i would look at junk yard for an intake valve spring surely one has a cracked head for a 3.8 as for the throttle position sensor it sounds like it is shot.i've never done it but i have heard of an air plug you screw in the spark plug hole and put air to it and it keeps the valve up while you change valve seals so i guess you could do this to change the spring. but if you can't find a junkyard that has a spring then a new one can't be that much.but i would also repace the throttle position sensor.