The National Highway Traffic Safety Administration said Monday that the steel straps holding up the truck's gas tank can rust and break, possibly causing a fuel spill and fire. No injuries have been reported from the possible defect.
The agency is looking into trucks from the 1997 through 2001 model years. NHTSA's investigations often lead to recalls.
NHTSA began looking at the fuel tank problem last year after it received 32 complaints. NHTSA and Ford have now received 243 reports of the tanks falling. In two incidents, a fire started when the fuel tank fell. One of the fires destroyed the pickup, while the other extinguished itself.
NHTSA said there was fuel leakage in 95 of the reported incidents. Nine reported sparks when the tank dropped on the road, which would increase the likelihood of a fire.
Ford said it's cooperating with the investigation. Company spokesman Wes Sherwood said owners with questions should contact their dealers.
The investigation isn't related to an air bag defect that caused the recent recall of 1.2 million F-150s from the 2004-2006 model years.
The F-Series has been the best-selling vehicle in the U.S. for more than three decades. It's been the subject of recent sniping between NHTSA and Ford because of air bags that could deploy at the wrong time. Ford announced in February that it would fix 150,000 F-150s for that problem, but under pressure from regulators, it expanded the recall to 1.2 million trucks last month. In that case, there were 98 reported injuries because of the problem.