Truck Repair: low oil pressure on N14, cummins n14, low oil pressure

Question
what could be a cause for low oil pressure on a 2001 cummins N14 435 E engine i have already replaced oil and filters but it does not improve oil pressure (from 1 at 650 rpm to 2.5 at 1700 rpm)ecm mileage is 300000 miles

Answer
Pressure regulator sticking or oil pump bad usually the problems that are not as serious as a bearing problem.First I would install a manual gauge on the oil pump & check pump.We take out the oil temp sensor & install a fitting where it is & rev engine up & should be around 65 psi.If not,remove suction tube from pump to pan.Stick a large screw driver in the oil pump & see if you can turn the gears or get some one to manually turn engine while holding the gears to see if gear slipping on shaft.If the oil pressure is around 65 psi,try a different oil rifle on the fuel pump side,if low go to oil filter head & check in oil filter & out oil filter ports.A broken piston cooling nozzle will dump oil pressure also.The oil filter in & out are around 20-40 psi at idle.Oil pressure that low will log a 143 & 415 code.The regulator is located on the bottom of oil pump.It hase one 5/16"(1/2" socket)& a big washer holding it in.Just remove bolt & push plunger plug upwards a little.Be careful because the spring will pop the plug out.The regulator should fall out or get it our easily with your finger.