I have a Dodge which has an electric cooling fan...

Dear Car Talk

Dear Car Talk | Nov 01, 1994

Dear Tom and Ray:

I have a 1983 Dodge 400 which has an electric cooling fan that is thermostatically controlled. The cooling fan stays on from eight to ten minutes after the motor is shut off. This seemed to shorten the life of the battery, so I took the car to a Dodge garage and had them fix it so the fan only works when the key is in the ignition. Will this have any affect on the engine? I don't understand why this is necessary in the car?

RAY: Great question, Bill. This is one of those simple systems in the car that nobody understands.

TOM: Yeah. I've been hoping you would explain it to me someday.

RAY: Here's how it works. When you shut off the engine, the coolant actually gets HOTTER before it gets cooler. That's because when you shut off the water pump, you stop the coolant from circulating. And all the heat from that huge mass of iron we call the engine gets absorbed by the coolant.

RAY: And that heat causes the coolant to boil, and some of it spills out of the engine and into the overflow bottle. When the engine cools down, the coolant will get sucked back into the engine, and you're ready to drive again.

TOM: But if you try to drive away BEFORE the engine has cooled down--like if you run into the store for a minute to grab a roast beef slurpy--when you come out, a lot of your coolant will be in the overflow bottle, and not enough of it will be in the engine. So you could overheat the engine and blow a head gasket or even crack a head.

RAY: And that's exactly what the cooling fan prevents. When it stays on for that extra eight to ten minutes, it cools the coolant in the radiator enough to keep it from boiling over, so it stays in the radiator where it belongs.

TOM: So by defeating the thermostatic control, you may get an extra week or two out of your battery, but you may eventually do serious damage to your engine. So I'd hook everything back up, Bill.

RAY: And by the way, how do find all that time to hang around your car listening to the cooling fan? Are you retired or are you a college professor?

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