Hyundai Repair: Elentra overheating, camshaft position sensor, radiator cap

Question
I changed the thermostat since it would not open properly. I changed the spark plugs since it was worn. I tried to read codes with OB2 and it read P0340 which is the camshaft position sensor. But it can not prevent the car from starting so I left the engine running and after several round he has finally started. The tank violently spits out the antifreeze and the engine overheated and each time I filled the radiator is the same problem ... what I can do more ... compression on the cylinder is correct ... I tested the thermostat and it is correct, it runs fine, but it has difficulty to start especially when the radiator is full ... I thought trying to leave the radiator cap off to let the engine air purge, but does it make a difference?? I do not know what to try especially my 2 cars were broken at the same time and I can not afford to go to the garage! Please help me!

Answer
Unfortunately, Sam, I don't have good news for you.  The symptoms you list are consistent with the cylinder head being warped.  This is a common problem with Elantras that have been severely overheated.  Every time I've had one with these symptoms, the cylinder head has been warped so badly that it needed to be replaced.