Pull the intake manifold and valve cover, and watch the valves and make sure that they all are moving while cranking the engine over with the starter. Removing the manifold makes it possible to remove the valve cover much easier.
Using a flat prybar through the hole on the right side of the bell housing turn the engine so the valves for cylinder # 6 are in overlap (Exhaust valve closing and intake valve opening). Now line up the pointer with TDC mark on the flywheel (Make sure you are turning the flywheel counterclockwize CCW when viewed from the rear) with the pointer. When cylinder 6 is on overlap, cylinder # 1 is on compression stroke.
Now remove the small cover on the side of the injection pump ( the little cover that is held on with 2 small screws) and check if the two timing lines are aligned with each other. With the valve cover removed you can also check the valve lash (0.027).
This will tell you if the pump is in time, that all valve are moving and that the valve lash is correct. If ever thing checks out, it time to remove the cylinder head.