A while back my (lovely wife's) 1997 Disco I oil light started flickering at idle.
148,000 miles.
Running Castrol 10w-40.

(mea culpa - I've been terrible at regular oil changes)

So, installed an oil pressure gage to start diagnosing.
Two, actually. Used the glowshift sandwich adapter and connected and electrical gage and a mechanical gage to help convice myself they were accurate. Readings matched, so I'll assume they're correct.

Pre- any repair work, readings were something like:
Cold 2k 40psi, idle 40psi
Hot 2k 30psi, idle 10, or 9, sometimes 8. (and the light flickers)

Changed to rotella 15w-40 and the huge Napa filter, no improvement in oil pressure.

Replaced oil pump. The old pump really didn't look bad. Not broken.
Tooth clearances were just barely out of spec (about .012 instead of .010). Tooth clearances on the new pump (from AB) were about .010.
Clearance to straight edge across pump in casing were within spec.
Pressure relief valve looked okay.

Put it all back together and... no improvement in oil pressure

Switched to 20w-50, and of course the pressure went up so the idle was about 12psi, so at least is was above 10.

Then it had a bad head gasket, so I went ahead and did an engine rebuild.

Replaced:
Main and big end bearings, .010 over, and ground crankshaft.
Camshaft and camshaft bearings.
Lifters.
Rockers and shafts.

Oil pump still looked fine, clearances still okay.

At startup after the rebuild, I used straight HD30 oil and ran for about 20 mintues at 1500-2000 rpm, then drained and changed filter and filled with Rotella 15w-40.

And... the stinkin' oil pressure isn't any better.

I'm flumoxed.

Can't really figure out what else could affect the pressure.

Per my understanding, the high pressures when cold show that the pump is working.
Since the 2000 rpm and idle (cold) pressures are about the same, it seems the relief valve is working.
Since the 2000 rpm (hot) pressure isn't too bad, doesn't seem there are restrictions in the return flow. (well and also because I had the engine completely apart and cleaned everything and blew out all the passages, etc)

It's the hot idle pressure that concerns me, although both the hot 2000rpm and hot idle are lower than I'd expect after a rebuild. Why would it still be that low after replacing everything that affects oil pressure?

Thanks.

 

What physical measurements did you take on the oil pump?

 

I measured per the rave manual:
Tooth to tooth was about .010
To a straightedge across the gears and casing was about .002 or less.

 

I had very similar issues with my 97 disco. Did everything,including new oil cooler lines, etc.

Turned out to be blocked oil cooler in radiator.

Quick check is to remove cooler lines, plug both ports and fire it up.

WIth the cooler lines removed and the holes plugged, the system design naturally routes oil through filter right after pump, then into passages in block.

I couldn't get anymore than 12 lbs at idle and when I rev'd it up, pressure dropped.

Notice oil filter had plenty of crap in it, so after realizing that everything goes through the cooler before the filter normally, I bypassed the cooler, fired it up and it went to 50 lbs in an instant.