24th Sep 2016, 19:36
This page explains everything you need to know.
25th Sep 2016, 03:09
I had an uncle who bought a 1981 Coupe DeVille.
The V8-6-4 system worked as designed by GM.
After you accelerated to a steady speed above (about) 40 MPH, the system transitioned from 8, to 6, to 4 cylinders within about a second, a event punctuated by a slight period of vibration during the transition. You could watch the active cylinder count in real-time on the Climate Control display in the dash - entertaining in the early 80's :)
My uncle liked to cruise @ 70-75 MPH, which required 6 active cylinders to provide the required power, rather than the 4 cylinders that sufficed below 70 MPH. A 90-degree V-8 is well-balanced device that has 4 power pulses per revolution. Cut in 1/2 the active cylinders to 4, and it's only a tad rougher subjectively. In 6-cylinder mode, you now have an odd-fire V-6, the kind Buick walked away from 4 years prior.
Hence 70 + MPH is vibratory experience that does not suffice for a Buick - let alone a Cadillac. Like Buick, they probably could have re-tuned motor mounts, driveshafts, engine accessory mounts, etc, to help bury this kind of behavior, but this was a rush-job.
This infrequent vibratory mode unique to 6-cylinder-mode is not (to my knowledge) harmful to the engine/vehicle, but it is a slap in the face to people who paid Cadillac money to get something not-quite-Cadiilac from a Noise, Vibration, and Harshness standpoint.
The rest of the car held up well - unlike the 1984 Sedan DeVille he bought to replace it. HT 4100. Ugh. A story in itself.
Stay below 70 MPH and enjoy :)
Keep an eye on oil changes - if after an oil change you can't get oil pressure, or it takes a LONG time to get oil pressure, replace the oil pump - it's getting tired, and losing its prime during oil changes. What you don't know is that it's also losing oil pressure during normal operation, and will one day take out the engine. This is a congenital defect with the 368-425-472-500 engine family.