Lets get some sensibility here folks. Firstly as Chief has stated, you may have a burnt intake valve.
You can't have a valve "hang". That would instantly have a very audible an frightening hammering noise
followed by much smoke everywhere and the need to rebuild
the engine. There is no wondering about that fault.
It won't be timing. Timing doesn't just slip in a Diesel and even if the timing did change, it couldn't do what you describe.
There is one other possible, but it won't be black smoke, more blue and that is sump compression
blowing through the sump breather which connects to the air intake manifold. However, to pressurize the sump, you have to have bad rings and thus you wills see much smoke at the exhaust as well. So seeing as that is least likely, lets go back to the intake valve. There is a chance you may hear it when the engine is running, but the best way to test for the problem is with a leakdown tester. This is a gauge that you connect to the cylinder at the injector port. Compressed air is forced into the cylinder. The gauge reading tells you how much air is escaping somewhere. Your ear helps you detect where the air is escaping too. If it is a burnt valve, you hear the air escaping back into the intake manifold.
Burnt valves can be caused by several issues. Incorrectly set valve clearance and even a faulty injector. If the spray pattern is wrong, it can spray onto or in the direction of the valve and create too much heat on or around the valve face.
I suggest you do a leak down test. If you have a burnt valve, the longer you leave it, the more damage you are causing to the valve seat which is par of the head
Oh and Cylinder pressure, think more in terms of thousands of PSI, not hundreds. You can not get an over pressurized cylinder burn. Unburnt fuel that has taken one complete trip around so as it remains in the cylinder when the injector squirts again, does not burn the same as the new injected fuel does. Diesel has to be atomized before it will ignite. unburnt fuel that has touch any of the internals will not burn. In fact unburnt fuel can actually flood the cylinder and rob enough heat to stop the next dose of fuel igniting at all. Instead the fuel burns much slower and very much later in the cylinder cycle and results in blue smoke. The same blue smoke you often see at start up of a cold engine. That blue is caused by Diesel and oil
being heated and burnt off outside of compression.