If you have some brake cleaner, while the car is running spray brake cleaner along the intake manifold connection to the head and if it changes the idle the gasket is bad. I have seen it way too many times thankfully I myself have only had to do one of them and hope to God I never have to do another. I looked up the book hours on the job and it quotes a bit over 6 hours. Not fun at all. I would have rather pulled the motor to do the job. No joke.
And yes it can cause an evap code because your vac would be very low and the evap system wont function right.