Is running the engine to purge extra dirty oil while changing it a good idea?

Dear Car Talk

Dear Car Talk | Dec 01, 2003

Dear Tom and Ray:

I do most of the light maintenance on my vehicle, and I've added a step in my oil changes that I think is beneficial in the long run. Here's what I do: I drain the warm, dirty oil, just like everybody else. Then I add this step: With the drain plug still out, I start the engine and let it idle for about a minute. I usually manage to get an extra cup or so of oil purged from the oil pump and related plumbing. My question is: Am I doing more harm than good? I change the oil every 5,000 miles, and I figure the more dirty oil I can remove when changing the oil, the better. -- Ken

TOM: You want the good news first, Ken? You are significantly cutting down on the number of future oil changes you'll have to do on this car.

RAY: The bad news is that it's because you're going to croak the engine. This is a terrible idea, Ken.

TOM: You might have noticed that while you're running the engine to extract that last cup of oil, the oil light on your dashboard is on. You also might have noticed that there's a warning buzzer or chime that's sounding. Those are what we call "idiot lights," Ken. And they're speaking directly to you, buddy.

RAY: The light and buzzer come on when the oil pressure is so low that serious damage to the engine is probably occurring. When they come on, you're supposed to turn off the engine immediately to limit the damage.

TOM: Here's why: The key components of the engine are made of metal. And when the engine is on, they're spinning, or moving up and down hundreds of times a minute, even at idle. When they're moving, they're rubbing against other pieces of metal. To prevent these things from destroying each other, there's an elaborate system that forces pressurized oil between all of these moving parts whenever the engine is running. But that oil-pressure system can only work if there's A) oil, and B) pressure. You have neither.

RAY: Without oil, even for a minute, those parts scrape against each other and wear down. Then the damaged parts no longer fit together perfectly. Those gaps between parts make it harder to keep the oil pressure up, and it's a downhill spiral from there.

TOM: So it's absolutely not worth it, Ken. Leave that extra cup of dirty oil in there. It's irrelevant, especially when diluted with 4 quarts of brand-new oil.

RAY: And just cross your fingers that you haven't done too much damage yet. You can't -- in good conscience -- sell this car to anybody else. So you're just going to have to hang on to it and drive it into the ground. But don't worry, Ken. It won't be a long drive.

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