Low oil pressure....sometimes

[DougAMiller]

I'm certain that I had 8 quarts in it at the time. I'm not sure of the angle that I had it pitched down, but I'm sure I can measure it. It was the combination of the angle of my driveway and the height of my ramps. I know the wheelbase and the height of the ramps, so all I need to do is determine the angle of the driveway. Should be able measure it tomorrow.

 

[iamdub]

Isn't that what he did?

Maybe. He said he did the overfill test- but how much did he overfill it? Then he backed the rear up on ramps- that's not much of an angle. IMO, based on this, there's a chance the O-ring still didn't get submerged.

 

[iamdub]

20-30 degree nosedive is steep as hell. 20 degrees is a 36% grade. Bear in mind that's a slight bit steeper than Baldwin street

Another consideration is that as soon as you turn the car on, oil's going to be out of the pan and circulating/dripping through the engine, which will cause the level to drop.

if anyone could get some detailed measurements of the pan, perhaps we could calculate the rough volume to fill it. I'd crawl under and do it myself but I don't have a tape measure with me

Lol
That definitely puts a better visual on it! Like I said- rough estimate. I literally just held my finger up to your pic, parallel with the flange of the pan then rotated my laptop. At 2100, I'm not gonna have the sharpest mind to picture a protractor's scale over the screen.

 

[Reebok59]

Food for thought. I found this on the GM Truck forum last night. I found it interesting enough to share here. (Posted by some other guy, not me)

Start guote.

"I had similar issue with my 2005 Suburban 1500 270k miles. On cold starts, the oil pressure was very slow to rise and would cause the oil light to come on. Once at operating temperature, the pressure would stabilize slightly below normal at cruise RPM. When the engine was still below normal operating temperature, the pressure would drop off very low at idle RPM to the point that the engine would begin to knock.

Based on everything that I have read, I have a worn O-ring on the oil pickup tube.

Here is what I did and the problem has completely gone away.

The o-ring is above the oil level. When the oil is cold and thick, the oil pump would suck in air around the o-ring rather than suck oil up the pickup tube, causing the very low oil pressure on cold starts.

I simply changed my oil from 5w30 to Mobile 1 0w30. The 0w allows the oil to flow much easier when cold, allowing it to be sucked up the pickup tube rather than the pump sucking in air around the worn o-ring.

Go figure. It worked! The oil pressure is perfect now, even on cold starts."

End quote.

 

[Reebok59]

There may be a hybrid test procedure buried in the quote above. Could simply try 0w30 on next oil change and if that solves oil pressure issues, I would think it confirms an o-ring problem.

 

[DougAMiller]

There may be a hybrid test procedure buried in the quote above. Could simply try 0w30 on next oil change and if that solves oil pressure issues, I would think it confirms an o-ring problem.

I still like the overfill idea the best, just need to know how much additional oil needs to be added to submerge the o-ring.

That being said though, there are a couple of other things that need to be considered. One has already been mentioned here, that is how much oil gets sucked up out of the pan and is circulating through the engine. That could expose the o-ring if there isn't enough extra oil to account for that. The second thing that has occurred to me is the reason that it is a problem to overfill an engine. And that is the fact that rotating components that contact the oil in the pan will whip it into a froth, which doesn't provide sufficient lubrication for the engine. Now for a brief pressure test at idle I wouldn't worry too much about this, but the whipped oil is going to be lighter than oil that isn't and therefore will rise to the top. This frothy oil likely would not seal the o-ring well and since it has air in the oil it is still going to result in air being sucked in past the o-ring.

OK, after writing that I have about convinced myself that the lighter-weight oil might be a more reliable test.

 

[madbuilder]

I see the rationale of switching to a 0w30 synth if you suspect the O ring. The pump is probably just a few inches above the oil (?) so the suction depends more on viscosity (in turn, on temperature) than it does on opposing gravity.

If that test gives a greater cold idle pressure, I'd take it as conclusive enough to do the repair.

 

[iamdub]

I think the lighter oil test is valid, but limited to the size of the leak in the O-ring seal.

 

[iamdub]

I'm certain that I had 8 quarts in it at the time. I'm not sure of the angle that I had it pitched down, but I'm sure I can measure it. It was the combination of the angle of my driveway and the height of my ramps. I know the wheelbase and the height of the ramps, so all I need to do is determine the angle of the driveway. Should be able measure it tomorrow.

I'm wondering if one more quart or an even steeper pitch would've yielded different results. Knowing the angle you had yours at would be really helpful. Knowing the angle at which your engine sits in the bay and the angle of your Tahoe at ride height are other factors to consider. One could fill a spare/junk oil pan with 6 quarts of water then tilt it to the net degree after calculating your factors. Then, add more quarts until the water was at the flange. This would tell you if you had enough oil or slope as well as determine a relatively accurate figure of how much slope and how much overfill is needed for anyone else to soundly test this going forward.

 

[HiHoeSilver]

I'm wondering if one more quart or an even steeper pitch would've yielded different results. Knowing the angle you had yours at would be really helpful. Knowing the angle at which your engine sits in the bay and the angle of your Tahoe at ride height are other factors to consider. One could fill a spare/junk oil pan with 6 quarts of water then tilt it to the net degree after calculating your factors. Then, add more quarts until the water was at the flange. This would tell you if you had enough oil or slope as well as determine a relatively accurate figure of how much slope and how much overfill is needed for anyone else to soundly test this going forward.

Will wait for you to report your findings on this.