Why is my car revving to 4000 RPMs in neutral?

My car started idling very high a week ago. When it starts and is sitting in neutral, it goes to about 4000 rpms. When I put it in first, it kicks down to around 2500. Every time I shift gears, it revs back up to 4000, when the clutch is depressed, then back to 2500 when in gear. I can basically drive it down the road without using the gas pedal at all! Hard on the clutch and brakes. A friend took the vacuum line off the map sensor and the idle kicked down temporarily. Not sure what that means though. It's back up to 4000 now. It's leaking coolant a bit from the water pump so I'm wondering if that has anything to do with it. The mechanic can't figure it out and said he can't plug a diagnostic tool in.