What Causes Water Leakage Under the Car Chassis After Turning on the Air Conditioning?
3 Answers
Here are the specific reasons for water leakage under the car when the air conditioning is on: Evaporator Drainage: The car's cooling system is equipped with an evaporator inside the dashboard. When the air conditioning is turned on (A/C button ON), the surface temperature of the evaporator becomes very low, causing moisture in the hot air inside the car to condense on the surface of the evaporator's heat-absorbing fins. The condensed water droplets are collected by the plastic housing of the evaporator and drained out of the car through a pipe. Compressor Condensation: Sometimes, the exterior of the compressor (low-pressure area) installed in the engine compartment can condense moisture from the air, causing it to drip onto the ground. Lack of Insulation Foam: If the low-pressure pipes are not covered with insulation foam, the surface of the pipes can easily condense moisture from the air, and water droplets often gather and drip from the lowest point of the pipes.
My old car often does this too—I park it and find a puddle of water under the chassis. I asked an experienced mechanic and learned it's completely normal. When the AC is cooling, condensation forms on the surface of the evaporator, just like the water droplets on a cold soda can. The car has a dedicated small drain tube to let the water flow out under the chassis. As long as the water is clear and colorless, and the puddle is under the front passenger side, there's no need to worry. However, last time I noticed the front passenger floor mat was completely soaked. The repair shop found the drain tube was clogged with poplar catkins—that's something you need to clear out promptly.
To address such leakage issues, first identify the type of fluid. In most cases, it's clear-colored air conditioning condensate near the front axle. If the fluid appears blue-green or pink, be alert for coolant leakage—especially if you detect a sweet smell, which could indicate a cracked radiator hose. Last week, we handled a case where a customer installed a low-quality underbody shield that deformed the AC drain pipe during bumps, causing water to backflow into the cabin air filter. Remember to regularly check for leaf debris or mud near the drain outlet—using a thin wire to clear it is the easiest solution.
