GM CEO Dan Akerson is being grilled by a Republican House leader over Volt battery concerns.

The nation’s top automotive regulator is denying his department attempted to hide a potentially serious safety problem with the Chevrolet Volt.

Despite waiting months before revealing that one of the plug-in hybrids had caught fire after a May crash test, Transportation Secretary Ray LaHood said his department needed time to see what actually caused the fire in a yard used to hold vehicles after they were tested.

The incident – and a second test-related fire, last month, involving a Volt – have raised concerns about the safety of the Chevy hatchback’s battery pack.  But while the DoT’s automotive arm, the National Highway Traffic Safety Administration, has begun a formal investigation of the Volt, the initial delay has led critics to question if there was a political motive at work.

“Absolutely not true,” countered LaHood when asked about the criticism, which is largely being fueled by Republicans, including California Congressman Darrell Issa.

The contention is that with the U.S. Treasury still owning about a quarter of General Motors’ stock the White House – to whom LaHood reports – was reluctant to criticize the maker.

“We have opened an investigation into battery-related fires that may occur some time after a severe crash,” Countered LaHood, who continued to insist, “Chevy Volt owners can be confident that their cars are safe to drive.”

The formal investigation could lead to a recall of the Volt should a serious defect with its lithium-ion battery pack be found.

Both NHTSA and GM have noted there have so far been no fires involving Volts actually on the road.  The incidents so far reported involve fires that occurred weeks after the vehicles were crash-tested.

Initial indications point to a problem with battery coolant that could slowly change properties after being exposed to the air in a crash that penetrates the Volt’s battery case.  But GM is looking at other possibilities, including the electronics used to control the battery pack.

The maker earlier this year revised its protocol for handling a Volt after a collision that damages the battery pack, requiring that the batteries be “de-energized,” or discharged.

Meanwhile, the maker has indicated it is working on ways to further strengthen the pack and make it even less likely a fire could follow a collision.  A fix, according to published reports, could come within weeks.

But in the politically charged Washington climate the GOP-controlled House is looking to hold its own highly public investigation.  A committee hearing has now been scheduled for next month on the Volt fires.

Cong. Issa has meanwhile asked GM CEO Dan Akerson to disclose when it first discovered a problem with the Volt batteries – and to answer questions about what the lawmaker described as “battery system safety deficiencies.”