What is the oil pressure supposed to be?

[Adamds86]

Hello all, got a question about my 1973 GMC K25, 350 with 4 speed SM465. I was driving it today and noticed that at idle or stop sign, when warm, the oil pressure is bottomed out. While running it goes up to the 1st line. So my question is if this is normal? What pressure is it supposed to run at? Also I am pretty sure I have a mechanical pressure gauge, I am wondering what range the marks go by, 15 psi maybe?

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here is a pic of what mine looks like...this isn't mine it's just a pic of one.

Thanks

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[yevgenievich]

Verify with a different gauge, but generally around 10psi at idle and 10psi per 1k of rpm's

 

[bucket]

If the oil pressure actually does turn out to be that low, it should still be fine to drive. As long as its not clicking, clacking, tapping or knocking, it probably has enough oil pressure to keep it alive.

 

[Jrgunn5150]

My dad sold me an old Highboy once, a 77 with a 351M, that had zero running pressure. I went back to my parent's and bitched at him about it, he said it had been like that for year's, figured the gauge was flaky. I went and go another mech gage, same result.

I lost alot of sleep over it for about a month, then I just embraced it, it ran fine for the 8 month's or so I drove it, daily, about 60 miles.

So while I agree, it "should" be 10 psi per 1,000 rpm, and low pressure when warm is a sign of increased bearing clearances, or wear. It's probably fine.

 

[CSFJ]

Try a new sending unit. Bought a truck not too long ago with the same syptoms. Turned out the sender was bad.

 

[Adamds86]

Thank you everybody, I started it cold and it jumps right up to 30 PSI, SO, I am going to get a remote gauge to check it and if it is more PSI on that gauge I will try a new sending unit. I will update my findings. Thanks

 

[Green79Scottsdale]

The viscosity of the oil can have an effect too. Conventional 30w type oil is thicker and will show a higher pressure. Synthetic 5w-30 type is thinner and will show lower pressure.

 

[MikeB]

I am going to get a remote gauge to check it and if it is more PSI on that gauge I will try a new sending unit. I will update my findings. Thanks

That's the best plan. Personally, I'd like to see no less than 10 at idle when warm, and no less than 30 when running down the road. If it's lower than that, I suggest 10W-40, or maybe even 20W-50 (assuming you are currently using 10W-30).

 

[Adamds86]

Just bought the gauge...and I also bought some 15w-40....figured it would help it if it was low.

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[chengny]

From the GM spec sheets on the small block engine series:

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[MikeB]

Just bought the gauge...and I also bought some 15w-40....figured it would help it if it was low.

Be sure to let us know how that works.

 

[Adamds86]

Be sure to let us know how that works.

Hooked up the gauge and it is low when it is warm. about 3 psi. I changed the oil and added the 15w-40. That definitely helped with the pressure. When warm and going down the road it is at about 28 psi and at idle its at about 7 psi. I haven't noticed any clicking, clacking, clattering at all so I am going to say it should be just fine. Thanks everyone

 

[MikeB]

Well, that's better but still a little low. I certainly would not rev the engine too high or pull a trailer! Next time try 20w-50. But it sounds like an engine rebuild, or at least main & rod bearing replacement, is in your future.

I bought a 55 Chevy car many years ago with a old 350. It ran well and used very little oil, but pressure was low like yours. Being on a budget back then, I dropped the pan (very easy on a 55 Chevy) and replaced the rod bearings and oil pump.

With that and 20w-50 oil, the pressure jumped significantly--around 15 psi or more at idle and at 1500-2500 RPM. I put another few thousand miles on that engine and sold it to a guy who put it in a old pickup. I know he drove it for at least two years before I lost touch with him. So it ended up being more than a temporary fix.

A machinist I know told me later that I should have mic'd the rod bearings, and had they been .021" or more undersized, instead of the nominal .020", I could have used .021" bearings for even more oil pressure.

 

[Adamds86]

Well, that's better but still a little low. I certainly would not rev the engine too high or pull a trailer! Next time try 20w-50. But it sounds like an engine rebuild, or at least main & rod bearing replacement, is in your future.

I bought a 55 Chevy car many years ago with a old 350. It ran well and used very little oil, but pressure was low like yours. Being on a budget back then, I dropped the pan (very easy on a 55 Chevy) and replaced the rod bearings and oil pump.

With that and 20w-50 oil, the pressure jumped significantly--around 15 psi or more at idle and at 1500-2500 RPM. I put another few thousand miles on that engine and sold it to a guy who put it in a old pickup. I know he drove it for at least two years before I lost touch with him. So it ended up being more than a temporary fix.

A machinist I know told me later that I should have mic'd the rod bearings, and had they been .021" or more undersized, instead of the nominal .020", I could have used .021" bearings for even more oil pressure.

Yea I know that I am going to have to eventually rebuild it or replace it. It currently has 162,000 miles on it which in my opinion is a lot for an old GM 350 small block. I only drive it once in a while and i don't think I will ever pull anything with it (even though it is meant to pull, 4x4, SM465 4 speed with 4.56 gears lol) I will wait til this fall and when I have some cash to do a rebuild on it and maybe give it some better performance parts. Thank you so much for all of your time and help.

 

[Camar068]

So to add to the info on this thread, what if you had an engine that ran ~30psi at idle and say ~50psi down the road....then all of a sudden idle dropped to zero and 30 psi down the road. No clanking, knocking or anything. After verifying it's not the sender and no oil leaks, keep cruising 'til it drops? Up the oil weight as some have mentioned? I've replaced 2 engines through the years cuz I'm kinda anal about oil pressure.....can't stand seeing zero.