I like mine. I've had 3 3126 motors, this one is a 2000 & it's been the best one to date. 555,000 on it & it's still going strong. I think I need to change my wire harness as i'm noticeing a little issue lately.
The truck needs Valve adjustments every 75000 to 100,000 miles. Change the oil every 5000, to 8000 miles is a must as they get older. Run some type of fuel injector cleaner though it on a regular basis.
The cold startup & warm up for mine makes a big diffrence. At this trucks advanced years & mileage it's deveolped a miss when it's first started up in the morning & it can take up to a half hour depending on the weather before it smoothes out. I always let it warm up before I go.
They say if a CAT motor isn't leaking than somthing is wrong. Well mine doesn't leak because I fixed all the leaks & it runs fine.
I've dumped alot of money in to this truck & maybe 20% of it has been engine related.
It pulls good enough on the hwy, top speed on mine the way it's geared is about 68-70 mph which is fast enough. It's ok on hills with up to 5000 pounds or so before I need to shift most of the time. When it's loaded to capacity I could be in 4th gear & barely pulling the hill at 25 mph.
The best years for this motor where 98 though 2002. After 02' they made changes to the motor that caused a lot of realibility issues. I think they were resloved by 2005, than CAT discontiuned the motor by 2007.