Why Is Brown Foam Coming Out of My Car’s AC?

Finding brown foam bits around your car’s AC ducts is not uncommon. Over my almost 60 years in the auto repair industry as an ASE and GM master technician, I have seen my share of these phenomena, particularly in cars that are 10 or more years old. Usually, it’s just an annoyance that you just have to keep cleaning. Read along, here’s what you need to know about what’s causing brown foam to come from your car’s AC.

What Is The Brown Foam Coming From My Car’s AC?

The brown foam coming out of a car’s AC is the foam (sort of like sponge weatherstripping) that seals the many different parts of your car’s HVAC (heating, ventilation and air conditioning) ducting system that has begun deteriorating. The foam may also be used as insulation around the blower motor fan, the heat/cooling exchanger (plenum) system, and blend doors.

Why Is the Foam Coming Out of My AC?

There are a few reasons the foam may be coming out of a car’s AC system. The most common is that as the foam ages and degrades, it becomes brittle and begins to break down and crumble. As the hardened foam falls apart into small pieces, air from the blower motor pushes the foam through the AC and heater ducts, finally making its way out of the dashboard AC vents/louvers or heater ducts onto the floor. Exposure to wild temperature swings of the HVAC system, dirty or clogged cabin air filters, dirt and debris in the outside air intake, and moisture buildup in the HVAC duct system helps speed up the aging process.

Another common reason foam comes out of the AC is the failure of the adhesive securing the foam to the individual HVAC duct parts. The loose foam then gets blown out of the AC system.

In climates where the outside temperatures reach 100 degrees F (with interior temperatures of 125 degrees F) in summer, firing up the AC quickly generates air temperatures of 40 degrees F or 0 degrees F in winter, with the heater outputting 180 degrees F can also cause the foam to fail prematurely. Basic maintenance can help keep foam seals pliable and sealing properly. This includes:

• Replacing the cabin air filter.
• Cleaning the AC “drain” weep hole at the bottom of the HVAC evaporator case.
• Removing leaves, dirt, dust and debris buildup from the under-hood air intake vent, usually located on the passenger side of the car under or near the wiper arms.
• After driving with the AC on, with the engine off and key “on,” running the AC blower fan for a minute to help dry out any residual moisture left in the AC evaporator case.

What to Do If Foam Starts Coming Out of Your Car’s AC?

First, do all of the above maintenance items. Then, use a compressor (ALWAYS wear safety glasses and a respirator when working with compressed air) or vacuum to clear away any loose foam from the dashboard. Carefully remove the AC vents/louvers.

• Caution: Made of plastic, like foam, vents/louvers can become brittle and crack while being removed. Vacuum out the ducts and the entire interior, including the vents/louvers, and clean everything with disinfectant wipes.

If the system is working okay, just keep up on the maintenance, clean up the foam and continue driving. If you feel comfortable removing the dashboard or sections of the HVAC system ductwork, you can also replace the foam. But in my experience, this is best left to the pros.

Is It Safe To Keep Driving With Brown Foam Coming From My Car’s AC?

It depends. If the foam is dry and simply around the vents, then it shouldn’t pose a safety issue. However, tiny foam particles (microplastics) floating around the passenger compartment from the HVAC ducts will negatively affect air quality, especially if you suffer from allergies or asthma. It also clearly signals that the HVAC system needs attention and should be inspected to prevent system components from failing prematurely and possibly impacting the HVAC system’s performance.

Resources

• Bill Kirkpatrick, ASE and Volkswagen Master Technician, former shop owner, and dealership service manager and retired a Pennsylvania Department of Education (PDE) certified automotive technology and small engine instructor. (Nov 23, 2024)
• Joe Simes, ASE and Toyota Master Technician, EPA Section 609 and PennDOT certified emissions and safety inspection instructor and inspector, and PDE certified automotive technology instructor at North Montco Technical Career Center in Lansdale, PA. (Nov 23, 2024)

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