BMW Repair: No Oil Pressure, oil filler cap, crankcase ventilation

Question
I have a 1998 740iL with the 4.4L engine. The problem I am having is that it will not maintain oil pressure. I have had a new pump installed and a new filter but still no pressure. I have ran engine cleaner through it and then changed the oil and filter. I have taken out the pump and cleaned everything very good and using an air compresser I blew out the filtered oil line and the non filtered line. After doing this and putting it all back together it will run and hold pressure for a while. I let it run for 45 minutes the last time and thought the problem was fixed but as soon as I got in the car and drove half a block to turn around it had no pressure again. I have also checked the sending unit but because the engine starts to rattle I am assuming that there is no oil pressure. The last time it also started to smoke really bad. I loosened the oil filler cap and the smoking stopped. I am going to guess that this problem is with the crankcase ventilation system but have no idea on how to check this or replace the parts that are needed... Any help would be appreciated... Thanks..  

Answer
Dear Richard,
If the engine overheats due to lack of oil pressure, then the oil vapor in the oil pan would overwhelm the crankcase ventilation system. But the crankcase ventilation system is totally independent of the oil pump.
If you can, remove the oil pressure sensor and install a suitable oil pressure meter. If there is no suitable pressure,
check backwards down to the connection with the pump (I hope that you have good O-rings installed where the oil pipes connect). Since you said that you have just installed a new pump, I have no reason to suspect a faulty oil pressure control plunger (piston, spring,sleeve and circlip) which is also called the relief valve, or a faulty overpressure safety valve. A new pump provided it has good input pipes and good output pipes (to be basic about it) would have good pressure. So in your case, if the pump is good, either it is not getting a good supply of oil or it is losing pressure somewhere.It could be in the pipes/tubes, oil gallery, wornout bearings, cracked head or block, etc. I am sorry to say this but most people end up replacing the engine in cases like yours.
Tino