My 1993 cadillac deville feels as if the rpm's are running high (idle seems too high) the service engine soon light ocassionally comes on and I get the error code E30. Anybody have any ideas??

The error is indicating that the Throttle Position Sensor (TPS) is out of the normal range with respect to the Idle Speed Control (ISC). Try something really simple and inexpensive first by removing the air ducting between the air filter box and the throttle body. Clean the throttle body interior and throttle plate with a cloth sprayed with WD40. Turn the throttle plate by pushing it at the top with your fingers and clean the throttle throat behind the plate as best you can. Reassemble the ducting and ensure that there are no air leaks at the junctions and no splits. Turn the key to position 2 (engine not running) and depress the accelerator pedal to the floor and then back up again. Start the car and nine times out of ten that should fix it. The TPS measures the opening angle of the throttle plate. The TPS is device consisting of a circular carbon track with a slider, corresponding to the throttle plate, sweeping along it. It as three wires to it, two to provide the 0 to 5 volt range along the length of the carbon track and the third to measure the voltage at the contact point of the slider. To check the TPS measure the voltage of the middle pin of the connector when the ignition is at position 2 (engine off) as you push the throttle plate open and closed by pressing on the accelerator pedal. The voltage should alter in a smooth continuous way. Any drop outs of signal indicate a worn carbon track and the TPS should be replaced.