Question: Why does my car have a rough idle when the air conditioning is on?
Answer: A rough idle when the air conditioning (AC) is on can be a common issue in many vehicles, especially older ones. This problem can be caused by several factors, including a cracked or leaky intake manifold gasket, a dirty or faulty idle air control (IAC) valve, or an exhaust gas recirculation (EGR) valve that’s stuck open.
When the AC clutch engages, it adds an extra load to the engine. So if there are underlying issues such as a vacuum leak or a dirty throttle body, the engine may struggle to maintain a stable idle—causing the RPMs to drop and fluctuate.
Common causes and solutions for a rough idle with the AC on
There are several reasons why your car may idle roughly when the AC is running. Here are the most common issues and how to fix them:
Cracked or leaking intake manifold gasket
- Cause: A cracked or leaky intake manifold gasket can cause unmetered air to enter the engine, disrupting the air-fuel mixture and causing the engine to idle rough.
- Solution: Inspect the intake manifold gasket for cracks or leaks. If any are found, even small ones, replace the gasket to restore proper sealing and prevent unmetered air from entering the engine.
Dirty or faulty IAC valve
- Cause: The idle air control (IAC) valve regulates the engine’s idle speed. A dirty or otherwise faulty IAC valve may not adjust the idle speed correctly when the AC compressor is turned on.
- Solution: Clean the IAC valve and the passages where it mounts. If cleaning doesn’t resolve the issue, you’ll need to consider replacing the IAC valve.
Stuck open EGR valve
- Cause: An EGR valve that’s stuck open can allow exhaust gasses to enter the intake manifold, disrupting the air-fuel mixture.
- Solution: Inspect the EGR valve to make sure it’s functioning properly. Clean or replace the EGR valve if it’s sticking open.
Faulty PCV valve
- Cause: A malfunctioning positive crankcase ventilation (PCV) valve can affect the engine’s idle by allowing excess air into the intake manifold.
- Solution: Check the PCV valve for proper operation. If it’s clogged or stuck, replace it to ensure proper ventilation and idle control.
Vacuum leaks
- Cause: Vacuum lines around the throttle body or other parts of the intake system can develop leaks, causing the engine to idle rough.
- Solution: Inspect all vacuum lines for cracks, wear, or disconnections. Also pay close attention to your tachometer, which tracks your vehicle’s RPM’s, as an RPM needle that bounces up and down might indicate a vacuum leak. Replace any damaged lines and ensure all connections are secure.
Dirty throttle body
- Cause: The buildup of carbon, grime, or dirt on the throttle body’s butterfly valve over time can restrict airflow and interfere with its movement.
- Solution: Clean the throttle body to remove any carbon deposits, which should ensure the butterfly valve moves freely and smoothly.
Faulty O2 sensors
- Cause: Oxygen sensors monitor exhaust gasses to adjust your engine’s air-fuel mixture. Faulty O2 sensors can cause incorrect adjustments, leading to a rough idle.
- Solution: Test the O2 sensors using an OBD-II scanner and replace any that are not functioning correctly.
Worn spark plugs
- Cause: Worn spark plugs can cause misfires, especially if they’re under additional electrical loads from the AC compressor.
- Solution: Inspect your spark plugs and install new ones if they’re worn.
Sarah Gray is an insurance writer with nearly a decade of experience in publishing and writing. Sarah specializes in writing articles that educate car owners and buyers on the full scope of car ownership—from shopping for and buying a new car to scrapping one that’s breathed its last and everything in between. Sarah has authored over 1,500 articles for Jerry on topics ranging from first-time buyer programs to how to get a salvage title for a totaled car. Prior to joining Jerry, Sarah was a full-time professor of English literature and composition with multiple academic writing publications.
Everett Cook is an award-winning journalist and editor with more than 10 years of experience across a variety of industries. In editing for Jerry, Everett’s mission is to help readers have a better understanding of the costs of owning or leasing a car and to better understand their vehicle in terms of insurance and repairs. Prior to joining Jerry, Everett was an editor for Axios. His previous work has been featured in The New York Times, The Los Angeles Times, The San Francisco Chronicle, The Atlantic, Atlantic Re:think, The Boston Globe, USA Today, and others. He’s also been a freelance writer and editor with experience in SEO, audience building, and long-term content roadmaps. Everett is a proud graduate of the University of Michigan.
