Solving Automotive HVAC Problems
This can be caused by two main things. The first is air in the cooling system. In the second half of this article, I talk about how to handle this issue. In addition to air in your cooling system, you could have a blockage at the drain tube for the AC system. Under the evaporator is a drain that is supposed to allow moisture from evaporator condensation to exit the HVAC system. It’s the reason you see your car dripping water during the summer when you run your AC. This is due to the humidity collecting on the evaporator in your HVAC system. It’s the same thing that happens to a glass of ice water when you let it sit out. The AC drain is there to help remove this moisture. If it becomes clogged due to debris, it can allow the water to collect in your dash. This can manifest as a swishing or sloshing noise when you make a turn, and in some cases you might even get water leaking into the passenger compartment.
This is actually really easy to fix. All you need is some compressed air. Find the AC drain tube and blow compressed air into it. Watch out; once you clear the blockage you might get a bath. One way to help prevent this is to remove any debris you find under your windshield wipers at the base of your windshield. This is the fresh-air opening to your HVAC system. Any debris that collects here can work its way into the HVAC system and cause problems like this. It’s less likely with vehicles equipped with a cabin air filter, as those filters are designed to trap debris before it gets into the HVAC and causes a problem.
Here’s a video on the process of unclogging an AC drain.
The biggest complaint I get is no heat or intermittent heat. The number-one cause of this is air in the cooling system. Whenever someone comes to me with a heater problem, the first thing I check for is air in the cooling system. Here’s a video on how to purge the air from a cooling system.
If you purge the air from your cooling system and you get your heat back, don’t consider it fixed just yet. There might be a leak somewhere causing the air to enter the system. Here’s a video showing how you can find coolant leaks in your cooling system.
Once you’ve found and repaired the leaks, your heat should be restored and you can go on your way. Read the Overheating Problems article to get more detail about finding and repairing cooling system issues.
This is covered in the Overheating Problems article, but I’ll touch on it here. If you find that you don’t have enough heat coming through the vents, it could be because your engine’s thermostat is stuck open. If this happens, your engine will run much cooler than it should and you won’t have good heat at the vents.
Don’t just replace the thermostat if you don’t have good heat. Look for other causes first. One way to check if your thermostat is working correctly is to feel the hoses going to the radiator. When the engine is at operating temperature, the top hose should be hot and the bottom hose should be cool, or cooler than the top hose. If both hoses are cool to the touch with the engine at operating temperature, your thermostat could be stuck open. The only way to really find out is to remove it and inspect it. If you find it’s stuck open, replace it, bleed the cooling system, and recheck your heat output. Don’t skip bleeding the cooling system. Remember the number-one cause of no heat is air in the cooling system. One of the main causes of air in the cooling system is not bleeding the air out after servicing the cooling system. I recommend original equipment thermostats for best results.