Rick's Free Auto Repair Advice

AC hissing noise

AC hissing noise — What causes it?

An AC hissing noise when your car is running and AC is on can be perfectly normal or a sign of a problem. AC systems work by pressurizing liquid refrigerant and then releasing through a very small opening. Depending on the type of system used by the carmaker the metering device can be an orifice tube usually located in the engine compartment or an expansion valve often located in the cabin area. As they release the high-pressure refrigerant they make a slight AC hissing noise, similar to the noise you hear when you depress the nozzle on a spray can. However, a louder hissing noise can be caused by a contaminated AC system

When does the AC hissing noise occur?

The AC hissing noise is most noticeable when you shut off the vehicle and you’ll hear it coming from under the dash area. That’s the sound of the high-pressure refrigerant flowing through the metering device and it will continue until the pressure is  “equalized” the same pressure on the high and low sides of the system.

If AC hissing is loud enough to hear when you’re driving

That can be a sign of system contamination. If the AC system has had a leak and you’ve recharged it with a DIY AC recharge kit, chances are you have air and moisture in the system. Air in an AC system reduces its efficiency. Moisture in the system causes two problems; ice and acid and sludge formation.

Any time you have a refrigerant leak, air gets into the system and that air carries ambient moisture into the system. When moisture flows through the orifice tube or expansion valve, it can freeze, causing the metering device to make more noise; enough AC hissing noise to hear while you’re driving. In that case, the only option is to fully evacuate the system, locate and fix the leak and then refill.

See this post for more information on what causes obstructions.

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Posted on by Rick Muscoplat



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