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Mercedes-Benz has been hard at work cleaning up its diesel act after restricting U.S. sales of its oil-burning sedans three years ago because of concerns over exhaust emissions.

The German luxury carmaker recently returned its larger diesel sedans to all showrooms except those in California after revising the engine "prechambers" to lower exhaust particulates.It hopes to be in all 50 states within 18 months, as it re-engineers its diesel engines to meet the new Califonria standards.

New for 1991, and driven for this review, is the 350SD Turbo, one of three diesel-powered Mercedes sedans that include the mid-size 300D 2.5 ($41,000) and long-wheelbase 350SDL Turbo ($57,800).

Based on Mercedes' standard-length S-class sedan, the 350SD Turbo cost $54,800 as tested, including a $350 delivery charge. Two options were added to its already full complement of features: automatic locking differential ($1,050) and passenger-side air bag ($630).

While that is admittedly expensive, it actually puts it in the middle of Mercedes' price range, which now extends past $97,000 when gas guzzler and luxury taxes are added.

(Mercedes will break into the six-figure price range this August when it introduces its first completely redesigned S-class sedans in 11 years.)

For 1991, both the 350SD and 350SDL S-class sedans are powered by a 3.5 liter inline six-cylinder engine, the largest diesel motor ever offered by Mercedes. While it generates only 134 horsepower at 4,000 rpm, its torque - the more critical measure in a diesel - is a hefty 229 pound-feet at 2,000 rpm, the same as Mercedes' 4.2 liter V8 engine musters at 3,600 rpm.

Fuel mileage is 22 city/25 mpg highway, quite respectable for a fully trimmed 3,805-pound sedan with a four-speed automatic transmission. One can travel nearly 600 miles between fillups of its 23.8-gallon tank, vs. a maximum cruising range of 476 miles for its gasoline-powered counterpart.

Just about every nuisance associated with diesel car ownership has been engineered out of Mercedes' latest offerings, which go a long way in clearing the air about oil-burning cars.

Drivers no longer need to shiver behind the wheel on cold days waiting for the glow plugs to warm up before getting under way. The 350SD was ready to fire up in less than 10 seconds, even on the coldest Michigan morning.

Driver ergonomics and passenger comfort are excellent. Workmanship and materials were found to be impeccable as expected for a car costing this much.

Mercedes hopes these new models will attract repeat buyers from its base of 400,000 diesel car owners in the United States.

Only 5.2 percent of all Mercedes cars sold in the United States last year were diesel-powered, down sharply from 79 percent in 1982. Mercedes blames the falloff on less availability of its models and easing concerns about the availability and price of gasoline.