NHC250/M939A1 No oil pressure on startup

Trecker said:

Hello together,
I have an M934A1 here in Germany. On motor start I noticed that oil pressure needs at least 10 seconds to start to build up. I the truck was standing for days/weeks it takes even longer.
Investigating this problem I found oil from high pressure side returning throught the pump back into oil pan. The oil filter housing is only filled to a third after some time standing. I could not find any part that could prevent this issue in manuals.
So I have following questions:
1. Is this the time for building oil pressure normal?
2. Is it normal that oil can flow back into the oil pan?
3. What can I do to solve these problems?

Oil pressure is perfectly normal after start-up and under hot conditions, so the oil pump should be ok?

I would be thankful I somebody can help me with this problem!

Greetings from Germany

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Mullaney said:

.
I would hazard a guess that you are just fine.
The gauges in those vehicles are not always "top of the line" either.
You might plug in a mechanical gauge in the oil gallery on the side of the motor - to verify your oil pressure.

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Trecker said:

Is there any way to improve this? Pump and motor always loosing 'prime' does not seem so good.

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