According to the Environmental Protection Agency, buyers who choose the Bluetec 4Matic over the gasoline V-6 version of the ML350 will save $450 a year in fuel. That said, Mercedes's diesel S.U.V.'s require periodic fill-ups of another precious fluid. And with dealers marking up this pollution-fighting liquid like champagne on New Year's Eve, refills can cost owners $200 or even $300, wiping out a chunk of the fuel savings.
Like other larger turbodiesels in America, the ML must carry a large onboard tank of urea crystals in mineralized water, in a solution typically branded as AdBlue or DEF, for Diesel Emissions Fluid. The solution is automatically injected into the hot exhaust stream. Fluid vaporizes into carbon dioxide and ammonia, which enters a catalytic converter to turn smog-forming nitrogen oxides, a bane of diesel engines, into harmless nitrogen and water.
But fans of diesel powertrains have complained on Internet forums about high prices for replacing the urea solution. Consumer Reports received an eye-popping $317 dealership bill to refill a Mercedes GL-Class with about 16,500 miles on the odometer. That included $241 for 7.5 gallons of solution, which worked out to $32 a gallon.
Nor can owners simply keep driving their diesel after the juice runs dry: the E.P.A. thought of that, too, and it has required models to shut themselves down after a certain number of starts if owners ignore low-fluid warnings. Each gallon of solution is good for about 1,500 miles, and Mercedes recommends topping off during regular 10,000-mile service visits.
Handy owners, however, can avoid buying steeply marked-up fluid from dealers and refill on their own at much less expense and without labor charges. Such do-it-yourself jobs are more convenient because Mercedes moved the filling spout on the refreshed M-Class to behind the fuel door from the cargo area .
At Silver Star Motors in Long Island City, Queens, a Mercedes service adviser quoted me a price of $7 a quart for the emissions fluid, or $28 a gallon.
But retailers including AutoZone and Napa Auto Parts offer 2.5-gallon jugs of DEF for around $13, barely $5 a gallon, meaning that some dealers are marking up the fluid more than 600 percent over retail prices.
The do-it-yourselfer, then, could fully refill the Bluetec for less than $40. Silver Star's adviser said owners need only remember to top off before the urea tank is nearly empty, which can require resetting the computer sensor that monitors the fluid level and system.
Donna Boland, the corporate communications manager for Mercedes-Benz USA, said in an interview that the dealer servicing also includes a flushing of the system. She added that owners who opt to replace the DEF on their own should follow the product instructions explicitly and be certain that the solution is viable, as it can crystallize if it lingers on a shelf for too long.
BMW, for its part, gives its diesel owners free urea fill-ups under its four-year, 50,000-mile scheduled service plan. Volkswagen covers refills for three years and 36,000 miles. For BMW, the plan would pay for four annual fill-ups. In contrast, a Mercedes dealer's four premium pours would pluck roughly $900 to $1,200 from an owner's pocket. Another tale BMW value.