What is the valve on cover of a 1999 Jeep Grand Cherokee?
That's likely the PCV valve (Positive Crankcase Ventilation valve). It's a very common location for the PCV valve on many vehicles, including the 1999 Jeep Grand Cherokee. However, there might be other small valves or sensors on the valve cover as well, depending on the engine. To be absolutely certain, you'd need to consult a repair manual specific to your Grand Cherokee's engine (e.g., 4.0L, 4.7L, 5.9L).