Low oil pressure/Bad filter?

[keepondancin]

I have a Westerbeke 63B with about 800 hrs. I changed oil and filter in June. It was a year where life got in the way, and really didn't use the boat, but did run the engine at about 1000-1200 rpm to give the batteries a little charge, (first time for that in 30 years). Went to decommision it in the early fall. Started the engine, all was good, dropped off the mooring and in a few minutes the engine alarm came on, and the pressure was way low. Grabbed another mooring and sat and regrouped. It was hard to check the oil on the stick, it was so clean, but it was at the proper level. Started up again, and the pressure rose, then dropped down again. I had TOWBOAT/US tow me to the boat yard. I had a mechanic come on board with a pressure gauge to check the pressure, hoping maybe I just had a bad sending unit, or an electrical issue. Nope, his gauge matched mine. A good guy, we went over everything it could be, and his suggestion was start with the cheapest and easiest things first. I changed the oil filter, and as I usually do, i put some oil in the filter. Started up and full oil pressure. Idled for awhile, good pressure. Tied off tight to the dock, started up, put in gear, ran for over an hour at maybe 1500-1800 rpm, all is good. Mechanic stopped by, he had seen me running the engine, and we talked again. I always thought, and he agreed that a blocked filter would bypass and allow pressure to stay up. Both being old guys, we each never saw this happen with filters. Went to my Napa store, discussed it with them, (I use NAPA Gold filters, by Wix, supposedly), and they said the same thing and had never had a problem that they could recall. Has anyone ever experienced this happening, with a bad filter? Of course I am concerned what will happen at launch time, and it will always be in the back of my mind every time I run the engine.

[Lepke]

2 common causes for low oil pressure in good engines is debris sticking to the relief valve in the filter or oil pump. After that, maybe the oil strainer in the pan. If it was my engine, drain the oil into a clean pan and maybe strain it thru a tee shirt and look for any big particles. If nothing found, put the oil back in.
I've never seen the problem in an oil filter, but a couple times in the oil pump relief valve. There's a big difference in oil filter relief valves. Utube has videos on oil filter internal differences between brands.

[keepondancin]

These are all things I thought of. It just is interesting that as soon as I changed the filter, pressure was right up there. It makes me think, and hope it was just the one defective filter. Not sure that just changing the filter would dislodge anything from the oil pump or the pickup screen. I will attempt to screen drained oil when I change it again. 800 hrs after 18 years, at least 18 oil changes, should be not much sludge.