I'm having an argument with my boyfriend about when to...

Dear Car Talk

Dear Car Talk | Jul 01, 2002

Dear Tom and Ray:

I'm having an argument with my boyfriend about when to check the oil in my car. He thinks it should be done when the engine is cold. I think it should be done when the engine is warm but has been sitting a few minutes to let the oil settle a little. Who is right, and why? -- Holly

TOM: That's an excellent question, Holly. And it turns out that you're both right. We've always told people to check the oil level first thing in the morning, when the engine is cold.

RAY: There were several reasons for this. First of all, when the engine is cold, all of the oil has had a chance to drip down from the top of the engine and settle in the oil pan, where it's measured by the dipstick. That way, you're getting a true and accurate reading of exactly how much oil resides in the engine.

TOM: Second, when the engine is cold, you don't have to worry about scalding your fingertips.

RAY: And third, when you check the oil first thing in the morning, you have the added convenience of being able to use your highly absorbent pajamas to wipe off the dipstick.

TOM: But then we noticed that sometime in the past decade, Ford Motor Company and General Motors started recommending that people check their oil when the engine is warm.

TOM: "Warm?!" we said. "How can this be?" So a few years ago, we called Ford, and they told us that very few idiots like us were going out first thing in the morning, in their bare tootsies, and checking the oil anymore. Most people, they said, tend to check their oil when they stop for gas, when the engine is warm. So these companies simply recalibrated their dipsticks to read correctly in a warm engine, before every last drop of oil has had a chance to drain down into the oil pan.

RAY: "But won't that result in an inaccurate and greatly varied reading, depending on how long you wait after turning off the engine? How can this possibly work?" we wondered.

TOM: So we went down to the Car Talk Plaza garage and pulled the dipsticks on the two cars we happened to be test driving: a Chevy TrailBlazer and a Volvo S60.

RAY: With the engines cold, both read "full." We ran them both for a minute to circulate the oil, then shut them both off and checked the dipsticks again every minute for five minutes.

TOM: Within a minute, the Volvo dipstick read "full" again. So in that case, it clearly doesn't matter when you check it. Do it whenever it's convenient.

RAY: But after five minutes, the dipstick on the TrailBlazer still indicated that the engine was half a quart low, meaning that some of the oil was still at the top of the engine. Did this mean the engine WAS half a quart low? Or is the owner's manual giving lousy advice?

TOM: We don't know. So here's our advice: If you're anal-retentive about things, you should do this experiment yourself and see how your car performs. If there's very little or no difference between the cold and warm readings after letting it sit for a few minutes, then you can check it whenever you want to.

RAY: And if the results differ, like they did on the Trailblazer? Fortunately, it still doesn't matter a lot. Even if you were to add an extra half a quart when you didn't need it, nothing terrible would happen to the engine.

TOM: But in cases where there is a difference between hot and cold readings, we'd have to suggest that you follow the instructions in the owner's manual. First of all, the manufacturer presumably knows exactly what situation the dipstick was calibrated for. And more importantly, if the company does screw up in the manual, it'll be paying the warranty costs.

RAY: And in most cases these days, the manual will tell you to check the oil at the gas station, after the car has been shut off for a few minutes. And that seems reasonable to us.

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