Bloodvenom

So my car started doing a lunging movment for about a week and felt like i had some cyclinders down or misfiring. So I had my car's CEL read plus it checked out by a friend of mine and we noticed that the intake manifold was cracked and leaking down into the a few of the cylinders.

After completely changing the intake manifold, new spark plugs, boots, etc, the car is idling extremely low, yet when i gas it normally its in perfect working order.

Do any of you know why my car would start doing this after the changes? At first I thought it was maybe my sensors and cleaned them off a bit and drove it around to kind of settle my car, or at least get a CEL, but neither has happened.

Any idea's?

Any help is appreciated.

The car is a 2000 GT 4.6L