Low oil pressure after oil filter change:

My 340 had about three-four hours of cruising time after the cam break in, so it was time to dump the break in oil and filter. I changed them both (was a Napa Gold filter/Driven Break In oil to another Napa Gold filter/Driven 10-W30 High Zinc) and went on a test drive. The oil pressure was fine before leaving the garage (22’ish at warm idle and 50’ish at cruise) but after a few WOT downshifts, I noticed that the oil pressure at cruise was down to 25’ish. So, I drove it back to the garage while listening and being careful, but I did not hear any extra noises (solid cam) coming from the engine.

Once the engine cooled off the next day, I checked out the wires on the Holley fuel pump safety switch and found a loose wire. So, I snugged it back up and thought that I found the problem. I then started the car back up to find that the oil pressure was at 25’ish again at high idle which is low. I was waiting on the engine to warm up and checking for leaks (just replaced the speedo cable seal) and then I was under the hood, and I noticed my fuel pressure gauge was starting to jump around (this pressure is usually solid). After a few seconds of watching it, it was dropping fast, and I was waiting on the engine to die. Once the engine died, I shut off the ignition and started thinking about what is now causing the low oil pressure and the low fuel pressure? I have a manual prime button for the electric fuel pump, so I pressed it for a few seconds and my fuel bowls filled up like normal. So, the fuel pump works while using the manual push button, but it is not working now while the engine is running.

I decided to replace the Holley safety switch since it drives the fuel pump. While doing so, I pulled the pipe/adapter setup that I have on the back of the engine that mounts the oil pressure sensor and this safety switch. I cleaned up the pipe/adapter (nothing inside that would cause a low oil pressure reading) and remounted the oil pressure sensor and the safety switch. But now I am thinking out loud that neither one of these would make the oil pressure read low, but the switch does shut off the fuel pump if the pressure is too low. It did not lose all oil pressure, but now I am getting half of what I am used to seeing on the oil pressure gauge.

So now I am thinking of replacing the oil filter (I have a new Wix filter) since it was the only change to the engine before I made the next drive as when it started to lose oil pressure. I will change this filter this weekend and then see what the oil pressure is at startup.

But if changing the oil filter gives me the same oil pressure result, what are my next few steps? Pull the dist/oil pump drive shaft and inspect? Then plan on pulling the oil pan to replace the pump/pickup?

What do you think? I used a well known/good filter brand before and after the cam break in.


Thank you.