Why is "excessive idling" considered to be "severe duty"?

Dear Car Talk

Dear Car Talk | Apr 01, 2011

Dear Tom and Ray:

Why in the world is "excessive idling" defined as "severe duty" in all owner's manuals? I read in one column that idling is harder on the valve train than running at red-line is! What's the explanation? Lower oil pressure? Hotter oil? I'm crossing my fingers for an answer, as I've asked this question many times of many different people, and I still don't have any idea. -- Frank


TOM: Well, I happen to be an excessive idler, Frank. And my ex-wives will all tell you that having me around was definitely "severe duty."

RAY: "Severe duty," in automotive terms, is a collection of conditions listed in most owner's manuals. If you meet those conditions, it's recommended that you change your oil more frequently than everyone else.

TOM: In the old days, excessive idling was listed probably because cars ran so rich. Carburetors dumped buckets of gasoline into the cylinders. Some of it was combusted, and some of it wasn't. The gasoline that didn't combust just ran down the walls of the cylinders, and some of it snuck past the rings into the oil pan.

RAY: That diluted the oil with gasoline. And since diluted oil doesn't lubricate as well as pure oil, it makes sense to change it more often.

TOM: Nowadays, cars rarely run rich. But the reason they recommend more-frequent oil changes for excessive idlers -- like police cars and taxis -- is because it means your engine is running for more hours than your mileage indicates.

RAY: In other words, if you idle in front of the senior center for five hours a day trying to pick up chicks, like my brother does, you'll have added zero miles to your odometer, yet your oil will have been working to lubricate your engine for five hours.

TOM: And if you wait until 7,500 miles go by before changing the oil, your oil may be overdue for a change by then.

RAY: This is one reason why manufacturers like GM and Honda now have oil-change indicators on the dashboard. They measure the amount of time the engine runs and the conditions under which it runs, and then use an algorithm to figure out how much life is left in your oil.

TOM: It's a better system than using mileage alone. Not only do you not wait too long between oil changes, but you don't change the oil too OFTEN, either. So you save money and room in our landfills.

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