I've been meaning to update this thread since rebuilding my oil pump a couple of weeks ago.
+1 on Rastus' suggestion (I personally would not suggest using flush in a manner other than the manufacturer directions) if your problem is high oil pressure, you most likely have a blockage somewhere.
Jono83, the pressure relief valve is replaceable, but not repairable.
The pressure relief valve would NOT be the cause of LOW oil pressure.....by design. Are you sure your light is coming on for low pressure? What about high pressure? The only way to tell definitively is to put on a gauge). If your near the Parramatta area, you can come round and use mine). You may well find you are suffering from high oil pressure rather then low.....your symptoms support this (cold oil and higher revs generate your light), although the 'spanner in the works' is your gauge showing normal values (gauge issue?). Again, the only definitive test is to measure pressure with a gauge.
Regarding my issue, after an oil pump rebuild, there is no change.
I still see similar pressures, albeit with a noticeable difference in pressure at idle when the oil is at running temp. Still nowhere near within spec though.
There was no problem with the gears, valve (and obviously) the spring that I replaced with the rebuild kit.
The bottom cover of the oil pump, that the gears bottom out on, showed some wear, but nothing significant.
What I did notice when manually priming up the engine before start, was that oil flow through the pushrods for cylinder number 5 was significantly slower than the rest. This would suggest a partial blockage of the gallery/pushrod that feeds this cylinder. Is a partial blockage in the feed to this cylinder enough to send pressures so high? I don't know. But I'm inclined to think so. I was pushing it for time at the time, so did not even contemplate removing that pushrod and checking it out...obviously regret this now....but that's easy enough to do at some stage in the future (when the wife gets over the time out I took to work on the car this time :-P ).
The FSM details a procedure to blow out the oil galleries with compressed air, but this involves motor OUT to get galleries in the rear of the block....and strip to get other galleries. I don't have that sort of time as it's my daily drive.
So, while my problem is HIGH oil pressure, I'm just going to live with it and keep an ear out for any knocks that indicate a total blockage....and worry about it then.
If your problem IS low oil pressure on the other hand, then maybe, yes, your oil pump gears are worn (I can't imagine how this could happen, they're really chunky gears), or perhaps a leak or blockage in your oil pickup or pump (Good luck trying to get a new oil pick up in Australia (the FSM warns not to disturb the oil pick up and that a new one must be installed if the old one is disturbed or removed...reason: it's an oversize (6 thou) press fit, supposedly for a vacuum seal....I had no choice but to put my old one back on, which seemed to be a nice tight fit....and I smeared it with rtv to be sure....works fine, but time will tell)......None of the main stream Repco etc suppliers even listed one. Jeep quoted a 3-4 week wait for it to come in from O/S, so that could be a 6-8 week wait. Rockauto does sell one.....oh and you need a 'special tool' to fit the oil pickup...youtube it....you can get around it if you're extra handy.
To recap, you next step is to identify exactly which problem you have...high or low oil pressure. If low, drop your sump and investigate, potentially clean/replace pickup/pump gears. If high, run a series of flushes, at least two back to back.
High Oil Pressure damn it!
