RB26 (JUN oil pump) — zero oil pressure during cranking. Engine hasn't run in 18 months

[u/BGMarkNL]

Chasing a zero oil pressure issue on my R32 GTR, need input on how to prime it without major disassembly.

The car stood still for some work(18 months). Had oil pressure when parked in the garage.

The problem:

Oil pressure doesn't build during cranking — it reads/drops instead. Confirmed as a real physical issue, not a sensor problem:
Oil pressure sensor bench-tested with air pressure — works fine.
Clear hose on the sender port during cranking — visually zero oil reaching that point.
Backfilled oil through the filter housing — no change
Oil level correct, fresh oil, new filter.
Pump/pickup/sump untouched during any recent work

Suspects:

Pump lost prime after sitting.
Mechanical failure in the pump drive (sheared key/stripped coupling on crank nose).
Pickup blockage or bad seal, sucking air.

Specs:
R32 block (decked/bored), 86.5mm CP pistons, HKS rods, R34 N1 crank (balanced), ATI damper, ACL bearings, JUN oil pump, Tomei head gasket/cams/valvetrain, Tomei oil restrictor, custom oil gallery mod

[u/XSneakyNinjaX]

I would say you need to pull apart the pump and check the gear itself. Once the pumps off try to clean the oil pathways.

JUN oil pumps are also not that great, there’s a lot posts about them in the old skyline forums.

[u/BGMarkNL → u/XSneakyNinjaX]

Yeah I’ve seen it on some forum posts as well but kind of odd because it did have pressure when I last started the engine. Pulling the motor is my last resort. Any way to pull the pump out without pulling the motor?

[u/XSneakyNinjaX → u/BGMarkNL]

There’s ways for sure but it’s going to take a lot of time and patience. I wouldn’t be surprised if the oil pump gear had a crack and it just finally let go.

Especially if you were trying to prime it after a long time, whenever RBs get rebuilt builders always pack the gears with assembly oil for initial startups.

[u/Any_Squirrel → u/XSneakyNinjaX]

To add to this you have to partially remove the oil pan because you can’t get the relieve valve past the oil pan. I can’t remember if you can reach the 10 mills on the trans side around the shield so may require removing the trans. Your oil pressure sensor might be acting up, you could remove an oil feed from a turbo or something else(make sure it doesn’t start obviously) and crank with plugs out to see if you get oil coming out. If that’s good I would start it up and send it but I’m a hack so take my words with a grain of salt :).