The milky residue you see on the oil cap is a common symptom of a bad PVC system.
PCV stands for Positive Crankcase Ventilation.
All car engines have a PCV valve and it is located on top of the motor's valve cover. There is a vacuum line attached to it. It pulls out of the valve cover and if you shake it, you should hear a marble knocking inside it. If you shake it and don't hear anything, it's bad. It costs less than 5 bucks to replace.
· 4.0L is located in the valve cover near the brake booster
· 5.0L is located behind the intake manifold (engine rear)
Removal & Installation
1. Disconnect the oil separator hose from the PVC Valve.
2. Remove the PVC Valve from the grommet in the crankcase.
Grasp the valve and gently remove it from the grommet in the passengers side valve cover. Remove the valve from the hose by carefully twisting it out.
3. Inspect the grommet for deterioration.
4. If necessary, replace the crankcase grommet.
5. Install the PVC Valve.
6. Install the hose.