In your columns on the vices virtues of allowing a...

Dear Car Talk

Dear Car Talk | Nov 01, 1994

Dear Tom and Ray:

In your columns on the vices/virtues of allowing a car to warm up on a cold morning, I don't recall you discussing the use of an engine pre-heater. With the cold winters where I live in Montana, many cars use an immersion heater that replaces a coolant freeze plug to pre-warm the engine. Set on a timer, these plug-ins take a lot of stress out of cold starts with only one or two hours of heating. In the (usually!) warmer parts of the country, I would think they'd perform even more effectively than in the -20F of a Montana mornin'!

TOM: You're absolutely right, Glenn. But we want to make it perfectly clear that we are very much in favor of pre-heating an engine. What we are NOT in favor of, and what we have always warned against, is warming it up excessively AFTER it has been started.

RAY: Right. The greatest amount of wear and tear on an engine occurs right when it's first started. That's when the oil is cold and thick, and hasn't splashed around to all the places it's supposed to protect.

TOM: What the pre-heater (also called a "block heater") does is give the oil a head start. In the hour or two before you start the car in the morning, it warms up the engine coolant, which in turn warms up the oil. That allows not only easier starting, but also allows the warmer, thinner oil to get where it needs to go more quickly. This means there's less wear and tear on your engine, and that's good.

RAY: The other good thing about block heaters these days it that they're pretty inexpensive. In fact, we've seen them offered as an option on new cars for less than $50.

TOM: Now, if warming up the car BEFORE you start it is good, why is warming it up AFTER you start it bad? Well, once you've started it cold, the damage is already done. What you want to avoid is making it worse. And one way people make it worse is by revving the engine as soon as it starts. As soon as it turns over, they step on the gas...VVVRRRRRMMMM... VVVRRMMMMMM. That's the worst thing you can do, because you're putting a lot of strain on the engine at the very time it's least prepared to handle it. So that's definitely something to avoid.

RAY: Other folks think it's a good idea to just sit in the driveway with the motor running for five minutes before driving the car. That's also a bad idea. It wastes gas, adds to the pollution, and doesn't do the engine any good. The engine warms up most efficiently while it's being driven, not while its idling.

TOM: So here's our recommended procedure: If you have a block heater, that's great. But regardless, start the car and just drive away. Unless it's colder than 10 or 15 degrees out, just driving gently for the first couple of minutes is the best way to warm up your engine. If it's bitter cold out, or the car stalls when you put it in Drive, then you can warm it up for a minute or so. But warming it up any longer than that is a waste of time, a waste of gasoline, and probably an affront to your still-sleeping neighbors.

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