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They may be "junk", but so were the "diesels" GM was using because they converted gasoline engines for diesel use instead of building diesels. There's plent of gasoline to be had for some time 9if we are allowed to get it) and it makes it impractical to shift to something else at the moment. Gasoline produces more energy than NG, but building an engine specifically designed to leverage NG might change the equation some.

The main problem was with the head bolts since they were torqued on to gasoline engine standards, the diesels have higher compression. There were other sundry problems too but once the bugs were ironed out, the GM diesel was a good engine. I think much like the Volt, it was rushed out before it was ready although in the case of the Volt, IMHO, the latter is a dead end alley.