Why Do I Sometimes Still Hear Things Running In My Car After I Shut My Engine Off?

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Engine cooling fans and other systems are programmed to keep running in some cars even when the engine is off. Here’s what’s normal.

A loyal BestRide reader recently purchased a new 2018 Mazda CX-5. Once home from the dealership, she parked the new crossover in her garage, shifted to P, turned it off, and locked it. Once outside the car, she could hear a fan running in the engine area and asked us if this is normal. It is. And many other sounds from under the hood of a car that has been shut off are also normal. Here is a quick rundown of the things you might hear other than the ticking of cooling metals.

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Cooling Fans Run After Your Car Shuts Off

Engines are complicated machines that combine liquid and air cooling to keep the engine and associated components at their optimum temperature. Modern engines heat up quickly and so too do the exhaust treatment components, some of which need to be hot in order to function properly. Similarly, engines are built to run at their peak operating temperature not just to avoid damage, but for performance reasons. Many modern engines are smaller. That old marketing line, “smaller engines work harder” is true.

With engines having aluminum and other metals in contact with one another that heat and cool at different rates, and which have different thermal expansion rates, managing the temperature changes is more important now than ever. The Mazda CX-5 like many modern automobiles continues to cool the engine after it shuts off. The manual addresses this, saying, “The electrical fan in the engine compartment could turn on for a few minutes after the ignition is switched from ON to OFF, whether or not the A/C is on or off, to cool the engine compartment quickly.”

Headlight Noises

As crazy as this might sound to a new car owner, modern headlights make noise. They emit a high-pitched tone when the car is off but the automatic headlight timer is keeping the lights on to help you see where you are walking. Not all new cars make this sound. It depends on the headlight technology your car has. We have noticed it mostly in cars with expensive optional lights. The sound can be quite loud and can also end with a sort of final tone almost like a click or clunk. The power electronics that new headlights use require cooling fans or heat sinks to work properly. The sounds these make can be heard easily as you exit your vehicle and walk away.

Turbocharged Engine Sounds

Imagine an engine with a turbocharger spinning at about 100,000 RPM in direct contact with hot exhaust gases. On your way home, you are using that engine and its hot parts and the cooling fans and liquid coolant are doing their jobs keeping the metal parts at just the right temperature. Then you abruptly stop and turn off the car. The hottest parts of the engine and associated parts still have a lot of heat to handle.

Automakers need to manage this situation. We reached out to Volkswagen repair expert and owner of Brian’s Garage, Brian Mushnick, to ask about how turbos are cooled after a car shuts off. Brian told us, “VW and Audi use a secondary electric coolant pump to cool the turbo after shut down. Circulating coolant while static cools the bearing cassette to help prevent damage and oil burning (let’s remember no oil circulates while the engine is off but the turbo still spins.) They have been using this system since they went to the turbocharged gas engines.” This also explains why it is not necessary to idle turbocharged cars after you finish driving to let the car cool down. Your automaker thought of that.

Evaporation System Buzzing

Every modern gas-powered vehicle has a system to capture gasoline vapors when you fill up and also to manage the vapors from normal evaporation in your tank. The system has a charcoal filter, about twenty feet of plumbing, check valves, relief valves and solenoid valves. When you park, the system will sometimes make noises that sound like a small pump (but isn’t). The noises can be at the back of the car or near the rear doors. This is normal and this sound hard to predict. The note above is from the manual of a Subaru Outback.

 

Cars make funny sounds. Many of the ones you hear after you shut the vehicle off are no cause for concern.

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John Goreham

John Goreham