake manifold gasket
  • Damaged base carburetor gasket

Finding a vacuum leak: Finding a leaking intake manifold gasket, hose or a bad gasket in some other component can be tricky. Still, some simple methods can help you locate the source of the problem.

  1. Trace each vacuum hose with your hand. Make sure it is properly connected on both ends and the hose itself is not damaged or brittle.
  2. Use a length of hose to listen closely for the hissing sounds of a vacuum leak. Do this with your engine at idle and trace hoses and gasket with the hose as you listen.
  3. Idle the engine and use a soapy-water solution or carburetor cleaner. Use the solution to spray hoses and gaskets. Bubbles that appear on a section of hose or gasket or a change in idle speed will reveal a leaking spot.

Other symptoms that may point to a vacuum leak may include:

  • Misfiring
  • Poor acceleration
  • Stumbling
  • Backfiring
  • Hard starting
  • Faulty power brake action

A loose or torn air cleaner boot can lead to high idle.
A loose or torn air cleaner boot can lead to high idle. | Source

3. Leaking Air Cleaner Assembly

A torn or disconnected air cleaner tube between the mass air flow (MAF) sensor and the throttle body can also cause the idle to increase.

This is unmetered air that the MAF sensor is not reporting to the car’s computer, but the oxygen (O2) sensor will detect. The computer will respond to the lean condition detected by the O2 sensor by enriching the fuel mixture, causing a high idle speed.

Check the section of the air cleaner assembly between the MAF sensor for a loose or torn boot.

Check the PCV valve for an suck-open condition.
Check the PCV valve for an suck-open condition. | Source

4. Faulty PCV Valve

A faulty positive crankcase ventilation (PCV) valve may also cause an unusual high idle speed.

This is similar to a vacuum leak, since the PCV valve routes crankcase gases into the intake manifold where they combine with the air fuel mixture.

On some models, you can do a simple PCV test to check on its condition:

  1. Open the hood.
  2. Engage the emergency brake.
  3. Set the transmission to Park (automatic) or Neutral (manual).
  4. Start and let the engine idle.
  5. Using a pair of pliers, carefully pinch off the hose that leads from the engine to the PCV valve.
  6. If engine idle seems to drop back to normal, the PCV is stuck open. Replace it. You can also test the PCV valve itself, if necessary. Consult your vehicle repair manual.

Check the ECT sensor's resistance values if you suspect a fault.
Check the ECT sensor's resistance values if you suspect a fault. | Source

5. Bad ECT Sensor

The engine coolant temperature (ECT) sensor informs the computer when engine coolant is cool or at ambient temperature. A cold engine requires a rich fuel mixture, while an engine running at operating temperature requires a lean mixture.

If the ECT sensor gets stuck at a cold reading, the computer will add fuel to the mixture to keep the engine running properly at idle. However, this will increase idle speed even when the engine is running at operating temperature.

On most vehicles, you can check the ECT sensor at home by measuring the device’s resistance at different temperatures using a digital multimeter.

Throttle body mechanical problems can also lead to a high-idle condition.
Throttle body mechanical problems can also lead to a high-idle condition. | Source

6. Throttle Linkage Problems

The most common problem with the throttle linkage is a misadjusted or binding accelerator cable.

Check the throttle cable and linkage. On some models, you can inspect it by grabbing the cable and pulling it down from its housing. You should feel a bit of play on the cable. If there’s no play, the cable might be binding and preventing the throttle lever from closing, causing idle to increase.

Follow the routing of the cable and make the necessary repairs. Sometimes, it’s necessary to replace the cable.

Consult your vehicle repair manual to check the linkage in your car.

The following video shows how a misrouted accelerator cable can cause a high idle issue.

An Accelerator Cable Causes High Idle

Check carburetor idle adjustments.
Check carburetor idle adjustments. | Source

7. Carburetor Problems

Older vehicles equipped with a carburetor can also face high idle speed problems.

High idle problems may come from:

  • Misadjusted idle speed.

    From time to time, you may need to adjust the idle speed to correct rough or high idle speeds. Follow the instructions in your vehicle repair manual to adjust the idle mixture screws.

  • Clogged idle passage.

    This problem may cut fuel-flow rate. The increased amount of air in the mixture will lead to a high or fast idle speed.

  • Fast idle cam issues.

    A problem with the fast idle cam can lead to a fast idle as well. This may point to a need for linkage adjustment or a choke problem.

High idle may also come from a problem with the power circuit or accelerator pump. Consult your vehicle repair manual to inspect the carburetor in your car.

Car systems maintenance can prevent many car troubles.
Car systems maintenance can prevent many car troubles. | Source

8. Dealing With a Car Idling High

On most vehicle models, checking common components that tend to cause an engine to idle high is not too difficult.

Also, these parts are usually accessible. You may need to remove one or more components using some common tools, depending on your particular model.

Keep in mind that it’s not uncommon for a sensor to go bad and cause a high idle problem as well. A faulty sensor may trigger the check engine light, but not necessarily.

Even if you don’t see the check engine light on, though, scan and download trouble codes from the computer memory. If found, a trouble code can guide you in your diagnostic.

Your particular vehicle model may have other components that, when faulty, may lead to an abnormally high idle speed. Even a problem with the fast idle control system or the engine control unit (ECU–your car’s computer) can lead to an unusual fast idle.

That’s why it’s a good idea to have the repair manual for your particular vehicle make and model. You can get an aftermarket copy of your manual at relatively low prices through Amazon. Haynes manuals come with step-by-step procedures for many maintenance, parts replacement, and troubleshooting projects.

Also, the manual has the voltage, resistance and current specifications for those electrical parts you may need to test, and torque specifications you need to know when mounting components. So you’ll be able to do a lot of common troubleshooting and repairs tasks and recoup your investment in a short time.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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