Hi Bbforks,

Thanx for the answer.

The trucks are from 2008 and they have about 8000 hours.
Yes, the pressure is in the crancase.
My guy has been looking for vent everywhere and found nothing.
No hose coming to intake.
We have taken the trucks under maintenance just recently, from the customer documentation seems they used 15W40 Texaco.
I think it's OK.
What we have learned too is, that for some time they have been using water as a coolant.
I am afraid, this could cause some problems (rust), maybe the vaporisation caused the pressure.

Anyway, we must have a closer look on the cooler.

Any ideas, how to deaerate the system in this type of Yanmar?