SAN ANTONIO – Count an all-new turbocharged diesel engine for light-duty pickups among the potential casualties of an exhaustive product review under way at General Motors Corp., but the auto maker still plans to maintain a robust engine lineup in the segment even as buyers continue to flee the market.

John Schwegman, product group marketing manager-Chevrolet fullsize and midsize trucks, says long-standing plans to drop an all-new 4.5L V-8 Duramax turbodiesel into ’10 Chevrolet Silverado and GMC Sierra pickups fall into the review initiated by GM’s plea for taxpayer loans.

GM outlines in its recent viability report to the U.S. Treasury an intention to base its future product plans on “few, better entries,” specifically reducing nameplates from 48 this year to 36 by 2012.

“Everything is under review,” Schwegman tells Ward’s during a media preview here this week for the new Silverado/Sierra light-duty hybrid. “We’re going to make sure that when we reduce the number of entries that every entry is right for the market, has the right price (and) value for the customer, and we can make a profit on every entry.

“So it’s under review like the rest of the portfolio,” he says.

GM spent 3.5 years developing the unique turbodiesel and planned to begin production at its Tonawanda, NY, plant later this year. It also had hoped to include the compact, lightweight engine in future cross/utility vehicles, SUVs and even some cars.

Unlike conventional diesels, there’s a reverse flow of air and exhaust gasses entering and exiting the cylinder heads. Entry air flows through ports in the top of each cylinder head. The exhaust gases exit inward between the cylinder heads directly into the turbocharger.

The design would have been a market-first, but that distinction now may fall to a new 4.4L twin-turbo V-8 diesel BMW AG plans to offer with its X6 CUV and upcoming 750Li large sedan.

If GM axes the light-duty diesel, the engine will join a growing list of ill-fated oil burners.

Chrysler LLC recently killed plans to introduce a Jeep Grand Cherokee SUV featuring Bluetec clean-diesel technology from Daimler AG’s Mercedes-Benz car division. Toyota Motor Sales U.S.A. Inc. has put a diesel version of its Tundra fullsize pickup on hold at least until the market rebounds.

On-again, off-again plans by Honda Motor Co. Ltd. to bring a diesel-equipped Acura TSX midsize sedan to the U.S. next year are off again until 2010 or later.

In addition to BMW, Volkswagen AG and Mercedes are sticking with plans to offer diesel engines in upcoming cars and trucks.

But weak sales play only part of the role in GM’s decision to place the light-duty diesel under review. Schwegman says it’s awfully difficult to determine where diesel-fuel prices are headed. When the 4.5L Duramax was on paper, diesel-fuel cost was on par or less than gasoline, he says.

The average gallon of diesel on Wednesday stood at $2.24, or $0.36 more than gasoline, according to the American Automobile Assn. Last year, diesel’s premium over gasoline reached upwards of $0.70.

“So we have to look at that value equation and ask, ‘Is the market going to be the same as we projected even a year ago?’ That is going to come into consideration when deciding the timing on that diesel,” Schwegman says.

At the same time, GM remains committed to enhancing the 6.6L V-8 turbodiesel for heavy-duty versions of the Silverado/Sierra.

GM previously announced it would add a selective catalytic-reduction aftertreatment system to curb oxides of nitrogen emissions. Combined with a diesel-particulate filter, the engine will meet the Tier 2 Bin 5 and LEV 2 emissions standards coming in 2010. GM says the engine will be compliant in all 50 states.

“We’ll make what is already a clean-burning diesel even cleaner,” he says. “We haven’t released specifics behind that, but we will be compliant in January of next year.”

Schwegman also says GM will keep its engine lineup in the light-duty pickup market at five choices, despite a shrinking segment as personal-use buyers flee for more fuel-efficient vehicles.

According to Ward’s data, sales of large light-duty pickups fell to 1.6 million units last year, from 2.2 million in 2007 and upwards of 2.5 million as recently as 2005.

GM presently offers a 4.3L V-6 and V-8s displacing 4.8L, 5.3L, 6.0L and 6.2L in its pickups. Only the 5.3L V-8 is available across the model lineup.

“Near-term, we don’t see any deletions to the portfolio,” Schwegman says, looking out over the next 12 months. But, he warns, “the market has contracted, so if there is future complexity and engineering spend that isn’t justified on a powertrain, we’ll look very hard at that. Because with the resources we have, we have to make those dollars work hard.”

Schwegman speculates GM may have to decide between offering both a 6.0L and 6.2L V-8. The 6.2L arrived most recently to GM’s truck lineup.