My 2007 Grand Cherokee - 4.7 L, presents the following issue: when I start the engine, the rpm goes to 1200, (as if the engine were cold as first start of the day), ONLY occuring after refueling. I have to wait about 5 minutes to get the normal idle speed of 500 rpm. It happens every time I turn on the engine. When the tank becomes half of its capacity, the failure disapears. I never overfill the tank when refueling. Can you help me?