The probe affects Ford Escape and Mercury Mariner SUVs and Ford Fusion and Mercury Milan sedans from the 2009 through 2011 model years.
The vehicles can unexpectedly go into "limp home mode" at reduced power, the agency said in documents posted Monday on its website. NHTSA and Ford have received almost 1,500 complaints about the problem. There were three crashes and one injury.
NHTSA began looking into the cars and SUVs after getting a request from the North Carolina Consumers Council last year. Nonprofit safety groups and consumers can petition the agency to investigate vehicle problems.
The cars and SUVs haven't been recalled, but the investigation could lead to a recall.
The Fusion and Milan are nearly identical cars with the same engines, as are the Escape and Mariner. Ford scrapped the Mercury brand in 2010.
The North Carolina organization said it received two complaints about 2009 Escapes that had been diagnosed with throttles either stuck open or closed. The group said that owners reported repeated stalling and surging.
Ford told NHTSA that vehicles made from June 22, 2009, to Oct. 15, 2009, may have faulty printed circuit boards that control the throttles. Ford and its throttle body supplier, Delphi Corp., changed the circuit board manufacturing process after Oct. 15, 2009. That resolved the problem, NHTSA said in documents posted Monday on its website.
NHTSA said it analyzed 123 complaints about the cars or SUVs going into what's known as limp mode, in which vehicle speed was limited to 20 mph or 900 engine revolutions per minute. Drivers may interpret the limp mode as stalling, even though the engines still move the vehicles, the agency said. Vehicles usually go into limp mode when computers detect an engine problem. This allows drivers to get to a safe place while protecting the engine from damage.
Power surges appear to happen when the engine revolutions fluctuate to prevent stalling during limp mode, the agency said.
But even though Ford said the throttle problem with the Escape was resolved with the manufacturing change, NHTSA still decided to investigate vehicles from the 2010 and 2011 model years. The agency says it received 59 complaints of engine stalling in 2010 and 2011 Fusions. Eighty percent of the complaints were received starting in March of 2012, showing an increasing trend, the agency said.
In addition, Ford has received 27,505 warranty claims in which the throttle bodies were repaired or replaced, NHTSA said.
Ford said Monday that it's cooperating with NHTSA on the investigation.
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