Just call him Mr. Goodwrench.

That's what President Obama is telling American car owners and buyers, making them an offer they can't refuse: If General Motors or Chrysler won't honor their warranties, he will.

Playing pitchman for the ailing U.S. auto industry, Obama on Monday offered guarantees on the warranty of every new vehicle sold by the Detroit automakers during their restructuring efforts. Obama said with additional incentives for new car buyers, the industry could sell an additional 100,000 new cars this year. 

But Uncle Sam won't stand by warranties held by current vehicle owners or safety recalls, which can occur years after the warranty expires. That would leave uncovered about 10 million to 15 million GM and Chrysler vehicles bought in the past couple years, said Clarence Ditlow, executive director of the Center for Auto Safety. 

Under the so-called Warranty Commitment Program, an accounting reserve must be established that contains 125 percent of the projected warranty costs for each new vehicle sold by GM and Chrysler.

Automakers will contribute 15 percent of those costs from their own funds while the U.S. government will cover the remaining 110 percent from the Troubled Asset Relief Program, the Treasury Department Web site says.

A separate company will be established to manage the funds and will continue to pay warranty claims even in the worst case scenario: automakers go into bankruptcy or out of business. 

If that happened, the warranty administrator and the U.S. government will search for a third-party warranty service provider to assume responsibility for all the warranties covered by the program in exchange for the assets of the program.

"If you buy a car from Chrysler or General Motors, you will be able to get your car serviced and repaired, just like always," Obama pledged Monday. "Your warranty will be safe. In fact, it will be safer than it's ever been. Because starting to day, the United States government will stand behind your warranty."

Ditlow pointed out that plenty of legitimate claims are denied by warranty companies and the program offers no details about how the government would honor claims. 

He added that among the biggest unknowns are the ability to find third-party warranty service providers to perform the work and ensuring a continual flow of parts if the automakers go into bankruptcy or out of business.

"It's a good idea but it's not a proven plan to date," he told FOXNews.com. Obama "has the right concept but there's no guarantee in that warranty plan."

Ditlow said he received a number of complaints from Daewoo owners when that South Korean car company went bankrupt and GM bought some of its assets but none of its liabilities.

"For vehicle owners, if GM or Chrysler goes into bankruptcy, get ready to lobby to have your debt recognized," he said. "And that debt is your warranty."

The White House nor the Treasury Department responded to requests seeking comment.

But the National Automobile Dealers Association said it also isn't impressed by Obama's plan.

"We are encouraged by President Obama's commitment to putting the domestic auto industry back on the road to recovery but continue to urge the administration to focus immediately on correcting the dysfunctional credit markets," the organization said in a statement, adding that bankruptcy should not be an option.

"It would further erode consumer confidence and, therefore, our ability to sell at the retail level. Moreover, it would further exacerbate the availability of credit."