Car Sputters When Air Conditioner Is On: 10 Potential Causes

Why car sputters when ac is on? If the car’s engine sputters when the air conditioner is turned on, the most possible reason is that the A/C compressor is overloading the engine, which can intensify issues in the alternator, battery, or the idle air control valve (IACV). This intensity could cause RPM to fall, stimulate vibrations, or generate stall-like symptoms when idling or nearing stops. Possible solutions might comprise testing the charging system, repairing or replacing the IACV, examining the intake for vacuum leaks, cleaning the throttle body, cleaning the clogged EGR valve, replacing worn belts/pulleys, validating refrigerant pressure, reassessing engine mounts, and inspecting components of the ignition system. Adjusting engine settings or the idle RPM can improve handling of the A/C load. A professional evaluation is recommended for effective troubleshooting.

It’s frustrating when you turn on the air conditioning in your car and it starts to sputter, stall or shake. The sudden loss of power when you need cooling on a hot day is a real pain point.

In this quick guide, we’ll look at some of the common causes for a car that sputters when the AC is on and provide tips to help you diagnose and resolve the issue.

Note: Car AC system has a very important component called AC clutch. Understanding its components and working will help you troubleshooting car sputtering issue when ac is on. So, make sure to read my guide on that topic as well.

Here is another video recorded by the user in which you can see car stalls as soon as ac was turned on:

Bonus Read: Car loses power when turning corners

Some Key Insights for You
  • Turning on AC adds extra load on engine which can reveal underlying issues
  • Common causes include faulty idle air valve, bad ignition parts, low refrigerant, dirty throttle body
  • Compressor seizing when engaged strains engine, causing sputtering
  • Weak charging system forces battery to work harder to supply AC components
  • Worn mounts transfer extra vibration to chassis instead of damping it

My Personal Experience With Car Sputtering When AC Is On

Once, my 2001 Honda Civic started sputtering badly whenever I turned on the AC. After some testing, I diagnosed a failing ignition coil that was struggling to deliver adequate spark under the AC’s extra load.

I replaced all four coils and spark plugs with quality components. The sputtering immediately stopped and the Civic ran smooth once again, even on the hottest days with AC blasting. Proper maintenance and diagnosis saved me an expensive repair bill and got my car back to normal.

Bonus Read: Car shakes at idle but smooths out when driving

If you prefer a brief overview instead of reading the entire article, here is a summary table to quickly gather the information:

Causes Of Car Sputtering and Shaking When AC Is On

Let’s discuss several causes that lead to car shaking and stalling when AC is on:

1. Faulty Idle Speed Control Valve Causes Rough Idling When AC Engaged

clogged idle air control valve
clogged idle air control valve

The IAC is a valve that regulates the amount of air bypassing the throttle plate in the air intake when the engine is idling.

IACV is controlled by the engine computer and helps maintain proper idle speed. When closed, it reduces the amount of air getting into the engine. When open, it allows more air to get to the engine.

Opening and closing the valve continually helps keep the engine idling smoothly at the correct RPM.

In essence, here is how ECU controls IACV:

  • The engine computer monitors engine sensors like coolant temperature, oxygen levels, throttle position etc.
  • Based on this data, it calculates the target idle RPM and signals the IAC to open or close to reach that target.
  • Common target idle RPM is around 600-800. Higher when cold, lower when warm.
  • The IAC has an electric motor that moves a pintle valve to precisely meter air.

How IACV Becomes Bad?

Carbon buildup, contamination and wear can make the IAC valve stick or not fully close. This allows too much air into the engine, causing high idling RPMs and sputtering. If it sticks fully closed, it cuts off air and causes the engine to stall out.

Why Does the AC Affect Idling with a Bad IAC?

  • When the AC compressor kicks on, it puts an extra load on the engine.
  • This load reduces the idle RPM.
  • The computer senses the RPM drop and signals the IAC to open more to compensate.
  • If the IAC is stuck open already, it cannot open further to raise RPMs.
  • This results in sputtering, shakes and a rougher idle.

How can you diagnose a faulty IAC valve?

If you suspect that your IAC valve is causing car sputter when AC is on, there are some steps you can take to diagnose and fix it.

  • Check for trouble codes: The first thing you should do is scan your vehicle for any trouble codes related to the IAC valve or the idle system. You can use BlueDriver scan tool. Some common codes are P0505 (Idle Control System Malfunction), P0506 (Idle Air Control System RPM Lower Than Expected), P0507 (Idle Air Control System RPM Higher Than Expected), P0511 (Idle Air Control Circuit), and P1508 (Idle Air Control Valve Circuit Low). These codes can help you pinpoint where the problem lies and what needs to be replaced or repaired.
  • Check for physical damage: The next thing you should do is inspect your IAC valve for any physical damage, such as cracks, corrosion, loose wires, or broken connectors. You should also check for any signs of carbon buildup or dirt on the valve or its passages. These can interfere with its operation and cause it to stick or malfunction.
  • Listen for clicking: With engine off, have someone turn the key to “run” while you listen to the valve. Should hear rapid clicking as it cycles. No clicking indicates an issue.
  • Start the engine and unplug the outer idle ECU plug. If the RPM spikes to 2200-2500, then the ECU and IAC motor are working properly.

You can watch following Youtube video to test IAC valve:

How to fix?

If your IAC valve is dirty or damaged, you may be able to fix it by cleaning it or replacing it. To clean your IAC valve, you will need some carburetor cleaner, a rag, and a screwdriver.

You will have to remove your IAC valve from your throttle body and spray some cleaner on its pintle (the part that moves in and out) and its passages.

Then wipe off any excess cleaner with a rag and reinstall your IAC valve.

Do note the following things while cleaning IACV:

  • Ensure pintle valve moves freely after drying
  • Double check all electrical contacts are clean
  • Check for related vacuum leaks during cleaning
  • Reset ECU adaptive memory or do idle relearn after installing IACV

To replace your IAC valve, you will need a new one that matches your vehicle’s make and model.

2. AC Compressor Clutch Seizing When Engaged Can Stall Engine Operation

The air conditioning (AC) system in a car relies on the AC compressor to function properly. The compressor is turned on and off by an electromagnetic clutch mounted on the front.

When the AC is switched on in the car, the clutch engages with the compressor pulley that is driven by the serpentine belt via the engine. This causes the compressor to turn on and start compressing refrigerant.

However, sometimes the compressor clutch can start to seize or bind intermittently while engaged. This clutch sticking can cause several problems:

  • Belt Squeal: As the clutch sputters trying to turn due to binding, it causes the belt to slip and squeal loudly. This squealing noise tends to come and go as the clutch temporarily binds then releases.
  • Engine Struggle: The added drag on the engine from the seized clutch can make the engine struggle or even stall out entirely.
  • Overheating: Friction from the stuck clutch heats up the compressor and AC system, potentially leading to failure of the compressor or other components.
  • Excess Wear: The intermittent sticking puts strain on the clutch components and bearings as well as the belt, pulleys, tensioner and engine accessories. This can lead to premature wear or failure.
  • Lack of Cold Air: Periods where the clutch cannot turn due to seizing mean the compressor cycles on and off, providing inconsistent or warm airflow into the cabin.

Why AC compressor clutch binds?

  • Worn AC Clutch Bearings: Bearings allow smooth rotation of the AC compressor pulley When they wear out, the clutch binds up intermittently.
  • Low Refrigerant: Not enough refrigerant oil to properly lubricate the system or loss of refrigerant pressure can allow clutch components to stick and bind.
  • Overcharged Refrigerant: If the refrigerant is overcharged, it can cause the compressor to work harder which will cause additional strain on the engine. This will result in car sputtering when ac is on.

How to diagnose the problem?

  • Check for signs of oil leaks, damaged connectors or loose wiring related to the clutch. Any signs of rubbing or damage around the clutch pulley can indicate binding issues.
  • Inspect the belt condition for any glazing effects and test the belt tensioner for smooth operation. A worn belt that slips or a stuck tensioner could mimic compressor clutch seizure.
  • Feel along the belt path while an assistant turns the AC on and off. See if you can detect any binding feeling in the section that leads to the compressor clutch.
  • Listen for any squealing or squeaking noises from the belt with the AC on, indicating slippage from resistance.

Here is how to test compressor pulley:

  • With the engine off, remove or loosen the serpentine belt. This isolates the compressor pulley from the engine.
  • Try to spin the clutch pulley by hand. It should rotate completely freely with no resistance or grinding noises.
  • If the pulley spins freely, the issue is likely inside the compressor. If the pulley barely spins or doesn’t rotate smoothly, the pulley bearing is likely worn out.
  • If the pulley spins freely, test the compressor by trying to move it with a locking oil filter wrench. If the clutch doesn’t move, the compressor is likely damaged. I have provided an image below for understanding:
testing car ac compressor

3. Weak Ignition Coils or Aged Spark Plugs Reduce Combustion Efficiency

The ignition system is a critical component that delivers electric spark to the engine’s cylinders to ignite the air/fuel mixture and power the engine.

When the ignition system is weak or faulty, it can lead to engine misfires, particularly under increased load conditions like when the AC compressor kicks on.

When the AC is on, the compressor puts an extra load on the engine, which requires more power and fuel. This means that the ignition system has to work harder to produce a strong and consistent spark.

If the ignition system is weak, it may not be able to deliver enough voltage to the spark plugs, resulting in a misfire or a weak spark.

A misfire occurs when the spark plug fails to ignite the air-fuel mixture in one or more cylinders, causing a loss of power and a rough idle.

A weak spark occurs when the spark plug produces a low-intensity spark that does not fully burn the air-fuel mixture, resulting in incomplete combustion and reduced engine efficiency.

Also Read: Bad ignition control module symptoms

Bad ignition coils is one of those causes

Ignition coils act as transformers, taking the lower voltage from the battery and converting it to the thousands of volts needed to jump the spark plug gap to ignite the mixture in the cylinder.

As coils weaken over time from heat cycling, internal arcing, and normal wear, they lose the ability to deliver adequate voltage to the spark plugs to maintain a consistent spark.

This weakness shows up most when the ignition system is under greater strain. Turning on the AC adds load to the engine both mechanically through the compressor itself which the engine must turn against resistance, and electrically as the AC blower fan motors draw additional current.

More voltage and spark energy is needed from the coils under these greater demands. Weak coils cannot deliver, resulting in misfires.

Also Read: Check engine light flashing

Ignition coils and spark plug wires (in case of coil packs) can also corrode, due to which sufficient voltage isn’t generated for spark delivery.

corroded ignition coils and spark plug wires
corroded ignition coils and spark plug wires

Fouled Spark Plugs

fouled spark plugs
fouled spark plugs

Spark plugs can become fouled due to oil leakage through valve stems, coolant leaking through head gasket and excessive fuel leakage through fuel injectors.

How to fix?

You can try cleaning the spark plugs using the wire brush and spark plug cleaner to carefully remove any carbon deposits or contaminants from the electrode. Ensure not to damage the electrode or insulator in the process.

First, use compressed air to clean off any dirt and debris. Then, keep the spark plug soaked in the brake cleaner till all the gunk on the spark plug is washed away.

4. Weak Charging System Exerts Load On Engine Idling During AC Cycling

Bad alternator connection
Bad alternator connection

A car’s electrical system is powered by two main components – the battery and the alternator.

The battery provides the initial power to start the engine, while the alternator generates electricity to recharge the battery and power all the vehicle’s electrical components once the engine is running.

This interplay between the battery and alternator is key for proper operation.

If the alternator fails or cannot supply enough current, the battery must make up the difference. This can cause problems like sputtering, dimming lights, and stalling when extra loads are placed on the system.

Alternator Function and Failure Modes

The alternator contains a rotor, stator, diodes, and voltage regulator. As the pulley spins the rotor, the magnetic fields induce current in the stator windings which is rectified by the diodes into DC current to charge the battery. The regulator controls the output.

There are two main types of alternator failure:

  • Mechanical: Caused by worn bearings or slipped belts. Often produces noise or squealing. Look for glazing, cracks, fraying. 
  • Electrical: Bad diodes, stator windings, regulator etc. Causes low/fluctuating output.

Electrical faults are more common and can only be detected through testing, as the alternator may otherwise appear functioning.

To learn more, you can also read my guide on car keeps burning alternator.

Signs of Weak Alternator Output

Some signs your alternator may not be providing sufficient charge:

  • Dimming headlights when idling, AC or other loads are turned on
  • Battery not staying fully charged over time
  • Voltage gauge reading under 13.5 volts with engine running
  • Intermittent stalling, sputtering, weird electrical issues

Insufficient output causes the battery to become drained over time. This leads to performance problems.

Testing the Charging System

To diagnose, first load test the battery to see if it is holding a charge properly. If good, check alternator output:

  • Use a multimeter to measure voltage across battery terminals with engine off (12.4 to 12.7V is normal)
  • Start engine, rev slightly, check voltage again – should now read 13.5 to 14.5V
  • Turn on all accessories (lights, AC, etc) – voltage should stay in range, not drop below ~13.5V.

How Insufficient Charging Leads to Sputtering?

When the alternator cannot provide the voltage and current demanded by the vehicle electrical system, especially with added loads like AC, problems arise:

  • Battery discharges faster to make up deficit
  • Electronics may turn off or function intermittently
  • Ignition and injectors may misfire due to low voltage
  • Reduced power to spark plugs causes sputtering

How to fix?

To address weak charging issues:

  • Replace damaged alternator components like diodes, stator, regulator as needed
  • Check/tighten all connections to clean and ensure good contact
  • Replace worn accessory belt if slipping and not spinning rotor fully
  • Install higher output alternator if car has added electrical loads
  • Check installation alignments – proper alternator pulley alignment critical

5. Dirty Throttle Body Restricts Desired Airflow

dirty throttle body

A dirty throttle body can disrupt the smooth flow of air into the engine, leading to various issues.

Over time, the inner walls of the throttle body can accumulate dirt, grime, and carbon deposits, which negatively affect the movement of the butterfly valve.

As a result, the throttle body becomes restricted, limiting the amount of air that can pass through it. This disturbance in the air-to-fuel ratio can cause problems.

Insufficient air intake to the engine can result in an unstable and inconsistent idle speed.

Consequently, when you turn on the ac, the car may stall or sputter as the engine unable to receive the required air volume for proper operation.

Steps to Address the Issue

To rectify this problem, follow these steps:

  1. Perform a visual examination of the throttle body, inspecting for any signs of physical damage, wear, cracks, leaks, or loose connections. These issues can potentially impact its performance.
  2. Thoroughly check the throttle plate for excessive carbon buildup which obstructs its movement. Look for any signs of sticking or hesitation when manually moving the throttle plate back and forth. This can indicate a throttle body mechanism problem.
  3. With time, carbon deposits and dirt can accumulate, leading to deteriorated performance. To mitigate this, utilize a throttle body cleaner along with a soft brush or rag to gently cleanse the throttle body and remove any buildup present.

6. Refrigerant Is Overcharged or Undercharged Affecting AC Performance

The refrigerant in a car’s air conditioning system is vital for proper operation and cooling performance.

Both undercharging and overcharging the system with refrigerant can lead to issues like the AC compressor cycling on and off, reduced cooling capacity, and even engine sputtering when the AC turns on.

There are several reasons why the refrigerant charge level needs to be precisely calibrated.

Causes of Incorrect Refrigerant Charge

There are a few common causes of low and high refrigerant levels:

  • Refrigerant leaks: Small leaks over time will lead to low refrigerant. Finding and repairing leaks is crucial.
  • Improper recharging: Using DIY refrigerant cans often overcharges the system since the exact amount added is unknown.
  • Normal wear: Older components like seals and o-rings will eventually leak refrigerant.

What exactly is compressor cycling?

The AC compressor is the pump that circulates refrigerant through the system. Both low and high refrigerant charges can cause the compressor to rapidly cycle on and off.

Here’s why:

  • Low refrigerant: The low pressure safety switch disables the compressor to protect it when pressure drops too low. This causes the compressor to turn off repeatedly.
  • High refrigerant: The high pressure safety switch turns off the compressor when head pressure gets too high. This also leads to short cycling as refrigerant backs up.

If AC is low on refigerant and short cycling, it prevents the PCM from relearning new AC on idle trim settings, causing idle sag when AC kicks on/off.

How it would cause engine sputtering when AC turns on?

One of the most noticeable symptoms of AC issues is the engine sputtering when the AC compressor engages. There are a couple reasons this can happen:

  • AC load: The AC compressor places an extra load on the engine when it turns on. If refrigerant levels are off, it strains the compressor and overworks the engine.
  • Idle control: The AC idle speed may need to be relearned if refrigerant levels change. Low refrigerant can prevent this, causing a low idle when AC is on.

How to diagnose and fix?

Since DIY refrigerant recharging often causes issues, the best approach is to have an experienced AC technician handle diagnosis and repairs:

  • Use professional gauges: DIY gauges are often inaccurate. Pro-level gauges provide precise pressure readings needed for diagnosis.
  • Leak test: Dye and UV lights should be used to check for any refrigerant leaks that must be addressed.
  • Evacuate and recharge: The AC system needs to be fully evacuated to remove all refrigerant, moisture, and contaminants before recharging.
  • Charge by weight: Refrigerant should be added by exact weight, not estimation. This avoids over or under-charging.

Note: The ideal low-side and high-side pressures for a car AC system are 25-30 psi and 200-250 psi respectively.

7. Bad AC Condenser Exerts Strain On Compressor

clogged ac condenser

The AC condenser is a radiator-like component located at the front of the car, usually between the radiator and the engine cooling fan. Its job is to remove heat from the hot, pressurized refrigerant gas coming from the compressor.

It does this by circulating outside air over finned tubing containing the refrigerant, cooling the gas and condensing it into a liquid.

Some key points about the condenser:

  • Made of finned tubes with good heat transfer properties to maximize cooling
  • Refrigerant goes in as a hot gas, exits as a pressurized liquid
  • Condenser has a receiver/drier to collect the liquid refrigerant
  • Needs good airflow anytime AC is on to work efficiently
  • Prone to damage from road debris due to front location
  • Can get internally clogged over time

How a Failing Condenser Disrupts the AC System?

When the condenser fails or gets blocked, it can no longer properly remove heat and condense the refrigerant gas into a liquid. This causes high pressures in the system since the compressor keeps pumping out hot gas that can’t convert to liquid form.

The high pressures place extra load on the compressor, forcing it to work much harder. This can cause the engine and belts to lag, resulting in sputtering as the compressor struggles.

Some specific effects of a bad condenser include:

  • Refrigerant not fully condensing leads to reduced cooling capacity
  • Excessive compressor cycling due to high head pressures
  • Higher risk of compressor failure due to overwork
  • Engine sputtering as compressor drags down performance

Essentially, the condenser is unable to do its job, resulting in poor AC performance and strain on the compressor and engine. This manifests as sputtering or even stalling when the AC is switched on and the compressor kicks in.

Condenser Failure Modes

Car ac condensers can fail due to a variety of modes. Common ones include:

  • Refrigerant leaks: Small leaks becoming large over time
  • Structural damage: Fins bent, tubes crushed by debris
  • Clogged tubes: Dirt and debris blocking interior passages
  • Corrosion: Refrigerant acids and moisture eroding tubing
  • Failed fan: Fan not spinning to provide adequate airflow
  • Uneven condensation: Indicates some tubes not functioning
  • Blocked exterior: Bugs, leaves, mud blocking airflow

Note 1: Low pressure side of car AC system is between evaporator and compressor. The high-side pressure port is located between compressor and condenser.

Note: 2: In blockage testing of condenser, you would simply bring your fingers close to the condenser. If you feel hot air flow through the condenser fins, it means condenser is fine. If you don’t feel any airflow, it means condenser is clogged.

8. Clogged EGR Valve Affects Fuel Consumption

bad egr valve

The EGR valve, or Exhaust Gas Recirculation valve, is an emissions control device found on most modern internal combustion engines.

Its purpose is to recirculate a portion of the exhaust gases back into the combustion chamber. This has the effect of lowering combustion temperatures and reducing the formation of NOx emissions.

Some key points about how the EGR valve functions:

  • It opens at specific times to allow exhaust gas to enter the intake manifold and mix with the incoming air/fuel mixture. This lowers combustion temperatures.
  • The EGR valve is controlled either by vacuum, backpressure, or electronically by the vehicle’s computer. It does not stay open continuously.
  • Opening the EGR valve lowers combustion efficiency slightly. The computer adjusts fuel trim to compensate when the EGR is active.
  • EGR is usually active at idle, light load and cruising conditions. It is deactivated under hard acceleration for maximum performance as high loads need maximum oxygen.

How the EGR Valve Can Become Clogged?

Over time, carbon deposits can build up inside the EGR valve assembly and associated intake passages. This is caused by the following factors:

  • Soot and combustion byproducts circulating through the system
  • Oil vapors condensing inside the intake, exacerbated by oil leaks
  • Short driving trips without sufficient warm-up time to clear deposits
  • Stop-and-go driving that generates significant exhaust soot

How a Clogged EGR Valve Causes Sputtering with AC On?

When the AC is turned on, it places an extra load on the engine. The compressor cycles on which draws engine power.

This extra load requires more fuel to maintain smooth operation. A clogged EGR will prevent proper fueling.

The ECU detects the extra load from the AC and instructs the fuel injectors to deliver more fuel to compensate. But the clogged EGR is allowing less air in, creating an overly rich fuel mixture.

This overly rich mixture causes misfires and sputtering as the extra fuel cannot combust properly.

When AC Is On, a Clogged EGR Can Cause:

  • Stumbling, sputtering idle as combustion suffers from excess EGR flow
  • Surging or oscillating RPM as the computer struggles to find proper fuel trim
  • Complete engine stall in severe cases where EGR flow chokes the engine
  • Diminished power and acceleration due to poor combustion

How to test?

Remove the EGR valve and perform a visual inspection to check for any cracks or holes in the housing. A cracked or damaged EGR valve will need to be replaced.

To check if the EGR valve is working correctly, we will now use a hand-operated vacuum pump.

If the EGR valve can hold a vacuum, it indicates that the valve is functioning properly. However, if the vacuum pressure drops off after a while, it suggests a faulty EGR valve that needs to be replaced.

How to fix?

EGR valves can be cleaned. Here’s how you can do:

  1. To resolve the issue, get some carburetor cleaner and spray it into the valve. You’ll notice black crud coming out, indicating carbon build-up. After cleaning, ensure that the valve is back in its closed position.
  2. Inspect the holes: Check the feed and inlet holes of the EGR valve to ensure they are not clogged. Here’s a trick you can use: take an old speedometer cable and push it into the holes. If the feed hole is clear, it’s a good sign that exhaust gas is passing through. However, the inlet side may be clogged.
  3. Clearing clogs from the inlet: To clear the clogged inlet, attach the speedometer cable to a drill. Spin the cable like a roto-rooter to clean out any debris. This will help maintain proper airflow.
  4. Verify clearance: Use compressed air to check if the clogs have been cleared. Blow air into the valve from one end and ensure it comes out the other end.
  5. Apply lubrication: Before reinstalling the EGR valve, spray some WD-40 into the hole and let it soak for about half an hour. This will help lubricate the valve stem, preventing it from sticking.

9. Worn Engine or Transmission Mounts Cause Engine Sputtering Under Load

bad motor mounts

Engine mounts, also called motor mounts, are brackets that secure the engine to the vehicle frame or chassis. They are typically made of rubber or a combination of rubber and metal.

The rubber provides cushioning to dampen vibrations from the engine. Transmission mounts serve the same purpose for the transmission.

Mounts locate the engine and transmission precisely while allowing some movement from engine torque and road shocks. This prevents excessive vibration noise and harmonics from reaching the cabin. Mounts must withstand tremendous loads while providing proper vibration isolation.

When worn engine or transmission mounts are present, drivers are most likely to notice driveability issues when the air conditioning system is engaged. This is because the AC compressor places additional load on the engine.

With bad mounts, the extra torque and vibration from the AC kicking on can be transferred to the chassis instead of being dampened. This manifests as:

  • RPM surge or dip when AC engages: The abrupt change in engine load is not smoothed out.
  • Stalling: AC load combined with engine movement can stall engine.
  • Sputtering: Extra vibration exacerbates any existing fuel delivery problems.
  • Shake/vibration: AC torque amplifies vibration transfer through bad mounts.
  • Rattle from components: Bad mounts allow more engine movement to transfer energy to components.

What Causes Mounts to Go Bad?

There are several reasons mounts can deteriorate and fail over time:

  • Age and mileage: The rubber compounds degrade over years of exposure to road salt, weather, engine heat cycles, dirt, and oil contamination. The rubber loses elasticity and cracks or tears.
  • Damage from accidents or potholes: Major jolts can bend or tear mounts.
  • Oil contamination: Leaks allow motor oil to degrade the rubber.
  • Improper torque on mounting bolts: This stresses the components.

How to test them?

To perform this test, you will need an assistant. With the engine running and the parking brake engaged, have your assistant shift the transmission from park to drive and then to reverse multiple times. Keep pressing down the gas pedal during this test.

Observe the engine’s movement during each gear shift. If there is significant movement, it suggests a worn-out mount.

If you are still uncertain about the condition of your mounts, you can perform a road test. During the test drive, pay close attention to any unusual noises, vibrations, or engine movements, especially when the vehicle goes over potholes.

10. Malfunctioning Oxygen Sensor Provides Inaccurate Fuel Trims For Operation With AC Load

The oxygen sensor, also known as O2 sensor, is located in the exhaust stream. It generates a voltage based on the amount of oxygen detected in the exhaust gases. This voltage signal is fed to the engine computer.

The computer uses this data to constantly adjust the air-fuel mixture to maintain the ideal ratio for efficient combustion – around 14.7:1. This ratio helps maximize power and fuel economy while minimizing emissions.

If the exhaust oxygen content is high, it means the mixture is running too lean. The computer will respond by adding more fuel. Low oxygen indicates a rich mixture, so it will cut back on fuel. This feedback loop allows real-time fuel trim adjustments.

Why Oxygen Sensors Fail?

Oxygen sensors are subjected to high temperatures and harsh operating conditions. Some common reasons they fail include:

  • Age and accumulated mileage which leads to thermal degradation of the internal sensing element
  • Oil contamination from leaks
  • Damage or improper installation of the sensor itself
  • Failed wiring or connector issues that disrupt the sensor signal

What happens when you turn on AC?

When an oxygen sensor starts to fail, symptoms may only appear when the vehicle is under increased load, like when the AC compressor cycles on. This is because the extra demand taxes the ability of the compromised oxygen sensor to maintain proper fuel control. Signs can include:

  • Intermittent sputtering, hesitation, or misfiring when the AC engages: This is from the engine struggling to run optimally with a faulty air-fuel mixture signal.
  • Surging or oscillating RPM when idling with AC on: The sensor cannot maintain steady fuel trim.
  • Stalling: Severely lean or rich spikes from a malfunctioning sensor can stall the engine when AC is demanding extra power.

How to test bad Oxygen sensor?

The voltage generated by the oxygen sensor should fluctuate within a certain range as the air-fuel ratio changes.

If the oxygen sensor is not functioning properly, the voltage may not fluctuate as it should, and the engine may not run at its optimal performance.

For a healthy O2 sensor, the voltage should fluctuate at least once time for every two seconds. The voltage range should be between 850mV and 150mV.

If the O2 sensor’s reading is stalled in position, or switches abnormally high or low, your sensor has failed.

To test the O2 sensor, you should connect the positive lead of the voltmeter to the signal wire of the O2 sensor. If the O2 sensor has two wires, connect the negative lead to the negative wire of the sensor.

Final Thoughts

In summary, a car sputtering or stalling when the AC turns on is often an indication of extra load being placed on an underlying issue.

Possible culprits include a faulty idle air control valve, bad ignition components, low refrigerant, clogged EGR valve or dirty throttle body.

Thorough diagnosis and testing is needed to pinpoint the root cause. Addressing any engine performance problems and having the AC system properly serviced can help resolve sputtering and driveability problems that appear when the AC compressor engages. Maintaining your vehicle with regular upkeep is key to minimizing many of these issues.

Some First Hand Experiences Shared By Users In Different Communities

Our team conducted research across various online communities, forums, and subreddits to gather user comments and opinions on “car sputtering when ac is on”.

User 1 says:

I experienced sputtering in my 2015 Kia Sorento when the AC was on. The cause was quite unique – a faulty engine mount. The additional vibration caused by the AC’s load made the problem evident. It was unusual because engine mounts aren’t directly related to the AC system. Replacing the engine mount fixed the sputtering, and the car runs smoothly now, both with and without the AC.

User 2 says:

My 2019 Mitsubishi Outlander had an issue where it would sputter when the AC was turned on. After some diagnostic work, I found out that the serpentine belt, which drives the AC compressor, was worn out and slipping. This slip caused the sputtering as the compressor struggled to operate effectively.

User 3 says:

My 2018 Honda CR-V started sputtering when I used the AC. It was puzzling at first, but after some research, I discovered the issue was with the AC clutch relay. This relay was failing to engage and disengage the AC compressor properly, leading to intermittent sputtering. After replacing the AC clutch relay, the problem was completely resolved.

User 4 says:

In my 2013 Volkswagen Jetta, I noticed sputtering when the AC was on. The issue was traced back to a malfunctioning throttle body. It wasn’t responding correctly to the increased load from the AC, causing the engine to sputter. Cleaning and recalibrating the throttle body solved the problem, and the car now handles the AC load without any issues.

User 5 says:

My 2011 Dodge Charger had a strange issue where it would sputter only when the AC was on. After some investigation, I discovered the problem was a leak in the vacuum system. This leak was causing the engine to run lean, especially when the AC put additional load on the engine. Fixing the vacuum leak resolved the sputtering, and the car now runs perfectly with the AC on.

User 6 says:

I own a 2017 Hyundai Tucson, and it started sputtering when the AC was activated. Initially, I thought it was an engine issue, but after some inspection, I found the AC condenser was clogged with debris. This blockage was causing the AC system to work harder, putting strain on the engine. After cleaning the condenser and removing all the debris, the sputtering stopped. It was a simple fix but a vital lesson in regular maintenance.

Car Sputtering or Shaking When AC Is On: FAQs

Why does my car sputter when I turn on the AC?

Common causes include a faulty idle air control valve, bad ignition components, low refrigerant charge, clogged EGR valve, or dirty throttle body that struggle under the AC’s extra load.

What are the most common reasons an AC could make a car stall at idle?

Low refrigerant prevents proper AC idle speed relearning, leading to stalls. Bad IAC valves and ignition parts also commonly cause stalling due to poor idle control when AC is on.

How can I test my AC condenser if I suspect it’s causing sputtering?

Feel condenser tubes with AC on. If no airflow, it’s blocked. Use pro gauges to check for excessive high-side pressure indicating issues.

What’s the best way to diagnose AC compressor clutch seizure?

Listen for squealing belt, feel for binding. Isolate compressor and check for hard turning. Clutch should spin completely freely.

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