text: The first year models were available in base and Medallion trim levels; the base engine was the 318 Lean Burn V8 developing  and the  Slant Six was a credit option. A vinyl roof was standard on sedans, while a simulated landau roof cover was an option on coupes. For 1978 the Slant Six with a four-speed manual transmission became the base drivetrain, and the station wagon was added. The wagon featured "teakwood"-grain body appliques (a delete option) and was only available in base trim. The sedans and coupes continued largely unchanged, but a low-cost "Diplomat S" was added. This model was offered for one year, and the 3,322 produced represented only about 5% of overall Diplomat production in 1978. The V8's output dropped by five horsepower, but on the other hand, a   V8 became a new option.
  For the 1979 model year, the Diplomat received a minor facelift. The grille was now divided into three rows of eight rectangular openings with an internal crosshatch pattern and a broad band on top. The sedan and coupe taillights were also altered, now with a horizontal theme while divided down the middle. The lineup was rearranged, with the base model decontented and a Salon model (corresponding to last year's base version) fitting in the middle below the Medallion. The new base model did not receive the padded vinyl roof (sedans). Coupes had a new landau top design, with much smaller rear side windows. The station wagon came in Salon trim, without any actual changes in equipment. The Slant Six, meanwhile, was available with a single-barrel carburetor and . V8 outputs dropped by five horsepower, to  respectively, but optional was a four-barrel 360 producing . This was the last year for the 360 engine in the Diplomat.