Interesting Oil Pressure Change

[TallPaul]

In summer, my cold start oil pressure is about 54 psi. In winter (this am at 10F with no oil pan heater for example), cold start pressure was about 48 psi, same oil. Hot it will run around 44 psi, again same oil. To summarize:

10F: 48 psi
70F: 54 psi
180F (assumed temp): 44 psi

One would expect the 10F pressure to be higher than the 70F pressure since the oil is thicker, but it must be so thick at 10F that more is forced out the oil pump relief valve than at 70F. The other strange thing is that on a 10F cold start the pressure will go to about 48 psi, then as I drive it will rise a couple psi in the first mile or so before beginning to drop to it's hot pressure of 44 psi.

So at cold engine temps, thicker seems to force more oil into bypassing the system.

As a point of reference, but maybe irrelevant, at operating temperature, 10w40 runs 44 psi, whereas 10w30 runs around 36 psi. The engine seems sound, i.e., I don't have reason to think the bearings are loose, so I wonder if the bypass has a weak spring and that with a new spring, maybe 10w30 would give closer to 50-60 psi, which it has done in two 7.5L V8s I have owned.

Seemingly trivial, but maybe someone will make some interesting points about it.

 

[TheLoneRanger]

Those are all good, strong, uniform pressures, Paul. That's a very good engine you're reading from.

One thing that comes to mind is the temperature of the pressure sensing devices. I assume they're electrical, so the readings may not be exactly accurate in measuring from the cold and warm ambient temperatures......?

 

[KBFXDLI]

I have never had a decent oil pressure guage on any vehicle or boat. As long as the needle moves off zero and to somewhere in the mid range and I don't hear any banging under the hood I assume all is well in oil land.

 

[TallPaul]

Ooops. I left out the most important fact. I am running an AutoGauge (by Autometer) mechanical gauge that is divided into 2 psi increments on the gauge face (not to say it is accurate to 2 psi, but maybe).

Spec oil pressure for this engine (a Ford 4.9L inline six) is 40 - 60 psi hot at 2000 rpm. My hot readings are at 2000 rpm, which is freeway cruise 70 mph. As you can see, with the manufacturers preferred 10w30 it is running below spec, but the 10w40 gets in in spec. Regardless, the 10w30 would likely suffice.

 

[TheLoneRanger]

Quote:

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As you can see, with the manufacturers preferred 10w30 it is running below spec, but the 10w40 gets in in spec. Regardless, the 10w30 would likely suffice.

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OK, I see what you mean. Since the
10W-40 raises the pressure, I doubt if a new pressure relief spring would help. Really, I wouldn't worry a bit about it.

I've always considered oil volume more important than pressure anyway, so as long as it's got nominal pressure, it should be fine.....

 

[TallPaul]

Right. Mainly I thought it odd the pressure would be lower on a very cold morning and slowly creep up. Hmmmm, maybe it's not the pressure relief valve, but that the oil pump just can't pull it well when it is that cold. Yes, that seems a much more likely explanation and would also fit with the rise in pressure in the first mile or so as the oil starts to warm and the pump gets a better grip. Yep, I think that's it.

 

[Gary Allan]

You must be reading post filter. When I had two gauges in my minivan on the remote setup ..and it was cold with heavy oil, the upstream pressure would always be 82. With light oil I would have a startup downstream pressure of ...maybe 80. That differential would be barely detectable as stuff warmed. When the relief opened, I'd still see 82 upstream ..but maybe 72 downstream. The two numbers would converge in a short time. If the 82 fell faster and the 72 rose faster it would resemble what you're seeing. You too see an elevation in pressure as your warming it for a brief time ..then it retreats again. The normal reasoning would tell you that if this was the case, then the filter would alter your ultimate pressure as it got loaded ..but, again, as you warm up ..I've never seen much loading, in terms of PSID, in up to 9k-10k ..so

Your bypass valve may or may not be in use. It depends the spread. If you're only seeing <10psi difference ..then you may be bypassing some oil. Otherwise the differential just isn't there.

 

[TallPaul]

Yes, my pressure connection is a few inches down stream of the filter. So, it may be the oil filter bypass partly opening that is causing my weird cold oil pressure.

 

[Gary Allan]

Well, yes and no. The oil pump being in relief gives you the lower pressure ..the filter just expresses (give you a place to measure the difference in pump output and flow to the engine) it and limits it (the pressure) to the relief setting.

btw- if you want more pressure ..or a higher limit that is, just pull out that spring ..stretch it out ..stuff it back in ..and voila~! ..higher limit.

A lot of this I attribute to inertia. If the oil is too heavy, even if it wouldn't normally trip the pump relief based on visc alone, it doesn't want to move on a dime. So the pump spins its tires for a bit ..the downstream pressure, due to lower flow, is lower ..as the oil gets more fluid ..or gets fully in motion ..the differential goes away (or is reduced).

You should never see a difference greater then the bypass valve limit downstream.

That is, if you had a hollowed out filter, under the same conditions, I'd venture to say that you would see the same peak readings, when cold enough , between 20w50, 15w40, 10w-40, 10w30. The flow to the engine would vary with the weight ..but the pressure would be the same for all of them.


I say that if you start it and let it idle instead of driving ..you'll see your 54PSI before it starts dropping. If not ..then it really is a curious event.

 

[TallPaul]

Some (but I think few) engines have access to the oil pump relief valve on the outside of the block. Pretty sure mine is built into the oil pump and would require dropping the pan.

IIRC the 10w30 did give me about 54 psi cold, same as 10w40.

Yes, I think the pressure climbs from 48 towards the 54, but before it gets there the oil is warming up enough to drop the peak pressure and so it goes down before reaching 54 psi.

Hmmmm. Maybe I should let it idle for a while next time I have to clean snow off the windows, which should be today. If I have time (and remember) I'll do it.

Would be interesting to run a cover on the oil filter mount that merely is an open passage, i.e., no filter. Bob Winters did that and there is a very long old thread, but I think he was testing UOA, not pressure.

 

[Gary Allan]

Yes. I'm sure it would be a pan off thing. Some older engines had easy access.

I could be wrong (there's always a first time

) and the "inertia effect" has more of an impact in your situation. That is, that it never would have bled off any flow due to visc and it was all due to the stuff just not wanting to go from sitting still to the rate/volume that your pump demands at startup. The cheaper way to test this is to get one of those sandwich gauge adapters ..but that's still $$$ in the $35 range ..:shrug:

The oil pumps that throw a spin on this stuff are the newer Titan V8 oil pumps. They appear to have a "volume on demand" with limited pressure regulation. They don't have a high relief capacity ..but won't vary much from idle to near WOT at any normalized (or beyond a certain warming) temp ..even though the volume varies or so it seems