We’ve endured some really hot days lately, which makes me appreciate air conditioning that much more.
Last summer, one of my customers had a tricky problem with the air conditioner in her 2006 Saturn Vue.
She told me that while driving the car, white smoke or steam intermittently came out of the vents. This was unusual because there are only a few things that could cause smoke, steam or gas to escape from the vents, and they all have some sort of smell or leave behind a residue.
I tried to ask as many questions as possible. I wanted to know if it happened when starting the car, while driving it or after it’s been running for a while. I also wanted to know if there was an odor and if it left behind any residue (such as leaving the car floor wet).
The customer told me the smoke or steam lasted for approximately 5 seconds and then it was gone. There was no smell or residue, and it was truly intermittent.
Inside the dash there is a component called the heater box. It houses the heater core, evaporator core, blower motor, vents and controls. The most common cause of steam from the dash ventilation system is a leaking heater core. Once there is a small hole in the heater core coolant, drips or steam will escape into the ventilation system. This coolant or steam will fog up the front window and will drip onto the floor. This type of leak has a distinctive sweet smell. Yet this car showed no signs of coolant loss or residue coolant inside the car.
Another thing that could cause this is an electrical problem. A failing electrical component, wire harness, switches, relays, resistors or blower motor could produce smoke. Yet in those cases, there is always a heavy burning electrical smell. These types of electrical smells do not dissipate quickly. The smell would be there indefinitely, and some electrical components would most likely not work.
The problem could also be in the air-conditioning system. If the evaporator core had a leak, Freon gas could escape. Freon gas itself is virtually odorless but can smell like the oil it is mixed with. The AC oil lubricates the AC components. AC oil leaves behind an oily residue and in some cases can have a pungent odor.
Diagnosing the problem
So at this point we had test-driven the car multiple times without being able to confirm the complaint. We pressure-tested the cooling system and found no leaks. We checked for blown fuses and inoperative electrical components. We also evacuated and recharged the AC system to find out it was OK and had a full charge. There were also no leaks inside the engine compartment that might have burned off and sent smoke to the ventilation system.
This is when we started thinking outside the box. Because we did not have a coolant leak, electrical problem or AC Freon loss, we thought there might be a problem insider the heater box, such as excess moisture on the evaporator core.
To get a better understanding of excess moisture, here’s a quick explanation of the AC system: Low-pressure Freon gas is pulled into the AC compressor, where it turns into a high-pressure gas. Once this high-pressure gas leaves the compressor, it is hot and has to be cooled. It then flows into the AC condenser, where it is cooled by the auxiliary AC fan. After the Freon is cooled, it flows through a receiver dryer that filters out dirt and moisture. It then flows through an expansion valve. Once the Freon leaves the expansion valve, it is a liquid. The liquid Freon flows into the evaporator core. The blower motor then pulls in warm, moist air and blows it over the evaporator core, where it turns into cold air inside the car. Once the Freon leaves the evaporator core, it is a low-pressure gas again.
One byproduct of an AC system is liquid build-up on the outside of the evaporator core. If you’ve ever sat in a parking lot on a hot day with your AC on, you would likely notice a pool of water under the car. If the AC system does not have an expansion valve to control the flow of Freon, the evaporator core will just turn into a block of ice.
So we came up with the hypothesis that the expansion valve may be intermittently sticking, in turn freezing the water on the outside of the evaporator core. The valve would then unstick and the blower would vent the ice crystals out the vents and into the cabin.
Because we could not reproduce the problem, we explained our theory to the customer. She gave us the OK to replace the expansion valve. Problem solved.
I pride myself on making the most logical diagnosis with as many facts as possible. This was one of the rare cases in which we could not confirm the complaint up-front. By checking each system carefully and ruling out what was working, we were able to make the fix.