Why does a car feel like it’s going to stall when AC is on?

This blog will answer the following questions: Why does a car feel like it’s going to stall when the AC is on? What are the components of the air conditioning system in a car? How to check the air conditioning system in a car?

Why does a car feel like it’s going to stall when AC is on?

A car feels like it’s going to stall when AC is on due to the following reasons:

• Cylinder misfire
• Low idle speed
• Faulty Compressor 

Let’s take a closure look at each of the reasons

Cylinder misfire

A cylinder misfire occurs when the air-fuel mixture is not okay and the ignition system is not working correctly. The combustion process in a cylinder can completely shut down and also causes excessive vibrations depending on the engine.

A cylinder can also misfire if the compression pressure is too low compared to the other cylinders. The overall engine power and torque reduce significantly if there is a misfire in one or more cylinders.

For example, if one cylinder in a 4-cylinder internal combustion engine is not working. The remaining three cylinders cannot develop sufficient torque to keep the engine running smoothly when the AC is on. 

The air conditioning system puts an additional load on the internal combustion engine. The air conditioning compressor is connected to the crankshaft of the engine. When the compressor turns on the engine must produce additional torque.

The engine can even stall when AC is ON if there is a misfire in more than one cylinder. Combustion misfire due to low compression is very expensive to repair as it requires a complete engine rebuild.

The repair cost to rectify a misfire due to a faulty fuel injector or fault in the ignition circuit depends on the type of fuel injection method and the number of spark plugs used on the engine. However, it is not as expensive as a complete engine rebuild.

Low idle speed

If the car feels like it’s going to stall when AC is on, the idle of the internal combustion engine speed is too low. The engine control unit has an idle speed control system that increases the engine speed when the air conditioning compressor is switched ON.

The engine speed is increased by opening the throttle valve. There is no need for the driver to press the accelerator or throttle pedal. In diesel engines, the idle speed is increased by increasing the injection quantity of the fuel.

The idle speed control does not work if the throttle valve is faulty or the adaptation of the throttle valve is not done. The control unit actuates the throttle valve and learns its mechanical end stop for the idle speed control function.

Similarly, clogged fuel injectors can prevent the engine speed from increasing when the AC is on and the vehicle is idling. This problem is more noticeable when the vehicle is at standstill. If the AC is ON and the idle speed stays the same, the car feels like it’s going to stall.

The fault can be easily fixed after replacing or cleaning the throttle body or the fuel injectors in the case of diesel engines. If the throttle body is replaced, the teach-in of the new throttle body must be completed successfully.

Faulty Compressor

The air conditioning compressor is installed on the internal combustion engine and it is driven by a V-belt connected to the crankshaft. A car feels like it’s going to stall when AC is on if the air conditioning compressor is jammed internally.

The air conditioning system also gets damaged due to the small metal particles from the jammed compressor. As the refrigerant pressure is not within the specified value, the AC control unit keeps the compressor pulley engaged for longer periods of time.

Therefore, the car feels like it’s going to stall because the load on the engine is more than the engine speed can handle. More power is used by the compressor which affects the acceleration of the car.

The internal combustion engine has to work a lot harder if the vehicle is driven under full-load conditions. When the AC is switched off, the air conditioning control unit disengages the compressor pulley, and all the power produced by the engine control unit can be utilized for the driving force.

The compressor pulley can also get jammed and remain continuously engaged, this problem is more common in old cars with a centrifugal switch or mechanical pulley. In both cases, the compressor and the drier must be replaced together.

If the drier or receiver is not replaced small metal particles can damage the expansion valve and the evaporator coil. A receiver or drier is utilized to store the refrigerant so it can be quickly utilized to cool the evaporator coil without switching on the compressor.

What are the components of the air conditioning system in a car?

The components of the air conditioning system in a car are as follows:

• Air conditioning control unit

The control unit is used for the operation of the actuating motors in the air conditioning housing. It is also connected to the in-car temperature sensor and refrigerant pressure sensor. The signal from both sensors is used to control the pressure and the operation of the compressor.

• AC compressor

The AC compressor consists of multiple pistons, the refrigerant is compressed and high pressure is generated into the air conditioning system. It uses a special compressor oil to lubricate the moving parts.

The air conditioning compressor in a passenger car can generate a refrigerant pressure of up to 25 bars. However, only 11 to 14 bar of refrigerant pressure is sufficient in the system to bring the evaporator temperature below 4 degrees.

• Condenser

It is used to cool down the refrigerant that is compressed by the air conditioning compressor. It is located in front of the radiator and the engine fan is switched on when the AC is ON.

• AC housing 

The air conditioning housing consists of a thermal expansion valve, evaporator coil, blower motor, and air conditioning flap actuator motors. It is installed behind the dashboard of the car. There are access panels to clean the evaporator coil if a bad smell is coming when the AC is switched on.

How to check the air conditioning system in a car?

The air conditioning system in a car can be checked using a temperature gauge. There is also a built-in evaporator coil temperature sensor. If the system is working correctly, the temperature of the evaporator coil must be below 4 degrees.

The car should not feel like it’s going to stall when the AC is on. The refrigerant quantity can be checked with the help of an extraction machine and leaks in the system are detected with the help of UV dyes and pressure hold test.

Conclusion 

This blog explained the causes behind a car feeling like it’s going to stall when AC is ON and the various components in the air conditioning system. An OBD-II scanner can also be used to check for faults in the air conditioning system in a car.