VN V6 Oil Pressure Fluctuation

[89vn_Jai]

Hey guy's and gal's,

Just want an opinion on something I noticed.
The car is a first series vn v6 manual. Fairly 'usual' light mods, extractors, exhaust, CAI, blah blah blah.
The only internal parts that are not standard are the roller rockers.
I service her every 5000km's or less, using Penrite HPR 15 (15w-60) This was a semi synthetic oil but has now changed to a full synthetic. (Full synthetic used on last oil change).

The oil pressure usually sits on 40psi at cold start, about 50psi at 2000rpm and 70 or 80 at higher revs (3000-5000rpm).
This I understand to all sound pretty good.
A few weeks ago I noticed mainly during low to mid rev range, when cruising or decelerating the oil pressure fluctuates between 45 and 65psi. I am pretty sure it never used to do that. The needle on the gauge actually wavers back and forth. It is a good quality Speco Meter Gauge so I do not think the gauge is at fault.

When the car is revved to 3000rpm+ the oil pressure goes solidly back up and stays where it should.
The car is my daily so I am still driving it around and the problem does not seem bad enough to stop driving it but I am a little worried.
Could this be anything to with the oil pressure regulator spring in the oil pump?

Any input would be appreciated, especially from people who have had this problem before.

[hi_ryder]

could your pickup have a partial blockage? has the engine been fully rebuilt in addition to your other mods? ive seen some other threads where people were having oil pressure problems that turned out to be years of sludge built up in different areas of the block. have you had the inlet manifold off at any point? it can sometimes build up there too. just throwing some ideas around....

[89vn_Jai]

No I do not think the engine has been rebuilt. Would love to do that myself but I cannot afford it on my wage
I have done Inlet manifold gasket and motor seemed pretty clean Inside. Needs a new sump gasket anyway so I will definitely have a look at the pick up then.
I don't think that's the problem but I shall keep that in mind and check it out, thankyou

[Jxw]

while doing the sump gasket remove the pickup and replace the o-ring that seals it to the block, they get hard and crusty, unlikely but could be related to the issue, if you have the timing cover off check the state of the pump, clearances, wear, pitting etc, i've seen quite a few that start to chew out the aluminium where the outer part of the pump runs but they were usually high km engines, you could try and check if anyone has added a spacer on the pressure valve spring or clean that area out as i've seen alot of sludge build up in that area thru valleys that don't get alot of oil flow to see if that makes a difference and stabilise the fluctuation.

alternatively try going back to a mineral oil 20w-70 grade and see what happens, probably the cheaper easiest test to start with, drop your synthetic oil into a clean container and save it just in case, make sure there is no dirt or debris under there to fall off into the catchment.
how many km's has your engine done? when it's fluctuating the engine isn't hunting at all?

rather than fork out all the money to do a rebuild, you could cruise ebay and slowly accumulate all the partss you need, with the aud & usd so close atm it can be cheaper to buy bits from the states and post than to buy it locally, just need to do some research and match the casting numbers with what vehicles they had over there as we were between 6-12 months behind in release dates for the buick 3.8/231 changes

[NPreston]

I think your pressure is too high. I have a Speco gauge fitted to my much modified VN V6 and don't get above 60 psi, normally when hot around 50 or perhaps a little more. My engine is virtually new having been rebuilt and travelled less than 1000km, hard kms though as it is a track car.
Gregory's manual claims 40 psi at 1850 rpm when hot to a maximum of 50 psi when hot.
Sounds like a faulty oil pressure relief valve to me.

[89vn_Jai]

Thankyou for all the input guys! I think pulling timing cover off and also the pickup to start inspecting is the best move. After trying the thicker oil first. Might even pay to just replace the oil pump as this is the original I think (motor has done 240,000).
I might just start doing that, slowly accumalating parts for a rebuild