Car Shuts Off While Idling With AC On: 3 Primary Causes and Fixes

Car Shuts Off While Idling With AC On: Troubleshooting Tips

Refer to this table for quick identification of causes of car shuts off while idling and solutions.

NO	Potential Cause	Solutions
1	Seized compressor	Add refrigerant oil
		Replace AC compressor
2	Malfunctioning idle speed control valve	Clean the ISC valve
		Replace the ISC valve
3	Low refrigerant	Recharge your AC compressor

1. Seized Compressor

As you engage your AC, you activate your compressor clutch. This way, your compressor pulley connects to and spins with the engine pulley that attaches to the crankshaft.

Under normal operation, the compressor places a small load on the engine as it spins the compressor. However, if your compressor is seized and can’t spin freely, it acts like a brake to the crankshaft.

At idle speed, your engine doesn’t have excess power to overcome such additional resistance. This causes your engine RPM to drop to a point where it can’t maintain idle speed, making it stall out.

Typically, low oil causes compressor lock. The following are tell-tale signs that your AC compressor is seized:

• Slipping drive belt.
• Squealing or smoke coming from the drive belt.

Alternatively, you can turn the pulley/clutch manually to confirm if your AC compressor is fully seized. Do this with your engine off. Nonetheless, use caution as it can be difficult to move.

Solution

The solution depends on what caused the lock-up. If it’s lack of lubrication, add oil to the compressor using a refrigerant oil injector. Use the correct oil specified in your manual.

However, for broken parts like a piston, replace your AC compressor. Follow these steps:

• Step 1: Use a refrigerant recovery machine to discharge the refrigerant from your AC system to prevent release into the atmosphere.
• Step 2: Disconnect your compressor clutch electrical connector. To do this, first remove the connector locking tab.
• Step 3: Remove your drive belts by loosening the tensioners.
• Step 4: Use flaring tools to disconnect the copper refrigerant lines from AC compressor ports. Open the ports.
• Step 5: With your socket wrench, remove the bolts that mount your AC compressor to the bracket or engine.
• Step 6: Pry or wobble the compressor to break the seal from the mounting gasket. Remove the compressor.
• Step 7: Clean the mounting surface and position the new compressor with a new gasket.
• Step 8: Reconnect refrigerant lines, electrical connectors, and drive belts. Tension the belts as needed.
• Step 9: Run a vacuum pump for 45-60 minutes to evacuate moisture from your AC system.
• Step 10: Charge the system with the correct amount/type of refrigerant.

2. Malfunctioning Idle Speed Control Valve

The idle speed control (ISC) valve maintains the engine’s idle speed to around 700-1000 RPM in park or neutral. As the AC compressor kicks on, it places an extra load on your engine, causing the RPM to dip.

It’s the work of the ISC valve to sense this dip in RPM and open wider. This allows more airflow into the intake to increase idle speed.

So, if your ISC valve is faulty, it won’t open wider when needed. Therefore, the extra load from the compressor causes the RPM to drop too low.

Most modern engines have a safety feature that shuts your engine off if the idle drops below a certain threshold. In most cases, the computer shuts the engine off, usually at around 350-500 RPM, as a fail-safe.

Clogging over time is the primary reason for the ISC or idle air control valve not to open as it should. Primary symptoms for a malfunctioning ISC valve are:

• Rough idling
• Backfiring
• Difficult starting
• Hesitation on acceleration
• An O2 sensor code from unstable idle conditions

Solution

I recommend cleaning your idle air control (IAC) valve first. Usually, it clogs from carbon, grime, and dirt buildup.

However, if symptoms remain after a thorough cleaning, replacement is necessary. Essentially, the ISC valve should last the lifetime of your car. Still, the solenoid and the seals may fail prematurely.

Watch the following video on how to clean your idle air control valve:

3. Low Refrigerant

When the refrigerant level is low, your AC compressor has to work harder to cool the air. So, it will put an extra load on your engine, causing the idle RPM

to dip too low. Your car’s computer may interpret this as a fault and shut down the engine.

The symptoms of low refrigerant levels are:

• Lack of frosting or ice formation on the evaporator coil.
• Compressor cycling on and off more frequently than usual.

Solution

Evacuate the system for at least 30 minutes with a vacuum pump. This should remove moisture and what’s left of the old refrigerant. Afterward, weigh in a new refrigerant using the AC service unit’s scale. Adjust it if needed to reach optimal operating specifications.