[Vwdiesel] Poor compression Q's

[Todd Osterbrink]

Got low compression in 2 cylinders in a 1980 Dasher diesel.  Thinking bad rings, since the performance of the thing went from good to bad within a day.  Running bad, missing, lots of blue smoke, and starting hard.  Checked pump and cam timing, all in spec.  

What could cause piston rings on a 1980 Dasher diesel to go bad?  The engine has 85,000 miles on it since rebuild (which included new over size Mahle pistons, rings and such).  Could it be something else?