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The CCV is "the hockey puck" on top of the valve/cam cover. The o-ring can croak from heat and leak nasty black oil all over that area of the valve cover. Also, there's a drain-down hole in the valve cover below the hockey puck that can plug up and cause more leakage past the o-ring.
A hose comes off of the hockey puck that goes to the bottom of the big air intake hose that runs between the air filter box and turbo charger. The big air intake hose tends to rot out on the bottom due to oily air coming from the hockey puck. That lets extra unfiltered air in that alters the airbox sensor readings.
Some owners run the small hose coming from the CCV-valve/hockey puck down below the engine to atmosphere and plug the hole in the air intake hose. This is known as "the elephant hose mod" and it's not a good idea for two reasons. In cold climates the end of the hose can ice up and blow the rear main engine seal, and, it pollutes a lot.
The best solution is to add a Provent air/oil seperator to the hose coming from the hockey puck and keep the air intake hose from rotting out. Sasquatch Parts makes a nice kit to do that and they also have a rot proof silicon air intake hose.
ID Parts is a good source for 2.8 CRD stuff like the CCV-valve/hockey-puck.