Washington's current heat spell has taken its toll on Metro buses, triggering more engine shutdowns during the first few weeks this summer than for the same period last year, Metro officials said yesterday.

Metro has average more than 100 shutdowns per day in the last week alone, when the temperature hit 90 degrees or higher each day -- the longest hot spell so far this season. The shutdowns peaked at 156 on Monday, causing more than 200 uncompleted bus trips and numerous passenger delays.

Metro's diesel engine buses automatically shut down to prevent overheating and internal damage when the engines reach a certain temperature, said Phillip Price, director of maintenance for the Metro buses. Usually, the passengers must get off and board another bus, while one of Metro's seven specially trained mechanics comes to the stricken bus and resets the engine after it has cooled off.

Metro officials said they do not have total figures on the numbers of buses that failed to complete trips during the heat wave but said, for example, that 149 shutdowns on July 9 caused a loss of 299 scheduled trips.

Altogether, there were 1,362 shutdowns in the first 13 days of July this year compared with 1,319 for the entire month of July last year, Metro officials said.

In some cases, Metro sends a replacement bus to pick up passengers from a shut down bus. In other cases, the passengers simply wait for the next scheduled bus to arrive.

Bus officials also blamed high street temperatures, stalled traffic and multiple starts and stops by buses on short runs downtown for the large number of shutdowns.

"If it's 90 degrees outside, that means it is 95 to 110 degrees on the street," said Price.

Inside the buses, conditions are hardly better. Yesterday morning, for example, 14 percent of Metro's 1,541 buses traveled without functioning air conditioners.

When a bus sits still or pauses frequently in downtown traffic, the stops and starts can also cause the engine to overheat, Metro officials said. Buses traveled through the more open Maryland and Virginia suburbs do not have the same problems.

Officials said the engine shutdown fail-safe system, which costs about $8,000 to $10,000 per bus, cannot stop overheating altogether but prevents damage to the engine.

A spokesman for the American Public Transit Association said that summer heat poses problems for transit systems nationwide. Regularly practiced preventive measures, such as cleaning radiators with detergent solution could keep bus breakdowns at a low rate, he said.

"Like all maintenance, if it's not paid for now, you can pay for it later," said Frank Sihak, director of technical and research services.

The National Weather Service says relief is on the way to the Washington area for both passengers and buses. Below-normal temperatures with daily highs in the upper 70s and low 80s should prevail through saturday.

Yesterday's official high temperature of 92 degrees at National Airport may have marked the end of 90 degree-plus temperatures in the Washington area for the rest of this week. A spokesman for the Weather Service said late yesterday that a cold front is spreading through northern Maryland that should bring cooler temperatures and lower humidity today.

Elsewhere, a severe heat wave continued to grip portions of the Midwest with temperatures in the 90s and low 100s from Oklahoma to South Dakota. At least five heat-related deaths were reported in the Des Moines area of Iowa, and violent thunderstorms caused flooding in North Dakota.