Thanks again for all the replies. I called Wagner Equipment in Denver, and Chris in the tech department told me the following. The 3116 has the altitude limit sticker to let you know what altitude the rack and injectors are set to operate at. At higher than max altitude the engine will be overfueled for the amount of available air in the atmosphere. He said that while you might not notice a big power difference operating at high altitude, there will be unburned fuel in the engine that could run down the sides of the cylinders into the oil and could damage components that are contacted by engine oil. He said there is also the risk of preignition with all that excess fuel sitting around which could cause piston corrosion. In addition, less air in the atmosphere means less cooling to the engine, and the engine will run hot and have hot exhaust. He said if a truck were to live at high altitude that the engine rack and injectors should be reset for the appropriate altitude, essentially cutting the fuel back to the engine. While he could not give me a quote for the job, he said it would take about an hour for his people to get to the necessary area, then another hour or so to set the rack and injectors. He said that, in all, it would be a 3-4 hour job, and if this has been done on a truck it would not necessarily be indicated by any kind of modification sticker on the engine. He said those things are documented in a report at CAT, but if there is no such report, they could use a tool to determine what altitude the truck is set to run at.