Dear Tom & Ray: Can this be true, or are my wife and I just gullible? Our dealer claims that we need to add "urea" to our BlueTec 350 Mercedes engine. With 21,000 miles on the car, we have spent almost $200 with the dealer to add this "urea." Why? What is its purpose? Are we being taken for the ultimate luxury-car ride, or is this a legitimate cost? -- Dan TOM: Doesn't urine have urea in it? RAY: Yes, but only at about 1/10th of the concentration the car requires. So you can't recommend that Dan simply pee into the tank. TOM: OK, I'm crossing that off my list, then. RAY: Urea refills are a legitimate expense for this particular car, Dan. And shame on the salesman for not disclosing this to you when you bought the car. It's an added cost and inconvenience of ownership that you had a right to know about. It's like selling someone a dog, and neglecting to mention that you have to feed it every day. TOM: Here's why you have to add urea to this car: In order to get diesel engines -- which burn dirtier than gasoline engines -- to meet new emissions standards in the United States, Mercedes (like all diesel-engine manufacturers) had to add some kind of system to scrub more nitrous oxides out of the exhaust. RAY: The system Mercedes chose is a urea-based system. When the urea is injected into the hot exhaust, it releases ammonia, which helps break down about 80 percent of the harmful nitrous oxides. TOM: The problem is, you need a constant supply of urea. In order to keep car owners from simply letting the urea bottle run dry and forgetting about it, the EPA required that the car stop running if it runs out of urea. So you have no choice, Dan. RAY: Mercedes says that a 7.5-gallon supply should cover about 12,000 miles of driving. So it suggests that you get the urea tank refilled every 10,000 miles, when you bring your Mercedes in for its regular oil, filter and wallet service. TOM: Aside from the potential inconvenience of that, Mercedes estimates the expense at about $1,500 for every 100,000 miles you drive. RAY: Of course, like everything else you do at a Mercedes dealership, you'll pay more to get your urea topped up there. So if the expense is a concern, you might look around for an independent shop that specializes in German cars. You'll pay less of a premium there. TOM: You can buy your own urea (under the Mercedes brand name "AdBlue," or as another brand) and learn how to add it yourself, if you really want to cut your expenses. RAY: Isn't it a good thing you're saving all that money with your high-mileage diesel, Dan? Otherwise, how would you be able to afford all that urea? Which is cheaper, buying or leasing? Should you keep a car forever or dump it after three years, before trouble starts? Find out in Tom and Ray's pamphlet "Should I Buy, Lease, or Steal My Next Car?" Send $4.75 (check or money order) to Next Car, P.O. Box 536475, Orlando, FL 32853-6475. Get more Click and Clack in their new book, "Ask Click and Clack: Answers from Car Talk." Got a question about cars? Write to Click and Clack in care of this newspaper, or email them by visiting the Car Talk website at www.cartalk.com. (c) 2012 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.