DETROIT -- A 4˝-month pause in production of the Duramax diesel engine threatens supplies of profitable Chevrolet Silverado and GMC Sierra diesel pickups.

Not only is GM's pickup inventory lean, but analysts also predict that sales will start to recover while the Duramax engine plant is down.

GM is scheduled to stop building the current version of the Duramax in mid-December. Production of a re-engineered 2010 Duramax is scheduled to restart in late April.

During the pause, GM will retool the Moraine, Ohio, plant where the engine is built, and GM engineers will tune the 6.6-liter turbocharged diesel V-8 to meet tough new emissions rules that take effect Jan. 1.

The revised engine will add a system to inject urea into the exhaust periodically. The chemical reduces oxides of nitrogen or NOx. The pickups will add low-fluid alerts to prompt drivers to refill the urea tank.

Some dealers worry they may run out of diesel-powered versions of the Chevrolet Silverado and GMC Sierra because of the gap in production.

“We got a notice yesterday that starting immediately, we can no longer order the Duramax regular cab,” said Scott Brasher, general manager of Brasher Motor Co. in Weimar, Texas, near Houston.

Brasher still can order extended-cab diesels, and he expects to place his last order for them next month. He also has been buying diesel trucks from other dealers to pad his inventory.

GM Powertrain spokesman Tom Read said GM plans to build inventory of Duramax engines this fall. The Duramax for the 2009 model year can't be built after Dec. 31, but engines that already have been built can be installed in vehicles after that time.

Ford Motor Co. is also launching a new diesel truck engine next year. But spokesman Anne Marie Gattari said Ford dealers will not run out of diesel-powered Super Duty trucks.

Gattari said Ford will stop production of the 2010 model year F-series Super Duty in December and then switch to a new engine and revamped truck in the first quarter. Ford's re-engineered diesel pickup also uses urea injection to reduce harmful emissions.

As of Sept. 1, GM had a 60-day supply of Silverados and a 71-day supply of Sierras. GM won't say how many of those are diesel-powered.

“We've been communicating with dealers, and we feel very comfortable we can meet their needs,” said GMC spokeswoman Dayna Hart.

Chevrolet spokesman Brian Goebel said GM will build up diesel pickup inventory for the rest of the year. The company also uses the Duramax in chassis-cab models and large vans.

Brasher said many more buyers are taking a second look at the gasoline-powered version of the heavy-duty Silverado because of the high cost of the diesel engine and the higher cost of diesel fuel.

“We're having much better luck with the gasoline engine,” he said. “Right now sales are running 60 percent diesel, 40 percent gas. But three years ago, it was 90 percent diesel and 10 percent gas.”

GM is spending $70 million on the Duramax plant.

Urea or carbamide is an organic compound with the chemical formula (NH2)2CO. The molecule has two amine (-NH2) residues joined by a carbonyl (-CO-) functional group.

Urea plays an important role in the metabolism of nitrogen-containing compounds by animals, and is the main nitrogen-containing substance in the urine of mammals. Being solid, colourless, odorless, neither acidic nor basic, highly soluble in water, and relatively non-toxic, urea is widely used in fertilizers as a convenient source of nitrogen. Urea is also an important feedstock for the chemical industry. The synthesis of this organic compound by Friedrich Wöhler 1828 from an inorganic precursor was an important milestone in the development of chemistry.