Toyota 4Runner 1984-1995: Why is the A/C Blowing Fog?

So you turned your air conditioning on, but instead of cool air, you got a face full of fog. Now that you're over the initial shock of what happened, check out this article and get your air blowing clean as well as clear again.

By Brett Foote - January 22, 2016

This article applies to the Toyota 4Runner (1984-1995).

Many automotive problems might seem like voodoo, but nothing figures to creep you out as much as a fog that suddenly (and mysteriously) comes out of your air conditioning system. You might also notice that the air isn't as cool as it should be, and the problem might not be constant, coming and going every few minutes or even at different times of the day. Keep reading and we'll help you get to the bottom of the mysterious air conditioning fog.

Toyota 4Runner Why is the AC Blowing Fog

Material Needed

  • None (visual inspection)

Step 1 – Check your air circulation

High humidity can cause air to "fog" and come out of the vents.

If you live in a humid climate where your air conditioner is cooling air past the outside dew point, fog will appear inside the vehicle. This happens because the air conditioner is effectively condensing the water in the air as it blows through your vents. If your fog issue is only occurring during times of peak humidity (like after a nice August rainstorm), this is likely the cause of the issue and it's nothing to worry about.

AC vent
Figure 1. A little fog is normal when humidity is high.

If humidity and dew point isn't causing your problem, move on to Step 2.

Step 2 – Check your coolant level

A leaking heater core can cause some fog to appear.

The heater core is part of your engine's cooling system. As engine coolant gets up to temperature, it begins to flow into an ancillary passage behind the firewall and the dashboard called the heater core. The hot coolant is what gives your HVAC system a source of heat. Over time this box can wear out, causing a leak. While a leaky heater core is one particular cause of a foggy interior, you will generally also have a coolant smell inside the vehicle, which is pretty hard to miss. Check your coolant, and if it's a little low, look around the firewall and on the inside of the vehicle for signs of a leak. A leaky heater core is a worst case scenario, as the entire dash must be taken out to replace it.

leaky heater core
Figure 2. A leaky heater core can cause fog in your vehicle.

If your heater core isn't the cause of the issue, move on to Step 3.

Step 3 – Check your air conditioner drip line

A clogged up drip line can lock in condensation and lead to fog.

The drip line on your air conditioning system serves to let moisture escape the system. If your line is clogged up or dirty, the condensation has nowhere to go and can just simply turn into foggy air coming out of your vents. You should also have a small puddle of water under the vehicle after you run the air conditioner and then park it for a while. If not, make sure that your drip line isn't clogged up and is flowing freely.

Toyota 4Runner drip line
Figure 3. Check your A/C drip line and make sure it's flowing freely.

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