I have a very recent & rapidly declining oil pressure issue on an otherwise healthy engine at 126,XXX miles.  Cold engine starts at 33 psi @ 1300 rpm, but I can watch it drop to down around 5 psi @ 800 rpm only a few minutes later.  Is there a common/typical issue that crops up?

Yes. It's common for the screws on the backing plate of the oil pump to loosen and back out. This causes the backing plate to separate from the pump and oil pressure is bled back into the crankcase.

 

Have to remove the oil pump from the block to fix it. If it needs a timing belt, nows the time to do it.

Beergarage has a nice write up of what Fairtax describes here.  Also, great vid by Miles Fox

.

Check thrust bearings for play

+1 oil pump back plate coming off.

+1, most of them i've pulled have multiple screws loose, yours must be all of them.

 

nice catch - how did you catch this?    noise?

I just did my timing belt,, I had at least 3 screws loose in that backing plate.

Thanks for all the replies and links!  Figures, I just was in there doing the water pump.  I'll drop a line when I get to this next week.  At least the weather will be pleasant!

Good call.... all but one screw loose.  Annoying the factory never used Loctite on them.  65 psi at 1500 cold rpm, 55 psi at cold normal idle.  The Felpro kit's gasket instead of the sealant is a good idea, too.

55 is low for cold pressure. It should be above 70 at cold idle. Bypass valve pressure is 75psi so you should get close to that.

What pressure do you get at warm idle?

 

You didn't put a gasket on the backing plate did you? Should be no gasket there. A gasket will increase the clearance between the pump rotor and the housing and will lower the output pressure.