my dad has a 3116 in his snapon tool truck, that thing is a pig, but i know that there is something wrong with it, from the small knowledge that ive gathered on this engine, it sounds like it needs a overhead adjustment the valves and the injector syncronization needs to be checked. they also had issues with the injector bore seats in the head coming loose, which would alow compression to get into the fuel system, and it really kills the power, also they dont like the be idled, and generally seems to hurt them. i have some oil leaks to fix, the cam boxes are leaking, but they can only be taken off once the rockers, and push rods are taken off, just like the Cummins N14 engines, but the cats dont rely on gasket thickness to determine the fuel injector timing unlike the N14's, thats a fun repair to do! i did fix the leak on the driver's side of the engine, the oil return casting in the block for the HEUI fuel system has a plate cover with a large oring that seals the plate to the block, the oring was very stiff, brittle, and flat, it really leaked when the engine was cold, replaced it and no leaks there. sorry for the long post, I just thought i would share my teeny tiny bit of knowledge that i have on the 3116 engine.
wes