Not clear where you measure this-I have measured mine at the oil sender outlet and the rear main bearing the results being the same. It also says 'secondary' oil pressure (whatever this means!) should be measured and is 7psi at idle and 42 to 56psi at 3600 revs). I have the "proper" Yanmar workshop manual and it talks about 'standard' pressure (measured at the oil sender outlet) being 42 to 56 psi whatever the discharge volume i.e however fast the engine is reving. However, these are a lot of dosh new and would welcome thoughts and views. I am therefore tending to the view that is may be the oil pressure relief switch that on increasing revs (an increasing the oil flow) is opening and allowing pressure to drop and then closes when I go back to idle and oil flow decreases. However, if that was the cause I would have thought as I increase revs beyond 1500 rpm the pressure would fall further-which isn't what happens. I thought it may be on the suction side of the pump-eg the oil pick up reinstalled too close to the sump bottom or having an air leak which would explain why when revs increase the pressure falls because the pump cannot get enough oil to pump round. Normally I would expect pressure to rise with revs and then fall back as revs decrease. If I rev it further the pressure does not drop or rise but stays pretty static. However, when I increase revs to 1500 rpm the oil pressure will either start to rise for a second or two and then drop suddenly to 30psi or so or drop straight to 30 psi. When warm and at idle (850rpm) the pressure sits at 40psi. New oil (15w-40w mineral) and filter etc and correctly filled so this shouldn't be cause of the problem. Since it is a mechanical gauge sender issues shouldn't be relevant. I have hooked up a mechanical pressure gauge in place of the oil switch sender-not sure of accuracy but the issue here is relative rather than absolute pressures. We have a rebuilt Yanmar 3GM apparently running well.
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