1998 Cadillac Eldorado, Throttle Body:
When I start the car it revs up to 4000rpm's and keeps going up to higher rpm's, it will not go down. I disconnected the cruise control and it made no difference. Then I checked the butterfly, it was opening and closing, it wasn't stuck. I took off the top plate on the injectors and checked all the vaccuum lines and had no leaks. I put the car in gear and the brakes held, so the vaccuum booster was all right. Still it will not go to an idle.
Can you tell me what might be happening? The car ran fine the day before.

. Clean or replace the Idle Air Control Valve (IACV)--I suspect it's the real culprit. If you suspect the IACV is sticking, tap on it with a small mallot and see if the idle speed changes. The "stepper motor" inside the IACV regulates air flow, during idle. If cleaning the IACV doesn't solve the problem, replace the unit ($80 at AutoZone). Be sure to clean all ports from the IACV to the throttle body.

. Clean the throttle plate and throttle body with spray cleaner.

. Clean the air box and install a new air filter.

. Check for vacuum leaks.

. Examine hoses and Fast Idle Thermo Valve (ensure valve seat is tight), if you have one.

. Ensure the cooling system is full of coolant and properly purged of air.

Check where the air inlet tube attaches the MAF and air filter to the Intake/Throttle body.
If that comes loose or otherwise didn't get installed securely, that can allow unmetered air to enter and cause an elevated idle condition.
Have you checked for Diagnostic Trouble Codes (DTCs)? Is the Check Engine Light (CEL) on yet?