We have a 2000 Airstream 350XC built on a freightliner chassis with a 3126B Caterpillar engine. We bought it this past summer and it's the first diesel I have ever owned. I discovered that it had pure water in the generator and the engine and it was supposed to get downin the 20s that night so I drained them both. The generator was easy. I just followed the manual, it was clear. The chassis manual was not so clear about the coolant replacement. It mentions an "engine venting petcock" and says that air must be vented from the engine to prevent damage. I cannot find this petcock on the engine. The only info I can find on the internet says to fill the coolant system and run the engine. But I'm not sure they are talking about the same engine! Anyone got any knowledge of this procedure?