Iowa's GOP Sen. Chuck Grassley wants NHTSA to take a closer look at Toyota's problems with Unintended Acceleration.

Did Toyota get off too easily after it was largely cleared by a pair of investigations looking into the maker’s problems with so-called Unintended Acceleration?

That’s a possibility being raised by Sen. Chuck Grassley, an Iowa Republican, who has sent a letter to the National Highway Traffic Safety Administration, or NHTSA, questioning the results of two studies conducted for the agency, in particular one done by NASA that found no evidence of electronic gremlins that might cause Toyota vehicles to race out of control.

“Key questions about the cause of unintended acceleration remain unanswered,” the veteran lawmaker wrote in a letter to NHTSA Director David Strickland.

Those questions have been supported by a raft of internal corporate documents also in the hands of TheDetroitBureau.com that suggest Toyota may, in fact, itself be worried that there are still yet unknown issues with some of its products.

Both the NASA study and a second one by the National Academies Science failed to substantiate claims by Toyota critics that the maker’s electronic engine control systems might be the result of Unintended Acceleration – also known as Sudden Acceleration – where a vehicle might surge out of control, possibly leading to a collision, injury or even death.

Toyota became enmeshed in a safety scandal when, in October 2009, a California Highway Patrol Officer and three family members were killed when a Lexus they were borrowing raced out of control, skidded off a freeway, crashed and burned.

The maker subsequently recalled 8 million vehicles in a pair of recalls, one targeting so-called “carpet entrapment,” where loose floor mats could jam the accelerator pedal, and another related to potentially sticker accelerator assemblies.

Late last month, Toyota extended the carpet entrapment recall to include nearly 134,000 additional vehicles, 2010 Lexus RX350 and RX450h crossovers. (For more on that story, Click Here.)

Despite the number of vehicles involved in those two recalls – which forced Toyota to briefly shut down some of its main assembly plants while waiting for the necessary replacement parts – critics of the maker insisted it had not addressed the major problem.  But experts, including some on the NASA panel, also warned that it could prove near-impossible to track electronic gremlins that might only occur sporadically and then vanish without a trace.

Some pointed to a phenomenon known as “tin whiskers,” in which nano-sized threads have been known to grow at key junction points on electronic circuit boards.  Such a possibility was raised repeatedly in the 177-page NASA report.

“This is a serious issue,” Grassley wrote in his letter to NHTSA’s Strickland, asking how often and how far it has been pursued.

Traditionally known for its seeming bulletproof reliability, Toyota took some unexpected shots as a result of the 2009-10 crisis – which saw key executives including CEO Akio Toyoda grilled by Congress and led to record fines for failing to respond to the sticky accelerator problem in a timely manner, as required by law.

The safety agency took some punishment of its own, however as a result of the Toyota scandal. Internal corporate documents showed that NHTSA went easy on Toyota by allowing it to avoid an earlier recall that saved the carmaker millions of dollars.  That was a key reason Strickland agreed to empower two independent studies of Toyota’s Unintended Acceleration problems.

In the end, both NASA and the NAS attributed the vast majority of reported runaway car incidents – those not clearly linked to carpet entrapment or sticky accelerators – to driver error. In many cases that was backed up by evidence showing that, in one instance, a motorist mistakenly pressed down so hard on the gas pedal, rather than the brake, that it bent.

But the studies left open a slight window suggesting there could yet be issues unknown impacting Toyota’s electronics.

A spokesperson for NHTSA said the agency has received the Senator’s letter and “will review it carefully and respond appropriately.”

Meanwhile, the various legal actions that have faced Toyota by alleged victims of Unintended Acceleration continue to work their way through the courts.