Advance Answers: Questions About Vehicle A/C Systems

car air conditioner pictureWe explore a couple questions that have come our way regarding your car’s thermostat and how vehicle air conditioning systems work.

Updated: May 2016

In general, using logic and common sense when diagnosing problems with your vehicle — and when making repairs and addressing issues — is a good strategy. You eliminate what doesn’t make sense as you narrow down your diagnoses. Every once in awhile, though, what you should do is somewhat counterintuitive. Especially when it comes to vehicle A/C systems (which are part of a vehicle’s HVAC system, or Heating, Ventilation, and Air Conditioning).

What should I do when my car overheats?

Sometimes cars overheat (fortunately, not as often as they used to). This happens most often in hot summer weather — and if you’re in the car, you’re probably feeling pretty toasty yourself.

The first thing to do is to shut off your car’s air conditioning and open the windows to decrease the burden on your engine.

So far, that makes sense — but, the counterintuitive part is this: if your car continues to overheat, then turnon both your heater and its blower. It may be uncomfortable, but this transfers heat away from the car’s engine into the inside of the vehicle itself.

Why should I use my A/C system in the winter?

The best thing to do during winter is run your A/C. It sounds strange, but running your vehicle’s air conditioning system during the winter can make sense (just remember your gloves!). If you run your A/C throughout the year, the system stays more lubricated, helping to prevent leaks. That’s because the refrigerant contains oil that lubricates the system, including the compressor. It also keeps seals and hoses moist, which helps to prevent the dryness that leads to cracks and leaks in the system.

Your car’s A/C system is also much more efficient at window defogging than the heating system. So, turn it on to clear up the fog — and, if your A/C doesn’t do the job, check the compressor because this might indicate a problem.

How important is it to fix the thermostat?

The thermostat is a relatively small and inexpensive part with an important job: it senses and “reports on” the varied temperatures throughout the vehicle’s engine. The engine needs to run hot to burn fuel, but if it gets too hot, the thermostat signals the release of coolant to reduce the temperature.

If the thermostat isn’t working properly, the coolant can keep flowing until it’s all burned off, which can lead to overheating or even more severe problems, like a blown cylinder head gasket.

In cold temperatures, the thermostat prevents water from going to the radiator. This helps the engine warm up enough, even on bitter winter mornings.

If you think your thermostat might be operating at less than optimal efficiency, it’s often easier to replace it to see if that solves your problem, versus going through more complicated diagnostics. Check the original thermostat in your vehicle and buy a comparable thermostat replacement. You can also check your owner’s manual to see which type of thermostat the OEM recommends.

Your A/C: Car Myths Debunked!

Car_air conditioner

Myths – they’re everywhere, and particularly online. Plenty of those myths focus on cars, like the one about it being better to fill your tank in the morning because the fuel is colder and denser (it isn’t) and you’ll get more for your money (you won’t.) Or there’s the one about increasing your pickup truck’s fuel mileage by driving with the tailgate down to reduce wind resistant (false, as pickups are designed to be aerodynamic with the tailgate up).

I’d like to investigate two myths that always seem to crop up when summer rolls around, the temperature climbs higher, and the long road trip becomes commonplace. It’s this myth: a vehicle’s air conditioner causes the engine to work harder. Therefore, electing not to use the air conditioner and instead rolling down the windows when driving will significantly increase fuel mileage. And in a similar vein there’s this myth – driving with your windows down will significantly decrease your fuel mileage because of the increased aerodynamic drag the open windows create.

One myth probably has some truth to it and one is most likely false. Here’s why.

In a test conducted by Consumer Reports, they drove a Honda Accord at 65 mph and found that using the air conditioner reduced fuel mileage by three percent. In another test they drove at 65 mph but this time with the windows down and found no measurable effect on fuel mileage. In a similar test performed by Edmunds using a Toyota Tundra, they saw a decrease in fuel mileage of almost 10 percent when using the air conditioner as opposed to driving with the windows down and the air conditioner off.

There are many similar tests and results online, but here’s what I think is the bottom line. It’s a conclusion similar to that reached by many of the testers:

  • Using a vehicle’s air conditioner may result in a small decrease in fuel mileage. However, that decrease is negligible compared to the discomfort of not having air conditioning on a hot summer day.
  • Driving with a vehicle’s windows rolled down doesn’t produce any measurable impact on fuel mileage as a result of aerodynamic drag (but your dog will love it if he’s along for the ride!)

If you really want to improve gas mileage during an epic road trip this summer, pay attention to these fuel-saving strategies instead:

  • Slow down and avoid aggressive driving, such as hard accelerations and hard braking and increase fuel mileage by as much as 33 percent at highway speeds, according to the official U.S. government source for fuel economy.
  • Remove excess weight from the vehicle and avoid hauling bulky items on the roof because it increases aerodynamic drag.
  • Keep your engine in tune and tires inflated to the recommended air pressure for a three to four percent improvement in fuel mileage.
  • Consolidate trips or share rides with someone else.
  • Drive a fuel-efficient vehicle.
  • Get more fuel-saving tips.

It’s not a bad idea to brush up on your  A/C-testing skills either because cold A/C makes for a comfortable car temperature. If you think your air conditioning might be malfunctioning, measure the temperature accurately by sticking this A/C thermometer in the vent with the A/C turned on. It might be working fine or you might need a simple fix. Either way, you’ll have an accurate temperature reading to help you decide.

For me, even on a hot summer day, if I’m driving on back country roads or on the highway, I prefer having the windows down and the A/C off. There’s just something about fresh air that I love. But driving around town, having the A/C on wins hands down every time. What do you think?

Editor’s note: Visit Advance Auto Parts for helpful advice and even better values. Buy online, pick up in store—in 30 minutes.