UK Car Repair: Low oil pressure after some minutes, low oil pressure, pressure relief valve

Question
QUESTION: Dear Sir,

I have a Jag V8 engine. After engine rebuilding (new crankshaft and bearings) I found that after some minutes from start the low oil pressure lamp is lighting. I replaced the oil and filter (the oil was very dirty). For some minutes everything was okay, but after that the lamp shows the low (or high ?) oil pressure again. At new starts the oil pressure lamp shows the problem after start immediatelly (without some minutes delay). What is your opinion is it a jammed oil pickup, or bad clearances, or pressure relief valve is bad ?
I'm so flustrated because of this problem :(

Thank you for your help,
Istvan


ANSWER: Why was the oil dirty if the engine was rebuilt? You first have to check the pressure by fitting a gauge directly to the engine, it is possible that the sender unit is faulty or wrongly connected

---------- FOLLOW-UP ----------

QUESTION: Dear Sirs,

I'm thinking about that, if the oil pump is an excentered type rotary pump, is it possible, when I filled the engine with new oil, the oil closed the maybe too high clearances in oil pump so I had some oil pressure. But after a half day the oil is emptied from the pump and because of too big clearances there is no pressure in the dry oil pump to pick up oil from the oil pan ?

Kind regards,
Istvan

Answer
First, do not start the engine again until you find the problem.  In order to turn it over easier, remove the spark plugs. If it had pressure initially then the pump was not dry. if it now has no pressure at the switch then it is not picking up the oil (a pipe loose or disconnected) or there is a blockage after the pump.IO feel you need to go back in to the sump and start checking.