Subject: Re: Oil pressure fluctuating...
A long time since we had correspondence on this topic.
One of my oil pumps primes reasonably well. The other doesn't, yet it performs very well on the bench, when connected to a source of oil. Indeed it empties the container in no time, so it shifts a lot of oil.
I gave up with the non-return valve because it caused more problems.
The pressure relief valve merely recirculates the oil in the pump, it does not discharge the excess oil into the sump. It relies on the large pipe from the sump being unrestricted so the pump effectively 'marks time' at the correct pressure. The non return valve allows the pump to keep drawing oil at maximum volume so the pressure builds up and up in the pipes. My gauge was reading 100psi, with oil coming out of the top of the pump, even though the cap was screwed down tight, with a good fibre washer in place. When I removed the non return valve everything went back to normal, max 40psi and a constant 30-35psi when hot.
I know others haven't had this problem, so perhaps they modified the non return valve in some way or introduced a way for the relief valve to vent into the sump. Or maybe their oil pumps don't ever produce high pressure because they're worn.