If my memory serves me right, the first time I took the intake manifold off I had the same problem and it was simply I forgot to reattach the short small vaccum hose that connects the intake manifold to the MAP sensor which is attached to the "sound wall" by the master cylinder. If the vacuum hose is connected properly I'd still check to make sure the hose or the fitting isn't blocked with muck from your cleaning of the intake manifold or alternatively cracked and leaking.