I've got an 87 GN with 10K on a rebuilt engine. Recently the oil pressure has dropped some. On cold start up It runs up to 60psi. After it warms up it'll run anywere from 10-15 at idle and 25-40 depending on rpm (about 20-25 at 2000/30-35 at 3000/35-40 at 4000/and so on) After driving on the hiway for extended periods of time, when I come to a stop and go back to idle, the gauge goes clear down to around 5-10psi. This just started happening.....any ideas? think my sending unit may be failing or did something happen to the oil pump, huh? Thanks!
This is a very common complaint with the Buick V6 . The common minimum standard for oil pressure is 10# for every 1000 rpm . You may be able to raise the hot idle pressure a little by installing a plate on the oil pump cover. I don't off hand remember who makes these ; maybe someone else could help with that .
A lot of people will tell you that as long as you have some minimum pressure then ignore it and don't worry about it. Me, when I see the pressure dropping over time, even if it's above the minimum recommended, I worry, because I figure something is bad is happening and I'd rather find it and fix it sooner than later!
Had a similar problem with my rebuild and determined that it was the #1 cam bearing. What I understand is that the stock cam bearings were fairly hard, harder than the commonly available replacements. Combine a new softer cam bearing with a new cam that has the groove cut in the #1 journal (which the stock cam didn't have as I understand it), and you can get a lot of wear right there pretty quick. And since this is the first place oil gets to in the block, the oil pressure drops as oil hemorrhages out between the cam and the bearing, and the rest of the engine gets starved for oil.
I was doing a cam swap, saw the excessive wear right there, and replaced the #1 bearing with a harder bearing that TA Performance sells, and it brought my pressure back up.
Unfortunately, if this is your problem, it takes a lot of disassembly to find it. Not a trivial exercise to do just for fun. And there are plenty of other things that could cause it as well. Heck, higher temperatures during the summer could cause your oil pressure to drop, and it'll recover as things cool down! Oil pump could be getting worn, yada yada yada. This is just something I found on my car at about the same mileage on a rebuild that you are seeing.
John Estill
John was "right on" with his info and I would like to add a couple comments also.
One simple, easy check on the internals of you motor is to periodically cut the filter apart and check its contents for bearing material or other debris. A magnetic drain plug will tell you if metallic parts like a cam is going away.
I really like to get as much oil as possible removed during a change. The pint left in the bottom of the pan especially. We weld another drain bung on the outside of the pan next to the original. This way ALL oil is drained from the pan. Occasionally it is a good idea to drain the oil cooler and lines too.