2003 ranger 4.0 - 4 codes

A friend of mine has a 2003 ford ranger.. truck has been great. Then one day i started it and it ran like crap. Check engine light came on. The codes are as followed.
P0300, P0302, P0306 & P0316

I had changed the plugs & wires - no change . I replaced the coil pack -no change. I went ahead and checked for intake leaks. None. PVC hose was collapsed. I replaced that - no change. Does anyone have any ideas what else it could be? Am i looking at checking injectors? computer? It happened all of a sudden. So i am thinking something failed. Help Please.

P0300 indicates random/multiple cylinder misfire detected.
P0302 & P0306 indcate cylinder misfire in nos. 2 and 6 cylinders.
P0316 indicates misfire detected during start up - first 1000 revolutions.

NOte: All the parts you have replaced were common to the function of the engine as a whole. However, the CEL indicated only problem with cylinders 2 and 6.
So, check for ignition spark on cylinders 2 and 6 first.
Then check the fuel injectors for the same cylinders.

Coil has been replaced, and spark is present at 2 & 6 cyl. I also replaced the crank sensor. still - no change

try the cam sensor it is on the back of the motor

maybe you swapped #2 and #6 plug wires?

I found the problem, to my amazement. Broken valve spring!! Is that common?

Which cylinder? Not too common but it could happen.