Losing oil pressure when warm???

[thefarmboy21]

For a while now, I've noticed that the 350 in my 86 has been losing oil pressure when it gets up to normal operating temperature. Cold start it holds about 30-40 at idle (500RPM)  and 50-60 at 2000-2500RPM. It had 15w40 RotellaT in it when I first noticed, so I did an oil change thinking maybe it was a clogged filter and made no difference. Changed it again and dumped in straight 30 weight, thinking that it would hold the same pressure at operating temp as when cold.....NOPE even with the straight weight it gets warm and drops oil pressure. It's been noisy uptop in the lifter area for a while (sounds like an old mechanical injected diesel) so I'm wondering if one or more of the lifters have a hole in em and/or its wiping the lobes. Any ideas? Thanks.

[zieg85]

It is normal for reading to be higher when the oil is thicker.  You need to determine if the oil pressure gauge is accurate before you do anything.  Slop in bearings is usually the cause but your oil pump may be starting to fail.  How many miles do you have on the engine?

[thefarmboy21]

Yes I know it's normal for the psi to fluctuate with thicker oil and/or hotter temps. But even with SAE 30 it's dropping from 60 cold to 10-15 hot. That's shouldn't happen.

[boulky12]

Oil pressure in my truck is 60 @ idle once started, and comes down to 10-15 when warmed up, and she's never skipped a beat.

I always thought this was pretty normal.

[thefarmboy21]

Oil pressure in my truck is 60 @ idle once started, and comes down to 10-15 when warmed up, and she's never skipped a beat.

I always thought this was pretty normal.

I know an old school mechanic who's been working on diesels since he was a kid in his dad's big semi garage and he's always told me that an engine only needs 6psi to survive, but the point of the matter is that it's getting worse AND it's losing pressure even with SAE30 oil, which is even more odd. Either way, I still drive it and intend to do so until it's either blows or I get another one rebuilt for it. Just thought if it was something I could replace in the truck without rebuilding it, that I would.

The engine has been a good one considering what it is. I built it back when I was running demolition derbies and wanted a "loose" engine, but also one that was "fresh" and reliable. It's an old 4-bolt 350 from the early 70's with Pistons from two different engines in it haha. Two had too much piston slap, so I swapped a couple out and they're technically facing backward because they were from the opposite side of the other engine, so the notches that represent FRONT are facing the wrong way. Besides that, the cylinders were up to .012 over stock. And the crank took me like an hour to clean up with Emory cloth and WD-40 to clean the rust off of. I built it back in 2009 I think. Has a Comp Cams extreme 4x4 cam, Clevite 77 bearings, Melling oil pump, double roller timing chain and currently some swirl port heads and edelbrock intake and carb. I tore it down when we floated the valves in a derby and the bronze valve guides in the race heads started pushing through the heads So I decided to swap the heads and throw it in place of the tired 292 that was in the truck. It's had a rough life but it's always held good oil pressure until recently. BUT I didn't replace the came bearing back then and I know they were worn....I just didn't have the tools or knowledge back then. So between the cam bearings and floating the valves, the valve train has gotten rattley and I'm wondering if that's where my issue lies.

[bd]

Worn cam bearings are a good bet.  Install a high volume (not high pressure) Melling oil pump and hardened oil pump drive shaft.  While you have the pan off, at the very least check the rod and main bearings with some Plastigage.

[thefarmboy21]

Worn cam bearings are a good bet.  Install a high volume (not high pressure) Melling oil pump and hardened oil pump drive shaft.  While you have the pan off, at the very least check the rod and main bearings with some Plastigage.

What's the deal with HV oil pumps? I've read articles upon articles about them and seems like most things over read imply that unless you're making 400+ HP or have a large sump oil pan....they're not really that beneficial? BUT there are still tons of people who switch to them, so what's the reasoning???

[VileZambonie]

HV oil pumps are good for loose clearances.

[Stewart G Griffin]

What type of gauge are you using to determine the low oil pressure?   Although i feel 10-15lbs. at idle is ok.

[Irish_Alley]

rule of thumb is 10 psi per 1k is good.

[thefarmboy21]

I think it's a sunpro gauge...it's mechanical regardless.

 Ironically enough I drove it this evening after church and the pressure has dropped even more, to around 7-8 psi

Looks like I'll either be building my Vortec engine sooner than planned, or borrowing the 350 in my 77 for a little while.

[Stewart G Griffin]

Has the oil light come on?   i think as long as the ALL of the engine is getting oil  (volume vs. pressure) you're ok?

[Irish_Alley]

again 10 psi per 1k rpms is ok. so even if youre running 7 psi at idle (700 rpms) its still ok. gm oil pressure sensor for the fuel pumps only runs if oil psi is at 3 psi or more.

[thefarmboy21]

It's a mechanical gauge. The factory gauge isn't hooked up.

It doesn't concern me as much that the oil pressure is low, it concerns me that in the last 100-200 miles it's dropped 15 psi or more. Last couple times I changed the oil, the magnetic oil plug has been covered in minuscule particles. Usual looks like a grease glob, but it's obviously metal.

[bd]

It's been noisy up top in the lifter area for a while (sounds like an old mechanical injected diesel) so I'm wondering if one or more of the lifters have a hole in em and/or its wiping the lobes.

Ironically enough I drove it this evening after church and the pressure has dropped even more, to around 7-8 psi

...it concerns me that in the last 100-200 miles it's dropped 15 psi or more. Last couple times I changed the oil, the magnetic oil plug has been covered in minuscule particles. Usual looks like a grease glob, but it's obviously metal.

More than likely the cam lobes and lifters are wiped.  Metal is churning through the oil pump, clearancing the pump and holding the pressure dump (bypass) valve ajar.  Hopefully, as long as the oil filter hasn't been overwhelmed with fine metal particles that plugged it, forcing the filter bypass valve open, the filter has done its job and prevented metal from reaching the main, rod and cam bearings.  You need to park it and open it up for inspection before you blow the motor.