'88 F-350 Diesel nat. aspirated, Manual Trans. Engine will turn 90 deg then stuck. I removed the injectors a couple of weeks ago and put Marvel Mystery oil in the cylinders, but the engine still will not turn past 90 degerees. Any suggestions?

 

withdrew my post

 

maybe it broke a valve and the valve is now hitting the head when the piston comes up

 

Yep sound like a broke valve or connecting rod.

 

Take the valve covers off and look for loose rockers (bent valve) or signs of a broken valve. It could be a glow plug broken off and hitting the piston also. Probably may as well just pull the heads and see whats up.