Code 505 is "idle control system malfunction" which raises more questions than it answers. Are you sure that's the only code your're getting? This is presumably from the OBDII port under the dash. Have you tried reading the codes from the 38 pin connector in the engine compartment? We need Arthur or Doc here - can he not read the OBDI codes with a flash code reader on the 38 pin on a dual system (OBDI and II) car like this?