Why is the Low-Pressure Pipe of a Car Air Conditioner Hot to the Touch?
4 Answers
Causes for the low-pressure pipe of a car air conditioner being hot to the touch include compressor or expansion valve failure, evaporator blockage, insufficient refrigerant flow, or excessive pipeline pressure, as well as excessive fouling in the condenser copper tubes. Under normal circumstances, the low-pressure pipe of a car air conditioner should be cool, while the high-pressure pipe should be hot. If the low-pressure pipe becomes hot, the air conditioning cooling effect will be poor. Below is additional information: 1. Compressor or expansion valve failure: This can cause the low-pressure pipe of a car air conditioner to become hot. The air conditioner compressor plays a role in compressing and driving the refrigerant in the air conditioning refrigerant circuit. The air conditioner compressor extracts the refrigerant from the low-pressure area, compresses it, and sends it to the high-pressure area for cooling and condensation. The heat is dissipated into the air through the radiator, and the refrigerant changes from a gaseous state to a liquid state, with an increase in pressure. 2. Checking the refrigerant: If the expansion valve is blocked and the air conditioner is not working, check whether the refrigerant is sufficient. If it is insufficient, the system needs to be vacuumed before adding refrigerant, and then use a pressure gauge to check whether the pressure in the high and low-pressure pipelines is normal. 3. Identifying the high and low-pressure pipes of a car air conditioner: The thin one is the high-pressure intake pipe, and the thick one is the low-pressure pipe. You can also distinguish them by the letters next to the compressor connection. Some compressors are mostly marked with S or D to distinguish them, where S is the low-pressure connection and D is the high-pressure connection.
I've been repairing air conditioning systems for over a decade, and from what you're describing, it's almost certainly due to insufficient refrigerant. You mentioned the low-pressure pipe feels scalding hot, right? Normally, it should be ice-cold with condensation. When the compressor runs without refrigerant to compress, those pipes become red-hot like soldering irons, rendering the entire AC system useless. Other common scenarios include: a completely clogged expansion valve blocking refrigerant flow, or excessive moisture in the dryer bottle causing ice blockage in the pipes. If the condenser gets caked with mud and leaves, it can lead to poor heat dissipation, causing high-pressure heat to backflow into the low-pressure side. More severe cases involve compressor clutch slippage or internal wear causing similar issues. You need to immediately test both high and low pressures with gauges - a minor refrigerant leak is one thing, but a seized compressor will cost you a fortune to repair. Remember, when you notice cooling efficiency dropping, don't push it - unless you want blistered hands from touching those scorching pipes.
Last time at the tuning shop, the technician mentioned that abnormal overheating of the low-pressure pipe is essentially caused by a malfunction in the heat exchange cycle. When refrigerant is insufficient, the pressure on the low-pressure side becomes too high. The compressor struggles to suck in refrigerant but fails, causing the pipe to be directly heated by the exhaust heat. Insufficient heat dissipation from the condenser is even more troublesome, as high-temperature and high-pressure gas accumulates and can't pass through, ultimately causing all the heat to pile up in the low-pressure pipe. The weirdest case I've encountered was an expansion valve stuck in the closed position, blocking all the refrigerant in the high-pressure pipe. At that point, the boundary between high and low pressure completely breaks down, and the low-pressure pipe feels hotter than the engine coolant hose. I recommend you immediately check the high and low pressure readings of the AC system—anything over 30 psi on the low-pressure side is definitely abnormal. If all else fails, turn off the AC switch first to prevent the compressor from running dry and burning out the clutch. For older cars, especially check if the cooling fan is running, and remember to blow dust off the radiator fins with an air gun every month.
Having run an auto repair shop for thirty years, there are only two possibilities when the low-pressure pipe gets scalding hot: either poor heat dissipation or refrigerant leakage. The most common scenario is the radiator and condenser being clogged with insect remains or willow catkins, and if the cooling fan isn't running, the high pressure won't drop. There's also a simple method: pop the hood and feel the high-pressure pipe—if both pipes are extremely hot, there's a 90% chance the refrigerant is insufficient. I once handled a case in winter where the expansion valve was completely blocked, causing the high and low pressures to equalize, and the low-pressure pipe temperature soared to 90°C. A rarer issue is compressor failure leading to a loss of pressure difference. Before repairs, always turn off the AC, and if on the highway, open the windows immediately for ventilation. During inspection, focus on whether the condenser surface is deformed and if the electric fan runs smoothly. For DIY checks, spraying water to cool it down can be a test—if the temperature drops sharply, it’s a heat dissipation problem.
