The U.S. government has beefed up its investigation into Toyota's Corolla and Matrix vehicles because of a possible engine defect that might cause them to stall.

A 2007 Toyota Corolla sedan and Matrix wagons sit on the lot at a Toyota dealership. The cars are now subject to a more intensive investigation over engine problems that can cause the vehicle to stall. (David Zalubowski/Associated Press)

The National Highway Traffic Safety Administration said in a statement that it was raising its preliminary evaluation to more serious scrutiny, and called for an engineering analysis to look into the problems that could cause the engine to stall, not start or to shift gear harshly.

The investigation into the 2005, 2006 and 2007 model year Corolla and Corolla Matrix cars in the U.S. began Aug. 18, NHTSA said.

Toyota spokeswoman Monika Saito in Tokyo confirmed the NHTSA investigation, and said Toyota was co-operating fully, but declined to give more details.

Toyota is battling to patch up its reputation after recalling some 10 million vehicles around the world since October last year for a range of defects in floor mats, gas pedals and braking software.

A problem with the Corolla, one of Toyota's top-selling models, could be a further embarrassment for the world's top automaker, which long prided itself on impeccable quality controls.

Toyota is studying two possible causes for problems in the engine control units, both production defects, related to improper coating applied to circuit boards and a crack in the surface of a glass coating, according to NHTSA.

It is unclear whether a recall will be necessary.