With summer weather arriving, Metrobus garages are trying to prepare for a rash of "hot engine" breakdowns. Transit buses are complicated pieces of machinery but the explanations for most of the mechanical failures that put them in the shop, like hot engines, are fairly simple.

When a bus is moving through stop-and-go traffic, with a standing load of passengers and air conditioning on full strength, the motor temperature rises. If it reaches 230-240 degrees Fahrenheit, a safeguard device shuts off the engine automatically or a red warning light flashes on the dashboard.

Air conditioners themselves are tempermental machines and contribute to overheating by putting extra strain on the bus' main engine -- some drivers switch air conditioning off when going up long hills as a precaution.

In winter, the big villain is ice in the air lines. Metrobuses' brakes and suspension depend on high-pressure air (the buses rest not on springs but on inflated bellows). Airborne moisture enters the lines, then freezes. Loss of air pressure affects brakes and leaves the bus body resting on the chassis.

"Hot engine" failures can be curbed effectively through preventive measures such as cleaning the engine and radiator. Loss of air pressure can be combated by injecting antifreeze for the ice. When those things are neglected -- and often they are at Metrobus -- buses go down in epidemic numbers.

Other breakdown reports read like a list of ills that plague the car owner: "Won't start"; "Brakes"; "Won't go in gear." Still other problems are lesser ones, such as "no acceleration," that private motorists might simply put up with for a while, but bus drivers, with members of the public aboard, play it safe and call for another vehicle.

The poor showing of the tinted-window RTS buses and the accordion-sectioned articulated buses apparently grows from bad preparation by Metro and the vehicles' design. Mechanics complain the RTS' air conditioner is mounted on its transmission. That exposes the cooling unit to vibration and heat and means that when mechanics work on the transmission -- and that is often -- they must first remove the air conditioner.

The articulated bus has two engines for the mechanics to worry about -- in the rear is a four-cylinder motor that drives the air conditioner. Its radiator draws air from under the bus, sucking in trash that inhibits air flow and creates another "hot engine" problem.

Metro has learned its lesson on the sealed windows of the RTS and articulated buses. It now plans to spend $550,000 installing windows that can be opened so passengers can get some relief when the cooling unit goes down.