WASHINGTON, D.C. --- The National Highway Traffic Safety Administration has opened a preliminary investigation to learn whether Chinese-made valve stems on up to 1 million 2007 model Ford vehicles may pose a safety risk by cracking and leaking air.

NHTSA is looking at TR431, TR414 and TR418 snap-in stems on 1.05 million 2007 Ford vehicles. They are the Grand Marquis, F-150, Mustang, Edge, Fusion, Expedition, Explorer, MKX, MKZ, Milan, Focus and Escape.

The NHTSA website said the agency has received 37 complaints alleging that snap-in tire valves cracked and leaked air resulting in a flat tire on 2007 Ford vehicles. Eleven of those complaints alleged the damage required the tire to be replaced.

Back in June, Tech International, a Johnstown, Ohio-based distributor of the Chinese-made parts, recalled 6 million TR413 model valve stems and offered free replacements. Tech International also offered to pay for any tire damage caused by the defective part. The valves are manufactured by Shanghai Baolong Automotive Corp., a subsidiary of Topseal Auto Parts.

Ford Motor Co. spokesman Wes Sherwood told the Detroit News that Ford uses tire stems made by Shanghai Baolong as standard equipment on most new vehicles. But he added that the stems produced for Ford were made in a different part of the plant and with different materials than the ones recalled by Tech International.

"We do not think this is a safety issue," Sherwood said. He said that Ford has received complaints in connection with the stems, but none alleging injuries.

Senior Ford safety officials met with senior NHTSA officials in Washington to discuss the tire-stem issue on Sept. 10, the Detroit News reported.

Sherwood told the Detroit News that if owners of 2007 model Ford vehicles have concerns about their valve stems they can contact their dealer.

In May, NHTSA began investigating 23.5 million Shanghia Baolong-made TR400 series tire valve stems distributed by North Carolina-based Dill Air Control Products. Dill officials told NHTSA that valves made from July 2006 to mid-November 2006 may leak from cracks caused by exposure to air by a chemical. The issue has been linked to a "change in suppliers of a chemical used to provide ozone protection," NHTSA said.

Those valve stems may have come out of the same Chinese factory that made the recalled valve stems distributed by Tech International.

On Sept. 24, NHTSA upgraded its investigation into an engineering analysis. A total of 4,767 complaints have been filed about the stems. Another 23,000 cracked tire stems were discovered during a tire inspection program. Dill told NHTSA in July that it had replaced stems on 130,000 vehicles and recovered nearly 900,000 unused recalled stems. "We expect tire dealers to replace another 250,000 in 12 months," Dill said.