The on-again, off-again story of the super-sexy Cadillac Converj is on again.

So says John Voelcker of Green Car Reports, who cites an “inside source, a person close to the project” saying the plug-in hybrid — essentially a tarted-up Chevrolet Volt — is a go.

We’ve heard this a couple of times since General Motors unveiled the Cadillac Converj at the Detroit auto show two years ago. No less than an authority than then-GM vice chairman and vociferous Volt proponent Bob Lutz said early last year that the brass had approved it. Three months later, however, came word that GM had shelved the project.

It appears to be back. Voelcker’s source says management has approved production for 2013 as a 2014 model. It will feature “a Generation 1.5 Voltec drivetrain” with a little more oomph than the drivetrain underpinning the Chevrolet Volt.

We asked Cadillac spokesman Dave Caldwell about this. As usual, he essentially said, “Maybe.”

“We’ve said all along that we have enthusiasm for this idea… and luckily others do too,” he told us. “Our product plans are things we keep internal, and frankly, they are subject to change as is the case in any business.

“It’s not as if Converj has crossed some sort of milestone – so, no we cannot confirm that it’s certain to reach production,” Caldwell said. “But we can say that it’s a concept we remain interested in.”

There are compelling reasons for GM to pursue this project. First, and perhaps most importantly, it will help recoup the massive investment in the Volt. By spreading the technology among other models, GM can bring down costs. This important, because one of the Volt’s biggest drawbacks is its $40K price tag. Even at that price, General Motors is almost certainly losing money. It would be far more profitable to pack that drivetrain into a premium coupe.

The Volt has proven more popular than even the brass expected — GM is ramping up production sooner than planned — so offering what is essentially an upscale Volt could bring in new customers. And it would give Cadillac a much-needed green(er) car because, frankly, the Escalade Hybrid is a joke.

The new federal fuel efficiency standards — which will require corporate fleets to average 54.5 mpg by 2025 — provide another incentive for cars like the Converj.

Yes, Cadillac’s got the XTS Platinum Hybrid concept and we keep hearing rumors of a plug-in hybrid SRX, but so far there’s no word either project has been approved. Which brings us back to the Covnerj. GM’s already got the drivetrain. It could use a variation on the Cadillac CTS-V coupe for the body. The rest is just details.

We’ve said it before. We’ll say it again: Build it.

Photos: General Motors