oil pressure on the 77 has been acting funny

[84 400]

So here is the scoop. when I start the truck the oil PSI is right around 40 or half way on the gauge. after driving for 30 min it dropps to just above 0. It still rises when accelerating but goes back down at idle. Is this something I should be worried about. I am not sure how much I trust the factory gauge. the engine is not burning or leaking any oil. I have a mechanical gauge on my other 350 in my 56 willys and it stays around 40 PSI all the time.

the idle is turned down on the motor and I can't rember if it dropped so much before I lowered the idle. My friend says his GMC Idles at 5 PSI when running for a while. As I said it does rise when acellerating to around 25 or so PSI. Could the sender be shot?? Or does it sound like my oil pump pickup or pump itself is going. The motor is bone stock with about 28,000 miles and has been changed regularly so I do not suspect too much sludge build up but I have never dropped the pan eather.

My 84 idles at 60PSI and rises off the gauge under acceleration but it is a built motor with a high volume pump and what not....

Any input would be great. Thanks every one!

[James McClure]

84, Put a mechanical gage in and check the pressure. Factory gages aren't real accurite. I would think that if your pressure was 0 psi it would knock enough to get your attention, ya think? jim

[85Bowtie]

The rule is usually 10 psi for every 1000 RPM.

The gasket in your oil pump may be gone/deteriorated etc and you may be experiencing loss of pressure at the pump itself. This may be why it has good pressure at start-up, your oil is colder and thicker.

Or while your driving the crud in your pan is moving around and the pick-up is sucking it up against the screen, then when you park it, it releases back into the pan and creates a never ending cycle. But wth your low mileage I would'nt think so. Unless it's an original 28K truck and your valve seals have made their way to the bottom of your pan after all these years.

Try a mechanical gauge and see what it does. Then act on that.

[Jonboy]

If it is a 77, it should have a mechanical gauge. My Blazer acted the same way with the original 400. It would also selectively burn oil. Some days, it would burn a quart of oil in 50 miles. Others it would not seem to use any. I carried a case of cheap 20W50 in the back. Anyhow, when I pulled the engine apart, there was a ton of stuff in the pan. It looked like someone had resealed the pan with 2 or 3 tubes of silicone, and most of that was in the pan, along with most of the valve seals and a bunch of nylon timing gear teeth. I did a cheap rebuild (college budget, so it was rings, bearings, and gaskets only) and the engine never used oil after that. The oil pressure fluctuated a lot, too. I am betting on the oil pump pickup being full of debris. If that is the case, ther emay be some in the tube that goes to your oil pressure gauge, too.

[mainermikeyd]

Start with the mechanical gauge first but I will agree with Jonboy, had a similar problem with mine this past winter, after I did a oil change. A quick peek an your bottom end with the pan off, won't hurt anything, an you'll definately get to see if you have anything if at all in your oil pan!!!

[enaberif]

Originally Posted by 85Bowtie

The rule is usually 10 psi for every 1000 RPM.

Woah really? I have like 20psi @ 1000rpm

[billnorman]

I would be concerned. I would put a different mechanical gage on it. If it still drops off that much at idle hot, I would suspect a piece of foreign material in the oil pressure relief valve. A 28,000 mile motor should idle at 20+p.s.i. when hot. The pressure relief valve should break at 30-40- p.s.i. hot at cruise r.p.m.
I'd suspect some problem with the pump unless perhaps the wrong filter is installed or the oil is diluted with gasoline. A clogged pick-up screen usually shows up cold. Save your main bearings, keep good oil pressure.

[84 400]

It has been running rich. I would say idle rpm is definatly lower then 1,000 right now. Dang I am goning to have to look into this further soon.

[85Bowtie]

Originally Posted by enaberif

Woah really? I have like 20psi @ 1000rpm

Yes really. That is the safe amount to have, of course others will have more than that ratio at any given RPM but that is what's considered safe.