Hello and Welcome! I'm Billy and i will assist you with your inquiry. Feel free to ask if any further info is needed.
At idle, and since you have the oil vapor hose from the valve cover to the intake manifold, you will get suction from the oil cap.
And when the oil cap is removed, excess air gets to the intake manifold (without being counted by the MAF), so you have a lean mixture, causing the erratic idle.
In short, yes this is normal.
Thank you for your trust! Get back to me if you need more info...Thank you in advance for the positive rating...http://www.justanswer.com/car/expert-billy/