With the exception of the following, the 1990 Seville shared the changes made in the Eldorado for 1990. The Seville was offered in four new colors, for a total of 16. The four colors were light auburn, dark auburn, medium slate gray and dark slate gray. New side body reflexes were used. The corner turn signal lamps were relocated from the body to the bumper. A new white crystalline tail lamp lens was used.
Interior developments consisted of two new Sierra grain leather color choices, slate gray and dark auburn, for a total of nine. The Seville's seat style was revamped to provide for envelope-type seat back pockets and molded side panels.
The Seville touring sedan was now built entirely at Detroit/Hamtramck. It had new front and rear body-color low fascias with a black bead and a body-color rocker molding, also with a black bead. The rear view appearance was ukpdated by use of a larger cloisonne STS deck lid emblem and dual outlet exhaust. Larger 16 in. diamond- cut-finish, forged aluminum wheels with larger Goodyear Eagle GT+4 P215/60R16 tires were also introduced for 1990. The STS for 1990 also had a new 6KY69 body style code.
| Type | 90-degree, overhead valve, V-8, aluminum block and cast iron cylinder liners, cast iron cylinder heads |
| Displacement | 273 cu. in. (4.5 liters) |
| Bore & Stroke | 3.62 in. x 3.31 in. |
| Compression ratio | 9.5:1 |
| Brake horsepower | 180 @ 4300 rpm |
| Torque | 240 lb.-ft. @ 3000 rpm |
| Main bearings | Five |
| Valve lifters | Roller hydraulic |
| Fuel supply | Sequential port fuel injection |
| Transmission | THM 440-T4 automatic four-speed (includes overdrive and viscous converter clutch) |
| Steering | Power-assisted rack and pinion |
| Steering Ratio | 16.5:1 (touring: 15.6:1) |
| Front suspension | Independent MacPherson strut with coil springs, strut-type shock absorbers and stabilizer bar |
| Rear suspension | Fully independent with coil springs. Automatic level control and strut type superlift shock absorbers |
| Brakes | Power assisted front disc and rear disc |
| Body construction | integral body-frame |
| Fuel tank | 18.8 gal. |