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New cars are able to tell when it’s time for an oil change, but is it really accurate?

Click & Clack Ray Magliozzi
Dear Car Talk:
My Chrysler 200 is new enough to keep me informed of how much oil life remains. Can I trust it? It’s been 6,000 (easy) miles and 15 months since my last oil change (synthetic), and my car is saying that 25 percent of the oil’s life is left. I’m inclined to believe it since I think that automakers are overly conservative regarding oil change intervals. Should I change the oil when the car says 5 percent left? 10 percent? Or do you recommend a mileage or time interval? — Jeff
I’d trust it, Jeff. If we just do an “order of magnitude” check, synthetic oil can easily last 7,500 to 10,000 miles before needing to be changed. So, if you’ve gone 6,000 miles and have 25 percent left, you’re on track for an oil change at 8,000 miles.
That’s right on target.
In case you’re interested, the oil life monitor in your car is not actually “testing” your oil. The oil life monitor is measuring the conditions that affect the life of your oil. It plugs them into an algorithm and constantly produces an estimate of how much longer your oil should last. From the car’s computer, it collects information on things like the number of starts (individual trips), the engine temperature variations (driving conditions) and the number of miles you drive.
Through the years, engineers have created algorithms that are pretty accurate in predicting when the oil is spent. Remember, they have incentive to make sure you change your oil on time. If they’re wrong, and you’re under warranty, they could owe you an engine.
I’d say when you get down to 10 percent, it’s time to make an appointment for an oil change. It’s not an emergency at that point. Your oil is still fine.
But it’s like getting down to an eighth of a tank of gas: You want to know where a gas station is at that point.
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Hair-dryer heater idea may have been ahead of its time

Click & Clack Ray Magliozzi
Dear Car Talk:
I taught high school auto mechanics for over 30 years. In the 1980s, when I was explaining how the resistors in the fan blower worked, I compared it to a hair dryer. That gave me an idea. Why not add a hair dryer-type of heating element in the car’s ductwork for instant heat to the passenger compartment? It could reduce or turn off as engine heat increased until it was no longer needed.
I didn’t pursue this idea because batteries back then were overworked beyond their capacity, and heating elements draw a lot of power from the battery.
Fast-forward almost 30 years to today, and you have remote starting devices that are installed for hefty prices. These wear out engines, pollute, use gasoline and reduce gas mileage and have too many parts that can fail.
Today you have heated seats, heated rearview mirrors and mega sound systems guzzling up power from the batteries, so they can’t claim the battery can’t handle it.
I would guess that installing my system in the air duct would cost less than $20 at the factory and cost little to operate. Can it be done? — Raymond
Anything can be done, Raymond, but that doesn’t mean it should be done.
A typical hair dryer uses about 1,200 watts. And realistically, you’d need at least a pair of them, which would be about 200 amps at 12 volts. That’s a pretty significant load on the battery.
It wouldn’t kill the battery in 10 or 15 minutes, but if your battery were old or marginal, and it was a cold day (which it obviously would be), you could weaken it to the point that you’d have trouble starting the car. And wouldn’t that stink?
Of course, once the car is running, you could pull 200 amps from the alternator and the battery combined, and that wouldn’t be a problem. So it could be a way to get some heat right away, once you start the car — but before the engine is producing usable heat.
The larger issue is that heating the air is the least efficient way to keep the driver warm.
Take a typical sedan. Say it has about 20 cubic feet of interior volume. You’re taking up about 4 of those cubic feet, and yet you’re wasting a ton of energy heating up the other 16 cubic feet to 70 degrees.
In contrast, radiant heating (seat heaters, steering wheel heaters, rear window defroster) use far less power and deliver the heat precisely where it’s needed: to your tuchus and key surrounding areas.
But here’s the good news, Raymond: Your idea actually makes more sense for electric cars, which are getting more popular every day.
Here’s why: Electric cars don’t have internal combustion engines, which give off heat, and they already use electric heating elements to warm the cabin.
On a cold day, with a simple remote control, you could run the heating element while the car is still plugged into its charger. That would preheat the cabin without eating into the car’s battery reserve and driving range.
It’s a great idea, Raymond. You were just 35 years ahead of your time and working on the wrong propulsion technology.
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