Since Jim Black of Langley Park traded in his conventional Mercedes Benz two years ago for a diesel-powered model, life has been heaven on wheels.

The expense and frustration of trying to keep his gas guzzler running were just too great for Black. "That car was worse than a bad wife," he says."It was in the garage more than it was on the road.

"This is the finest car I've ever had in my life," Black says of the diesel Mercedes. "We've got 25,000 miles on it and we haven't spent 5 cents on it (for repairs)."

Impassioned testimonials like this are not uncommon from the ever increasing number of economy-minded Americans who are finding a friend in the diesel. And little wonder. Dr. Rudolf Diesel's rugged engine, originally designed for heavy trucks and buses, gets exceptional mileage, needs no costly tuneups and is built to last. And with Detroit just beginning to explore the diesel passenger car market currently occupied by Mercedes, Peugeot and Volkswagen, consumers now have a wider choice of diesel models than ever before.

The problems that go with driving a diesel car - slow starting and acceleration, noise and vibration, unpleasant-smelling exhaust, and the added expense of buying one - don't appear to bother the people who own one.

"It never takes more than 30 seconds to warm up," says one diesel Rabbit owner." And it rides really nice."

Driving a diesel car for the first time may be a little unnerving. In fact, Mercedes people refer to their diesel as a "two-day car" for the time it takes someone to get used to its driving characteristics.

The one factor which probably has steered a lot of people away from diesel is the popular nation that diesel fuel is scarce, that service stations with diesel pumps are few and far between. This doesn't seem to be the case in the Eashington area. With at least 44 diesel fuel outlets in the city and the surrounding suburbs, most diesel drivers in this area experience very little inconvience in terms of finding fuel. (See map on Page K3)

"There's diesel fuel all over the place, particularly if you have a CB," says another Mercedes diesel owner "I've travelled to the middle and the south of the country and it's no problem."

To make things a little easier, Mercedes and VW both publish guides to diesel stations in the U.S., Canada and Mexico. The American Automobile Association and many oil companies have similar directories.

You may still have to drive a little out of your way to fill up but, as one person put it, "If you have to drive four or five miles to get fuel every couple of weeks, it's no big deal. Besides, you have to be pretty dumb to run out of fuel when you're getting 50 miles a gallon."

Diesel fuel is usually easier and cheaper to come by on the highways where the prime customers are trucks. And prices vary from state to state. In Wyoming, for example, the fuel is cheaper than most places, but because there are no state or local taxes at the pump, drivers of diesels must pay a yearly registration fee of $50.

Diesel motorists are discovering that fuel is available but nowhere as cheap as they would like. One angry diesel owner says that the lowest-priced diesel fuel he could find in the area was 64.9 cents a gallon. "I was expecting it to be substantially less. It really should be less," he protests. "In cases where they're offering it at higher prices than regular that's outrageous, it's inexcusable." But despite high prices, he "would still go with a diesel engine."

The fuel which goes into most diesel automobiles is No. 2 diesel fuel oil, a relatively unrefined fuel which is in the same family as No. 2 home heating oil middle distillate oil fuel oils.

The reasons for the comparatively high price of diesel fuel oil are complex. But across-the-board increases in all petroleum products are attributable to one factor the increased cost of crude oil. Inflation also has taken its toll, with increased labor and refining costs.

"It's the old story," says a spokesman for a major oil company. "If you run a slaughter house, you have to kill a cow to get steak. You may have some limited flexibility in getting one fuel or another using one barrel of crude oil. But it becomes academic to say it's less expensive or more expensive (to produce a certain fuel)." As of a couple of years old, a 42-gallon barrel of crude oil would yield about 9.2 gallons of middle distillates.

The middle distillates have been deregulated since July 1, 1976. With no price ceiling and a comparatively light demand, and consequently little competition, diesel wholesalers and retailers can bring the price up. Generally, price increases of No. 2 diesel fuel have kept pace with gasoline price rises. (See chart on Page K1)

Guy Stephanelli, owner of a Texaco station on River Road in Bethesda, believes that the price of diesel at his pump, 67.9 cents a gallon, is too high but his wholesale costs are also high. "I'm the middle man," he says." And I pay the tank wagon (wholesale) price. My tank wagon price for diesel is close to the regular gasoline tank wagon price." But Stephanelli says his demand for the fuel is adequate. "We get quite a few Mercedes. This is one of the better areas for diesel."

Jerrald Bradley, a spokesman for the Gulf Oil Corp., explains that in a decontrolled market such as diesel fuel, it's easier to pass along cost increases to the consumer. "Through the free market, you can pass through all increases if the market would bear them," says Bradley. "The other thing is that diesel is not one of our major markets. Our prices are set by No. 2 home heating oil prices. They piggy-back with home heating oil. What they pay at the pump is controlled by the market, and there's no way to predict which it will do."

Oil company executives are wary of Detroit's sudden interest in diesel. The feeling is that the move to diesel engine production is a temporary solution to the problem of meeting the Environmental Protection Agency's fuel economy requirements, that it's a throwaway engine until something better comes along.

"The oil companies don't really feel we have a hell of a lot to gain from increased sales of diesel automobiles," says one oil company spokesman.

"When GM announced its big diesel plans a few months ago . . . that would probably double the number of auto diesel engines. But even so, it was very insignificant. We're talking about 1990, we're talking about billions of cars, and even if 5 or 10 percent of cars put out in 1990 were diesel, it would be a very small portion."

Another thing which worries the oil industry is the nagging question over diesel fuel emissions. As one spokesman put it: "I don't know if you can look at diesel now and say it's more or less polluting. But later someone is going to focus on diesel fuel's environmental problems."

About 5 percent of all stations nationwide carry diesel fuel. Right now most oil companies refine and sell a lot of diesel fuel, mainly for trucks, trains and buses. Automobiles consume an insignificant amount of diesel.