240D - Dead Cylinder

[DieselPaul]

So my dad has a 1983 240D automatic with around 245,000 miles. He's put about 20,000-25,000 of those miles on it. Oil changed usually at 5,000 miles with 5/40 Rotella T6, he religiously runs fuel additive in the car as well.

It started running rough all of the sudden. Runs great off idle, but idles terribly. Doesn't consume oil or coolant, still gets close to 30mpg mostly highway driving.

He drove it up to my shop to pull the injectors to send off to greazzer, pulled the injectors and compression tested it.

300 - 80 - 300 - 290.

Put ATF in cylinder 2 and when you crank it over the gauge will now spike to 200psi but it won't hold pressure. So it goes 200 - 0 - 200 - 0 - 200 - 0 as it cranks over (if that's indicative of anything)

Pulled the valve cover and the valve adjustment for #2 is dead on.

Why would a diesel all of the sudden have a dead cylinder? Car is well cared for and didn't really display any "going down hill" symptoms. We just assumed the nozzles were finally worn and needed done when it was idling rough.

Just wondering if there is something we have [not] done that killed this car.

Really nice rust free southern 240D in midnight over palomino that he adores, so we will be fixing it, but its annoying to have to put an engine in a car that ran so well right up until it didn't.