Symptom #1: The unexplained loss of coolant from your degas (overflow) bottle or cooling system. This can easily be explained when you have a bad EGR cooler - the EGR cooler is simply leaking the coolant back into the exhaust system, not outside of the engine. Most people make an expensive mistake of ignoring this warning sign. They assume that since they can not find or see the leak, it must not actually be a leak. The longer they ignore it, the more coolant that leaks out of the engine and quickly creates warning sign #2.
Symptom #2: All that white smoke coming out of the tailpipe is actually steam. As the coolant is dumped into the exhaust system, it quickly vaporizes and turns from liquid to steam and comes out of the exhaust.
Symptom #3: That "puking" of coolant out of your degas bottle is actually not a sign, necessarily, of a bad EGR cooler. Its more likely a sign that your engine oil cooler is plugged and is restricting the flow of coolant getting to your EGR cooler. (Click here for more information about this) The EGR cooler is not getting enough coolant flowing through, so the coolant that is inside it quickly begins to boil. When the coolant boils, it creates high-pressure steam pockets which billow out through your degas bottle, pushing and forcing coolant out of the lid. The EGR cooler should be replaced in this instance because its already been overheated and stressed - and very likely to fail in the near future.
How to Diagnose a Bad EGR Cooler: This method is easy to do and is a good "test" if you have any of the three symptoms above. To do this test, turn off your rig. When its all cooled down, remove the EGR valve (between your alternator and oil filter - it looks like a 7-ounce beer can with wires going to it) and look inside with a flash light. If it looks wet, gooey or steam cleaned, then you probably have EGR cooler problems.