Hi,
I was changing the oil and filter today on my gf's 96 Nissan Sentra with the 1.6L engine. After I had the old oil filter off, I shone a flashlight up on the mating surface on the engine wall, and saw the hole in the center, which lets oil into the engine from the filter, and also 2 holes near the outer edge of the mating circle, next to each other,. I believe one was the oil inlet hole, for letting oil into the filter, and the other hole the "oil filter bypass valve". This "oil filter bypass valve " hole had a rough lip protruding from its circumfrerence. I thought perhaps something had been there which had broken off, leaving this rough lip. Inside this hole was a very weak valve, held in place by a spring pushing it from behind, I assume. I was able to push the valve back in very easily with a pen. Seems like the filter would only need to provide a very weak resistance in order for the bypass valve to open and have the oil not go through the filter.
Am I on target about this hole? Is it the "oil filter bypass valve" ? Is it supposed to be so weak? Is it normal for the lip around it to be rough and uneven ?
Thanks