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My Car Idles Rough When the AC Is On

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

Check common sources of trouble that can cause a rough idle when AC is on.

Check common sources of trouble that can cause a rough idle when AC is on.

The Air Conditioning Shouldn't Normally Keep the Idle Speed Down

Does your car idle rough when you turn on the air conditioning?

A brief initial loss of rpm as the AC comes on is normal. When the AC clutch engages to run the compressor, it places an additional load on the engine. But the car's computer (PCM) should bring the idle speed back up.

If you lose over 200 rpm and idle speed doesn’t rise again, then there’s a problem.

Possible Causes of Rough Idle When the Air Conditioning is On

A rough idle when the air conditioning is on may be caused by:

  • A faulty or dirty IAC valve
  • A dirty throttle body
  • A faulty cooling or condenser fan
  • Other faulty systems or components, depending on the model

The following sections let you explore the most common causes. Each section has or links to a simple diagnostic or troubleshooting procedure you can do at home. The diagnostic can help you fix your vehicle faster and, if possible, save money in the process.

Index

1. Faulty IAC Valve

2. Dirty Throttle Body

Video: Dirty Throttle Causes Rough Idle

3. Problems with the Cooling or Condenser Fan

4. Distributor and Ignition System Problems

5. Adjusting Idle Speed

6. Further Investigating a Rough Idle When the AC is Running

The IAC valve is a common source of trouble.

The IAC valve is a common source of trouble.

1. Faulty IAC Valve

The powertrain control module (PCM) uses the idle air control (IAC) valve to control idle speed. Basically, the IAC allows a certain amount of air to bypass the throttle plate. The extra air enriches the air-fuel mixture during a cold engine start. It also helps increase engine speed when an extra load is placed on the engine, for example, when you turn on the AC or defrost system. However, a faulty or restricted IAC valve may keep the engine idle speed from increasing when it needs to.

The most common IAC problems are carbon buildup around the IAC valve and throttle passages, and IAC motor malfunction.

You can do a simple IAC motor diagnostic and check the valve and throttle bypass port for carbon buildup.

If you need to remove buildup from the IAC valve or passages, you can apply the same procedure used to clean a throttle body, as described in the next section.

Check and remove buildup from the throttle body, if necessary.

Check and remove buildup from the throttle body, if necessary.

2. Dirty Throttle Body

A dirty throttle body can also affect idle during AC operation.

A throttle plate is calibrated to remain almost closed during idle, while the computer regulates airflow past the throttle plate. However, if the throttle plate and bore are dirty, insufficient air will flow and cause trouble when you turn on the AC.

Cleaning the throttle bore:

  1. Pop the hood open.
  2. Remove the air cleaner duct that connects to the throttle body or air cleaner snorkel, depending on your particular model.
  3. Remove the air cleaner snorkel, if necessary.
  4. Grab a clean shop rag and spray it with carburetor cleaner.
  5. Use the damp rag to remove deposits around the throttle bore and plate, and under the plate.
  6. Carefully swing open the throttle blade using your hand. Don’t use screwdrivers or other sharp tools that may scratch the throttle bore. Scratches may cause performance issues later on.

Cleaning an electronic throttle control (ETC):

The electronic throttle control system (electronic fly-by-wire throttle body) is run by the PCM through sensor inputs.

You should be careful when cleaning buildup in this system. Carelessly handling the throttle blade can upset the throttle base idle position programmed into the computer.

  1. Pop the hood open.
  2. Gain access to the throttle body.
  3. Ask a helper to turn the key to the On position, but don’t start the engine.
  4. Ask your helper to slowly depress the accelerator pedal.
  5. Spray throttle body cleaner on a clean shop rag.
  6. Use the shop rag to clean the throttle bore and blade.

    A couple of points to remember:

    • Be careful not to move the throttle blade with your hand while cleaning. If necessary, only spray the throttle body and blade with throttle body cleaner.
    • When removing carbon deposits in an ETC system, use only throttle body cleaner. Other solvents may cause damage to the system.

If you need the PCM to relearn the idle base position again, consult the vehicle repair manual for your particular vehicle model.

Sometimes, you need to remove the throttle body to clean it. If necessary, replace the throttle gasket. The following video shows how a dirty throttle body and valve were causing a rough idle.

Dirty Throttle Causes Rough Idle

Make sure the cooling or condenser fan is working properly.

Make sure the cooling or condenser fan is working properly.

3. Problems With the Cooling or Condenser Fan

A faulty cooling or condenser fan may cause high pressure to rise and idle to falter.

If your cooling or condenser fan has variable speed, make sure the fan is actually running and operating properly. A simple test can help you do some quick circuit checks, if necessary. Or you can do some simple cooling fan troubleshooting as well.

Carefully check the distributor cap's central and outer terminals and the rotor's tip.

Carefully check the distributor cap's central and outer terminals and the rotor's tip.

4. Distributor and Ignition System Problems

If you have an older vehicle model with a distributor, check the cap and rotor.

  • The cap’s center and outer terminals can wear, become covered in carbon buildup, and burn. This may prevent a strong spark at the spark plug tip.
  • Thoroughly examine the terminals and the distributor cap for carbon traces and cracks. Carbon traces will shortcut voltage to ground.

    You may need a strong light to notice carbon traces on a black-colored distributor cap. Carefully examine the cap. A bit of carbon and wear can make it difficult for the 200 to 6000 volts to push a strong spark from the rotor’s tip to the cap’s outer terminals.

  • Inspect the rotor tip for burns, damage, and play. Replace the tip as necessary.
  • Check the spark plug wires and plugs.

Check your car manufacturer's recommended service schedule for the rotor, spark plug wires and plugs. If necessary, consult your car owner’s manual or vehicle repair manual.

Check the carburetor idle adjustments.

Check the carburetor idle adjustments.

5. Adjusting Idle Speed

If you haven’t found the cause of the problem, idle speed may need adjustment.

On older vehicle models with a carburetor, it’s possible to adjust idle speed. In fact, this is done on a regular basis on many carburetors.

Depending on your particular model, adjust the idle-speed screw and check the idle solenoid. Follow the instructions in your vehicle repair manual.

On modern vehicles, the powertrain control module (PCM) controls idle speed with the help of some input sensors based on temperature, airflow, and throttle position. A few models may provide a few manual adjustments. Check the decal inside the engine compartment or your vehicle repair manual.

Still, you can check on those sensors the PCM relies on to set idle speed, for example the engine coolant temperature (ECT) sensor, throttle position sensor (TPS), and mass airflow (MAF) sensor. A sensor or actuator running on the edges of its operating parameters may not cause trouble at idle speed until you turn on the AC on. Also, it may not trigger the check engine light.

Often, the fault may trigger a DTC.

Often, the fault may trigger a DTC.

6. Further Investigating a Rough Idle When the AC is Running

Usually, you can find the fault among the components or systems described in the previous sections. These are the most common. Often, a faulty sensor will cause the computer to store a diagnostic trouble code (DTC). The check engine light may or may not come on. Either way, scan the computer’s memory for DTCs. You may find a pending code that can guide you in your diagnostic.

If you still haven’t found the fault, consult the repair manual for your particular vehicle make and model. You can get an aftermarket copy of your manual online. Haynes manuals come with step-by-step procedures for many maintenance, component replacement, and troubleshooting tasks, along with specifications. So you can recoup your small investment soon.

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.

© 2020 Dan Ferrell

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