Checking the oil correctly may interfere with someone's romantic plans.

Dear Car Talk

Dear Car Talk | Oct 01, 1993

Dear Tom and Ray:

I teach at a major university about 200 miles from my home. During the work week my home away from home is in an apartment complex inhabited primarily by university students. Through my window I recently watched a student-age fellow check the crankcase oil level in one of the cars in the parking lot, aided by a sweet young thing -- His wife? His girlfriend? I don't know. His car? Her car? I don't know. One of us needs to enroll in Oil Checking, Course #101.

I have always thought, and I have taught my two children (both of whom are several years older than either the sweet young thing or the student-age fellow!), one should wait as long as possible after stopping the car's engine before checking the oil. Mr. Student-age Fellow had Miss Sweet Young Thing start the engine and let it run for a few seconds, revving it up a bit for good measure. Just as soon as she had turned off the ignition he pulled the dip stick, wiped it off with a hand full of grass, stuck it back into the crankcase, pulled it out again and looked at it. He added some oil. He repeated the entire process at least four times, except the last time he didn't add any more oil. Either the young student-age fellow or the old university teacher is misinformed. Will you tell us which it is? I sure wanted to educate him right then and there, but I didn't want to denigrate him in the eyes of the sweet young thing!

RAY: It's a good thing you didn't interfere with young love, Tom. I suspect that student-age fellow is intentionally overfilling the crankcase with oil. He's HOPING to break down in the middle of nowhere with Miss Sweet Young Thing.

TOM: But of course you're you're right about the oil, Tom. When you insert the dip stick, you're measuring the amount of oil sitting at the bottom of the engine in the crankcase.

RAY: And when you start the car, much of that oil gets pumped up to the top of the engine. Some of it stays up there until you turn the engine off. Then, it slowly seeps back down. But if you "check the oil" too soon after shutting it off, some of the oil will still be at the top of the engine. So you'll get an inaccurate reading, and you'll think you need more oil.

TOM: And if you dump more oil in based on that reading (like Student-age Fellow did) you'll put more oil in the crankcase than you're supposed to. And overfilling it can be just as bad for the engine as running low on oil.

RAY: Generally, if you wait four or five minutes after turning off the engine, enough oil will settle for you to get an accurate reading. But you'll get the MOST accurate reading when the engine is cold (like first thing in the morning).

TOM: By the way, Tom, I guess you have tenure, huh? How else would you have so much time to spend staring out your window at sweet young things?

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