On May 7, thermometers registered a moderately warm 86 degrees and Metrobus riders got a taste of things to come this summer.

More than 170 buses -- 1 in 10 of Metro's peak-hour fleet -- conked out on the streets, at least 66 due to overheated engines, the others suffering failures ranging from jammed transmissions to a smoking front wheel. One bus gave out three separate times, twice with an overheated engine, once with door problems.

Hot temperatures put Metrobuses out of action in unusual numbers, but even in fair weather the fleet's performance on the streets has been deteriorating steadily in recent years.

About 240,000 people ride buses on weekdays -- 90,000 more than use the rail system. For them, maintenance failures have meant increasingly common breakdowns, delays and steamy rides with broken air conditioners. Metro officials say there is no quick remedy in sight.

"I think that it's going to get worse in the next year or two," said General Manager Richard S. Page. "But I also think that we have the foundation to make it better."

Metro's own figures give evidence of the deterioration: In January 1978, for example, Metrobuses averaged 3,200 miles between breakdowns serious enough to cut short a trip with passengers aboard. In March this year, the figure had dwindled to 1,875 miles. In January 1978, 7.4 percent of the fleet was listed as crippled. In March this year, 12.1 percent was crippled.

There is no easy answer to why the bus system, which employs 3,600 people and will cost about $200 million to run this year, is running downhill this way. Conversations with managers, mechanics and union officials turn up the following explanations:

* Top management at Metro traditionally has favored rail in money, attention and prestige.

* In the rush to get daily quotas of buses onto the streets, mechanical problems are often fixed only when they cause a breakdown.

* The fleet and eight bus garages are getting older.

* The newest Metrobuses -- the sleek, tinted-window General Motors RTS-2s and the articulated buses made by the German company M.A.N. -- are less reliable than the older buses.

* The average bus runs more miles and needs more preventive work but the Metro board has not approved money to expand the maintenance force.

* Garage stockrooms are chronically short of spare parts, and parts quality is lower.

Absenteeism and union work rules keep productivity low, and many mechanics are poorly qualified and motivated, according to Metro officials.

Some of these problems were inherited from the four bus companies Metro took over in 1973. Some are the result of federal rules. Some are Metro's own doing. They are found in each garage in a different mix, creating vast contrasts in service quality, with buses from the inner-city garages in D.C. breaking down four times more often than Virginia buses.

To combat its problems over the short term, Metro has taken 35 steps ranging from rebuilding engines to special training programs. Its hopes for longer-term improvement rest on plans to rebuild 600 old buses and open four new garages.

The obstacle is that the important steps will require big money at a time when Metro and its prime source of capital funds, the federal government, are strapped financially. They also will require major changes in the time-honored ways things are done on the garage floors.

In the garages there is little optimism for the short term. Jerry Wolford, a mechanic at the Four Mile Run garage in Arlington, said: "We really do a pretty good job with what we've got." But without more mechanics "you're going to have buses running hot, buses breaking down," he said as he changed a radiator.

Page maintains that despite its faults, Metrobus has made important improvements, such as building 526 passenger shelters and air-conditioning the entire fleet. He further says Metrobus completes 99.2 percent of scheduled trips and service is as good as or better than service in other U.S. cities.

Ranking bus systems statistically is difficult because there is no standard yardstick of performance. Certainly many other cities are experiencing maintenance problems, too. New York buses, for example, traveled 510 miles between any type of breakdown in calendar 1981; Baltimore buses averaged 1,721 miles for the 12-month period ending April 30.

Page points out that Metro was founded in 1967 to build a rail system, but six years later Congress gave it an unforeseen job, absorption of the ailing D.C. Transit bus company and three smaller private lines in the suburbs. Metro officials acknowledge that some of today's problems have arisen because Metrobus has been neglected for Metrorail.

For example, nine years after the takeover, Metro is just now ready to open a new garage, the spacious Montgomery Division facility off Rockville Pike. "It just amazes me that this organization could build 44 rail stations in the time that we've taken to build one bus garage," said Page.

This neglect has meant that many mechanics continue to work in cramped, drafty garages with leaky roofs, clogged plumbing and inch-thick carpets of petroleum muck underfoot. Parts of Northern Division, a former streetcar garage, date to 1906.

In the meantime, bus service continues, seven days a week, 24 hours a day. In the hours after midnight on weekday mornings, garages gear up to get their scheduled quotas or "lines" of buses -- now about 1,500 areawide -- onto the streets for the rush-hour schedule.

Over the years, Metro often has favored the garage chief who successfully makes his line over the one who holds a few vehicles back to fix an oil leak before it gets worse or replace an ailing transmission before it shakes to pieces while the bus is hauling passengers.

From this, a short-sighted, firefighting approach to maintenance evolved. Buses leave on hot days with their radiators clogged and engine oil low. Later, they break down, forcing mechanics to interrupt preventive work on other buses and go after the crippled ones, guaranteeing more breakdowns the next day.

Buses now are passing through Metro's heavy-duty repair shop on Bladensburg Road at the rate of 900 a year, which alarms managers because it means the entire fleet is coming through every two years. A visit there on a recent day suggested that basic preventive work had been neglected. Engines were coated with grit, battery compartments full of corrosion, bodies dented and battered.

Metrobus chief Tom Black, who took his job 18 months ago, said he is trying to break this long-established cycle, stress maintenance and ensure that buses get their 5,000-mile inspections on schedule. Black reports progress on this front. When he began his job, 70 percent of the fleet was overdue, he said; the figure now is 10 percent.

Old habits die hard and not every problem found is fixed. One mechanic working beneath a bus at Northern garage said matter-of-factly: "They send a lot of junk out on the streets." In a recent week, that garage was operating eight buses that would not go into high gear and 14 with no reverse. Even then, it still was two or three buses short on its 280-bus rush-hour line on some days.

Even if maintenance improves, mechanics will be working on buses that are getting older and need more upkeep. Most Metrobuses were purchased between the early 1960s and the late 1970s. In 1979, Metro got its streamlined RTS-2s to update the fleet. That same year, it also got the articulated buses in an attempt to reduce driver payrolls by running larger-capacity vehicles.

Both new bus types have been plagued by breakdowns, apparently due to design faults and poor maintenance. Garage chiefs have grown so sour on the RTS-2s that they are politicking to unload them on the Montgomery Division. The Metro board is soured, too, and now plans to concentrate on rebuilding the older, more reliable buses.

About 600 General Motors buses dating to the 1960s will be stripped and rebuilt at a cost of $60,000 each. But there are two catches: The plan depends almost entirely on federal money at a time when the White House is trying to reduce spending, and, even if funded, will take five years to complete.

Similarly, hopes to modernize the garages hinge on federal grants. Plans call for three more new garages after Montgomery opens, one each in Fairfax County, Landover and Southwest D.C. Some existing garages would undergo major renovation, with the the whole program expending $180 million over five years, assuming the money is there.

Even as the buses are aging, they are traveling more miles per day. Over the past four years, miles run have held roughly steady in the vicinity of 52 million per year, while the Metro board has lowered the number of buses operating those miles from about 1,825 to 1,730, including emergency spares.

Buses that travel farther need more maintenance, Black argues. But the mechanics' staffing ratio has held at one maintenance employe per 2.65 buses, while some mechanics have moved to new jobs -- training, keeping up wheelchair lifts and articulated buses, for instance. The result is that about 90 mechanics, or roughly 12 percent of the force, have been diverted from line work in recent years.

For the fiscal year beginning in July, Page proposed hiring more mechanics and cleaners and putting more spare buses on the streets to fill gaps left by rush-hour breakdowns. The Metro board, seeking to control the growth in subsidy bills, granted some of his proposals but rejected many as too expensive.

Charles Boswell, president of Amalgamated Transit Union Local 689, which represents most Metro drivers and mechanics, agrees that staffing levels are a key to Metro's maintenance troubles. "The equipment is pretty complex these days," he said. " . . . They management know they're short-handed."

Some mechanics argue that regardless of how many hands are hired, many buses would remain crippled because stockrooms are chronically short of parts ranging from replacement transmissions to brake pads and light bulbs. The cause appears to be a bureaucratic bottleneck of accountants' forms, interdepartmental rivalries and last-minute ordering.

Metro is trying to streamline parts procurement but mechanics continue to complain about it. A recent staff meeting was told that lack of replacement transmissions had led 18 Northern Division buses to break down on the streets 51 times over a 19-day period.

There also are suggestions that in recent years, the quality of parts has gone down. "We used to get 100,000 miles between transmission changes; now we get 30,000," Black said.

Metro management also charges that mechanic absenteeism is another major reason why buses stay broken. On an average day, between 10 and 13 percent of the maintenance staff is absent on sick leave, worker's compensation, personal business and other forms of leave that supervisors feel are abused. Metro is trying to crack down on abuse of leave time.

Boswell of the Amalgamated Transit Union said attendance could be better but that parts and staffing shortages are the real problem. "They've got some awfully good employes here," he said.

The current work rules hamper productivity, Metro officials say. For instance, because the shops function on seniority, employes move from garage to garage or job to job based on their years on the job, not where their skills are most needed.

Another issue is certification. Mechanics now are promoted on the basis of written tests. Metro wants rigorous certification, for which a candidate would be given a disabled bus and be told to fix it. One supervisor at the Bladensburg bus garage complains: "You've got mechanics getting paid $12.53 an hour who can't tune an engine."

Those are union-management issues, but perhaps Metro's biggest complaint with labor is the contract's quarterly cost-of-living raise, inherited from D.C. Transit and remaining nine years later, board members say, due to federal rules designed to protect labor. Top-ranking mechanics now make a base pay of more than $26,000 a year. With wage scales like that, the board feels it is hard to increase the work force.

The three garages in Virginia -- Four Mile Run, Arlington and Royal Street in Alexandria -- have the best service record, managing in April an average 6,269 miles between road calls, breakdowns serious enough to disrupt service. Page said that service provided by these garages -- he rides a bus in from Virginia daily -- is among the best in the country.

Virginia's garages are larger, less crowded, more airy and generally located in more desirable neighborhoods. For these reasons, mechanics and operators tend to move there as they gain seniority. One mechanic with experience at Northern said moving to Four Mile Run was "like dying and going to heaven." In D.C., in contrast, garages are old and grimy and the majority are in poorer neighborhoods.

Another major factor favoring the Virginia garages' maintenance performance is that they run buses on less punishing schedules, with more miles on open highways and less off-peak service. That allows vehicles to return to the garage for a rest at midday.

In D.C., schedules are heavier because people are more bus-dependent. A bus often leaves before sunrise and stays on the streets until dark. The driver spends much of that time inching through crowded downtown streets. "He's making a stop every block with a standing load of people and there's bound to be a lot of wear and tear," says one Metrobus official.

The worst road-call performance in April was registered at Northern (1,358 miles), Bladensburg, where the troublesome articulated buses are based (1,401 miles) and Southeastern (1,454 miles).

Metro's two other divisions ranked in the middle, with Western on upper Wisconsin Avenue averaging 2,475 miles and Prince George's on Southern Avenue, 3,566.

Metro is concentrating efforts at the low-ranking garages. "We're headed in the right direction," said Northern's chief, Sherman Ramey. But "I don't anticipate any dramatic changes overnight." Neither does Tom Black. "It looks like a long summer," he said. "And it'll be compounded if we get adverse [unusually hot] weather like we did in 1980."