Diesels from the 80s were strictly mechanical. In the 90s there were still some engines (ex. Cat 3406C) that were capable of computer control but operated mechanically. Then came the computer control of fuel/air mixture. I still have a 2001 KW with a C-15 engine. Now there's some weird additive that has to be added to an engine to keep it running. That and regurgitation engines where the exhaust is plumbed back into the intake. Worked on a job with a guy who, at the time, had a brand new Peterbilt. He was forever having to park and fuck with it. My old KW is still worth a ton of money since it's low mileage (private carrier only) but still has the C-15, 550 hp engine. At 2,500 RPM it tops out at just a little over 129 mph.