They extended the warranty when the 1st problems started cropping up. The motor needed the anti-corrosion tabs (that were very common with GM engines in that era) added to the coolant as well.
Mom's '83 (I think?) shorter wheelbase Coupe deVille (the Tobaccolac) got 21+ around town and pushed 30 mpg on the freeway with the cruise control set at 80 mph. Never really was much of a "tire smoker", though.
This was one of GM's 1st cars with a "computer controlled" powertrain, although it was not too much of a computer: just a lot of electronic modules, which, BTW, worked pretty good! The idea was to increase gas mileage (which it did) without losing the plush Cadillac "RIDE". Frequent oil changes, transmission service, new coolant every couple of years. Fix anything that needed fixing. A couple good thousand mile road trips per year to keep things warmed up & working. No major mechanical problems with this car, although to be truthful it was being driven by a "little old lady" who happened to smoke a pack-a-day well into her '90's (MOM!) and only went to church sporadically...
Best part was these cars had the cooling & electrical systems, a/c, transmission, suspension, rear end, etc. of the much heavier and more powerful BIG, BIG BIG! V-8 engines. They were very hard to break with the 4.1 motor.
As far as the engine was concerned it was really a rolling test-bed for the transverse application (4.1, 4.5 & 4.9 varients) that were not replaced until the Northstar came along, much later.
My brother still has this car down in St. Pete where it is not so "vertically challenged" by any hills, or Ohio winters, salt on the roads (except sea salt), and so on... About 88K miles on her, original engine & transmission, a complete body & paint redo.
Chocolate metallic paint, deep dark chocolate leather interior and carpets are still perfect, new headliner, tires, shocks, bushings, etc. Car pretty much looks like it did when it came off the line. New SONY radio with CD (GASP!)!
Probably won't last for much more than another 25+ years!
Cheers!