The Cash for Clunkers program is running out of gas, a victim of its own success.

Designed to stimulate the economy by spurring new car sales and aid the environment by taking gas-guzzling vehicles off the highway, the innovative federal program has nearly burned through its $1 billion appropriation in less than two weeks.

Customers have swamped dealerships, sending their old gas gulpers -- 18 miles-per-gallon average or less -- to the crusher in exchange for instant rebates of up to $4,500 toward the purchase of a new, more fuel-efficient car or truck. A 10 mpg-or-more spread nets buyers $4,500 while a 4-to-9 mpg improvement merits a $3,500 voucher. More than 157,000 transactions had been finalized through early Tuesday, with an average fuel economy gain of 9.6 mpg per trade, according to preliminary government reports.

Now, it's up to Congress to refill the tank and keep the program running. The House did its part in a 316-109 vote Friday, pumping $2 billion from the federal economic stimulus package into the program.

But the effort could encounter a road block in the Senate amid claims that the program itself is a clunker. Some fiscal conservatives, Utah Sen. Orrin Hatch included, say the government should concentrate on restoring consumer confidence, not propping up a single industry with rebates.

While it's true that car dealers, manufacturers and buyers benefit most from Cash for Clunkers, a sustained effort to spur auto sales could help lift the entire economy. Plus, a more fuel-efficient fleet aids us all by curbing climate change, reducing air pollution and lessening our dependence on foreign oil.

But the Clunkers program could certainly use a tune-up.

In some cases the fuel efficiency gain -- a 4 mpg improvement nets $3,500 -- is not substantial enough to warrant such a large rebate. And the 18 mpg maximum for the clunker should be lifted, with rebates based on the net fuel economy improvement without an arbitrary starting point.

Also, there needs to be a component to allow low-income owners of gas guzzlers to take part by extending smaller trade-in rebates for the purchase of fuel-efficient used cars. While it might not benefit automobile manufacturers, it would still be a boon for the environment.

But for now, with the House in recess and the Senate preparing for its summer break, time is of the essence if Clunkers is to maintain its momentum. The Senate needs to provide some emergency roadside assistance to keep the program running, then fine-tune it further down the road.