Piston Slap: Going Out With Fans Blazing

Sajeev Mehta
by Sajeev Mehta
piston slap going out with fans blazing

Scott writes:

Why is it that the radiator fan turns on when I switch the heater knob to “Defrost” and not in any other mode? I have a 2001 Subaru Outback that does it even with the engine off. Turn the key to “On”, set the vents to blow on the windshield, turn the fan speed on any speed except off, and the radiator fan will turn on (and I’m not confusing it with the heater fan. Open the hood and it’s one of the radiator fans spinning). My 1999 Toyota Camry doesn’t do it with the engine off, but does when it’s on.

When I sit with it idling and have the vents pointed at the windshield, you can hear the radiator fan kicking in intermittently as needed. In both cars, with the selector in any other position, the fans don’t run nearly as often. Both cars are the base four-cylinder models with the typical three-knob HVAC layout (both lack automatic climate control). Thanks in advance.

Sajeev answers:

Scott, the answer is simple and complicated. The simple part: the radiator fans kick on because the A/C compressor is running. You need the fan to make the A/C condenser (that radiator thing that’s next to the engine’s radiator) more efficient.

The more complicated part? You need air conditioning when you use the defroster. Dehumidified air quickly removes dew/fog from the windscreen. Think of your HVAC control panel as a Triage department at your local hospital. Cold feet in need of heat? Tough: wait for the heater to get to work. But running the Defroster is an all hands on deck, like a head injury patient coming through the door. This isn’t a hangnail, you have impaired vision and the risk of an accident goes up high. Way high. Which is why a failing HVAC vacuum control unit always defaults to the defrost position.

I don’t know why your Subie runs the fan when the car is OFF and the Camry does not. Perhaps it’s concerned with blowing off the turbo’s extra heat, or perhaps Toyota engineers like wiring everything to the ignition circuit. Not a big deal, just be glad it works when the motor is running.

Bonus! A Piston Slap Nugget of Wisdom

Let’s talk drag racing. No really, you’re gonna love this. Heat is the enemy to any motor on the drag strip, especially turbocharged applications. Cooling down between runs ensures you won’t abuse your ride for diminishing returns. Do yourself a favor in the pits: run the engine with the hood open, set the HVAC to defrost, turn the temperature to full heat and run the dashboard fan at its highest speed.

This does two things: with the A/C running and the fan on, you force cool air through the radiator and out the open hood, lowering engine coolant temperature. And with the heat on, a second radiator comes online: the dashboard’s heater core dissipates heat like the front radiator, doing a great job if the HVAC fan runs at full tilt.

Sound silly? This trick kept my trap speeds level and my temperature gauge down after repeated laps, but don’t take my word for it: hit the drag strip and see for yourself.

(Send your queries to mehta@ttac.com)

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2 of 16 comments
  • Greg Locock Greg Locock on Feb 22, 2010

    Opening the bonnet will change the airflow patterns around the engine. The CFD engineer who signed off on your car's cooling system may raise his eyebrows at your conclusion.

  • Golden2husky Golden2husky on Feb 28, 2010

    In the old days, most Japanese cars had the A/C button that would disable the compressor no matter what setting the rest of the HVAC system was on. Most American/European cars gave you the compressor on Defrost no matter what. I believe most Japanese have adopted that process today.

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