Why is my car’s air conditioning system blowing hot air?


AAA Northeast Automotive Physician John Paul answers a question from a reader trying to figure out what’s wrong with his car’s air conditioning.

Photo by Joe Raedle/Getty Images

Q I’m retired and my 2012 Toyota Camry doesn’t drive much. I had problems with my air conditioner blowing hot air. The dealer found no problem. I normally keep the gas tank a quarter full. I filled the reservoir and that seemed to fix the AC problem. Could the performance of the air conditioner be directly related to the volume of gas in the tank?

A. The two should have nothing to do with each other. I suspect a sticky air mix door that for some reason stuck and then closed and let cold air into the cabin. If the dealer who checked your car was simply checking the A/C compressor operation and pressures—not the air duct temperature—they would have assumed the system was working normally.

Q In 2006, I had a Jetta GLI with a surprisingly smooth 2.0 turbocharged engine that produced 200 horsepower, even though it required premium fuel. A few months ago I purchased a low mileage Lincoln MKZ AWD also with a 2.0 turbocharged engine that makes 240 horsepower. I find it to be surprisingly fast and uses regular gas. The engine cover says Lincoln, but I believe in other applications it’s an Ecoboost. How do they make more power and do they do it with regular fuel?

A. More air equals more power. If the turbocharger operates at a higher boost pressure, it will produce more power. Typical VW boost pressure is 7-11 PSI and Ford Ecoboost pressure can reach 17 pounds of boost. If you add air scavenging (how quickly air moves in and out of the engine), engine timing and fuel delivery, compression, horsepower and torque can change significantly. It is this combination of engine timing, sensors, engine design, and turbocharger design that can allow for higher horsepower while using 87 octane fuel.

Q I have a 2016 Mazda CX-5 with 78,000 miles. Recently the car suddenly started shaking, sputtering and had no power at all. Apparently the spark plug coil failed. I was told that the spark plugs needed replacing at 75,000 miles and that worn spark plugs caused the coil to fail. I replaced them as well as the faulty coil. Should I be worried about this happening again with the other three original coils? The car is almost impossible to drive when this happens.

A. The dealer is right – the spark plugs were due for replacement at 75,000 miles. And it is possible for worn spark plugs to cause ignition coil failure. If the ignition coil has failed due to a worn spark plug, this may be a unique situation. Still, if it was me, and even with my frugal (okay, cheap) nature, and I was planning on keeping the vehicle, I would replace all three coils to prevent future problems.

Q I have a 2009 Volkswagen CC Sport with 65,000 miles. It is in good condition with many new parts, it is well maintained and drives well. The check engine light is on and after being reset by a mechanic, comes on again. This issue is causing problems getting my vehicle inspected. The dealer made some repairs to an oil separator. Also, the dealer says there is a main seal leak that could cost up to $2,000 to fix. We are old people and we cannot afford it. I need the check engine light to go out to have the car inspected. Can you help us?

A. The oil leak has nothing to do with the check engine light. At this point I would check the oil weekly so you can figure out when you need to add oil. The check engine light is on because one of the sensors is reading outside of its limits. The oil separator is a clue. There may be a leak in the system causing a lean operating condition. If either gasket is leaking, it will overwhelm the PCV system, cause the car to run lean, and cause the check engine light to come on. Resetting the check engine light without performing any additional repairs will ensure that the light comes back on.

John Paul is AAA Northeast’s Automotive Physician. He has over 40 years of experience in the automotive industry and is an ASE Certified Master Technician. Email your question to [email protected]. Listen to the Car Doctor podcast on johnfpaul.podbean.com.