How Do I Verify Oil Pressure?

[JohnC]

Also, run it up to 2000 RPM. The numbers from the mechanical gauge seem low, although it does seem to be consistent with the switch. If you don't get up to about 40 psi at 2000 I'd be concerned...

Maybe the pressure relief spring is broken...

[DmaxMaverick]

I agree. But the DG should not be indicating that high, if it is in fact remotely close. If the relief spring is broken, the DG should not indicate such a high pressure in any case. I too would be concerned if the pressure(s) doesn't indicate higher, even at 1000 RPM's. You need to see the the actual (or as close as possible) pressure at the bearings. The main gallery (where the OEM sender should be getting the input) should indicate a pressure closer to that delivered to the bearings and lifters. If your pressure is in fact that low, you have issues that need to be addressed to keep a healthy engine.

[DennisG01]

At 2,000 RPM's, the MG (at turbo) reads 32psi, while the DG reads 42psi.

My DG readings seem to be inline with what it was before the new engine and also with what I remember reading about here on the Page in regards to other member's trucks and their readings - of course this is if "IIRC" .

Also, correct me if I'm wrong, but isn't the GM spec, at idle, for minimum PSI about 10psi?

I would like to get a reading at the stock location for the pressure sender, but it looks to be too much of a hassle for my purposes.

[Hubert]

I am surprised at the readings. Its just not intuitive to me that the turbo location would have the same psi reading as filter head so far down stream and across filter media and all the loss of oil flow through various other paths. But pressure isn't everything. Flow is still unkown. All we really know is the orifices and path sizes are at least balanced well enough to still maintain pressure at turbo feed for its gpm flowrate.

I have to go back and relearn why the Bernoulli equation breaks down so and yields such poor estimates of pressure drop. I think its the size of the path and direction changes and diameter changes along the way that I am not accounting for.

Anyone want to say that and or this better. I think it relates to flow will always take the path of least resistance. Resistance is measured by psi so psi is the same for the system because of conservation of mass if pressure were lower somewhere more mass would flow that direction until it is restricted enough to empede flow. So as long as your feed flowrate exceeds all your exits by an appreciable amount it all equals out to flowrate follows path of least restriction. Flow rate will be different according to bearing clearances and path diameters but pressure remains the same???.

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I thought GM spec was 10 psi per 1000 rpm.
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I think if you measured say at remote mount line leaving the block and the remote mount line coming back to the block you might see a pressure drop due to friction but once you go down stream and are necked back down pressure increases again. And like I said above flowrate of oil pump still exceeds all the "exits" of the path so oil pressure is relatively the same measured for "the engine" but flowrate volume is considerably different.

[JohnC]

I think the velocity is relatively low in this system so the Bernoulli effect is minimal. The gallery size is large relative to the "leaks" so the system looks more like a hydraulic cylinder that a flow system. In a hydraulic cylinder, pressure is evenly distributed over the entire surface, so the pressure at the turbo end would be the same as the pressure at the pump outlet. Any pressure drops are proportional to flow rates; slow flow == minimal pressure differences.

The 10 psi/1000 rpm is a generally accepted guideline, not GM and not a limit. I'm sure GM has a published limit, in fact I seem to recal seeing it somewhere in the service manual, but the "for public consumption" limit is "not in the red". I myself would be a little leary of pulling a trailer at 65 mph with oil pressure just a hair above the red...

[Hubert]

I looked it up for reference my Helm Manual for my '97.... (pg 6A-258 Engine Mechanical Specifications).

oil pressure - hot

10 psi @ idle
40-45 psi @ 2000 rpm

69 Kpa @ idle
275-310 Kpa @2000 rpm

[Hubert]

Great now this is clear as mud.

Driving last evening I looked and yep my dash gauge reads 42 psi @2000 rpm hot according to spec. Which I think is pretty normal I think. How many have verified actual oil pressure? I wonder if Helm manual spec is the dash gauge number or an actual independant measurement????

Looking at the oil path diagram it looks like the right oil gallery is fed via the back cam bearing. And the turbo feed is at the front end of the gallery. The left oil gallery is fed from either the oil filter or the bypasses. The left gallery feeds all the other cam bearings and then flow down to crank etc or up the lifters etc. So I can still wonder if there is some loss of flow affecting the measurements. There is a note of a cup plug with 0.06" hole at the filter base item 7 in my helm book diagram. Not sure what that does. Anybody know?

John brings up a good point but if you guestimate 1-3 gpm flowrate. Which I think is in the ballpark??? Velocity of oil through a 1/2" diameter hose would be roughly 1.6-5 ft/sec (unless I had a math error; very possible) which is not all that slow of flow???. I still wonder if high viscosity of oil doesn't affect things.

I just can't understand/explain the left oil gallery being at a significantly higher psi than filter head on inlet side.

Dennis do you have routing of remote lines crossed? I think at engine base OUT goes to filter head IN then filter head OUT goes to engine base IN right?

Any other thoughts? Just bad oil pump, poor dash gauge, and/or loose clearances?

[ronniejoe]

The clearances are dead on the nominal and I have photographic evidence to document the assertion. The pump is a brand new Melling unit. There is nothing wrong with the engine. This is simply a measurement accuracy issue.

I was the one who suggested checking pressure at the turbo feed. I didn't expect much drop from the core where pressure is measured for the gage. Maybe there is more than I thought. At this point, to put all the guessing and all the speculation about a "bad engine" to bed, you should remove the plug at the back of the block and install a tap for a mechanical gage there. This location is accessible, it just isn't convenient. Gage calibration should be confirmed first to ensure accurate readings.

Have you considered that the pressure switch on your light might be out of calibration?

[DmaxMaverick]

Like said before, if you are going to compare pressures with differing devices, you MUST sample pressure at the SAME location, however inconvenient. Or, use the SAME device to sample at the different locations. There seems to be two objectives here (in regards to this discussion). One is the comparison of pressure samples at different engine locations, and the other is the comparison of different sampling devices. Keep that in mind. Unless you have all your devices calibrated to the same standard, which is an active process, not assuming anything is correct "out of the box", your sample data will be about as accurate as a tabloid news article (sometimes, they get one right, but it's pure luck or coincidence). In the end (as we now see), you will generate more questions than answers. The mere process of your testing will invalidate any data collected.

The GM gage system(s) are notoriously inaccurate, but they aren't generally that inaccurate. If it is inconvenient for you to access the correct sampling location, then you'll have to live with the results you get, and go on scratching your head. If you continue a relative comparison of apples and oranges, your result will yield the same unanswered question at the conclusion.

[JohnC]

The mechanical gauge has determined that the readings at the filter and at the turbo are for all practical purposes identical. What you have not determined is the accuracy of the mechanical gauge. Without doing that, it seems that all the speculation is just speculation.

[DennisG01]

First, I absolutely don't believe there is anything wrong with the engine - from the amount of care that Ron put into building it and the fact that the dash gauge is reading similar to what I saw with the old engine. I agree with everyone that is saying it's simply a case of whether the gauge readings are accurate. Although, both gauge readings do seem to be consistent. If I can find the time to get a reading at the unused port on the back of the driver's head, that should help quite alot to clear things up.

Knowing exactly what the readings are at various locations has never been a priority for me - I know some of you are actually more interested in this than me - and that's OK, I am interested to find out the results, it's just not my main purpose.

This thread has helped me quite a lot to understand more about this 6.5! Thank you!

However, for my purposes, I'm satisfied in knowing that everything is operating within specs, I'll just need a lower PSI switch. But, I will keep up on this and make an appointment at my shop to use their gauge. I can't promise anything, but I will try to get readings at that unused port.

Dennis do you have routing of remote lines crossed? I think at engine base OUT goes to filter head IN then filter head OUT goes to engine base IN right?

That is the way I figured it, too and that is the way I put it together - but I will double check.

Gage calibration should be confirmed first to ensure accurate readings.

Have you considered that the pressure switch on your light might be out of calibration?

Can I verify accuracy with my air compressor? I mean, PSI is PSI, right?

Yes, I have thought about the switch being wrong, although it does seem consistent with the MG (of course, at this point that's assuming the MG is accurate)

The mechanical gauge has determined that the readings at the filter and at the turbo are for all practical purposes identical. What you have not determined is the accuracy of the mechanical gauge. Without doing that, it seems that all the speculation is just speculation.

ditto.

[DennisG01]

FOR CLARIFICATION:

At Remote Filter Mount:
RPM MG DG
Idle 13 35
1000 17 38
1300 19 42

And, at 2000RPM, the numbers were 32 and 42.

OK. Final take on numbers gathering here: This morning I went to the garage that did my engine install and used their gauge. When I asked about the accuracy of it, he gave me a little smirk and said "it's dead on - we use it all the time on lots of cars and it's always been accurate". That was good enough for me - besides, I've known these guys for years and they have always proven trustworthy.

Over the weekend, I did the "high idle" mod, so that's why the RPM numbers below seem odd. Except, of course, the 2000 rpm, where it's all up to the steadiness of my right foot . I only took readings at the filter mount as we've already established consistency between there and the turbo.

------ MG DG
IDLE 12.5 33
1070 25 37
1360 32 40
2000 38 42

Having used a more accurate gauge, it's good to see that the readings have risen. I think we still need to keep in mind that the dash gauge is run off a (typically) less accurate type of pressure sending unit than a good mechanical gauge. In fact, the mechanic and I were talking for a minute or two while I was at the 1070 setting and I happened to notice the dash gauge jump up about 4 psi for about 10 seconds, then back down. During this time, his MG never waivered.

As far as getting a reading at the unused port we talked about, it's not gonna happen. We both looked at it while the truck was on the lift and decided someone would have to be Houdini to be able to easily do that. And I didn't want to pay the labor bill to have him do it any other way - he agreed.

I bought a switch today that is supposed to activate between 8 and 12 psi. We'll see how it works out. If the light is still coming on, I know of one more source for a 7 psi switch.

Again, I appreciate all or your comments throughout this somewhat "minor" post. Thank you!

[Hubert]

Thanks for the follow up. That looks much better and makes more sense than other mechanical gauge reading.

Interesting about difference at low flow (idle) but closer at higher flow.

FYI Lawson Products had several different oil pressure switches when I was looking through there book a while back. Lawson products dot com or something like that.

[DennisG01]

Well, so far so good with the 8-12 psi switch.

Interesting about difference at low flow (idle) but closer at higher flow.

FYI Lawson Products had several different oil pressure switches when I was looking through there book a while back. Lawson products dot com or something like that.

I thought it was interesting, too. Since the stock sender is electronic, who knows how GM has it calibrated. My guess is GM has it calibrated to purposefully read high at low RPM's so they don't get complaints when customers would see their dash gauge reading so low.

Thanks for the link!

[JohnC]

... When I asked about the accuracy of it, he gave me a little smirk and said "it's dead on - we use it all the time on lots of cars and it's always been accurate".....

Compared to what? I think the smirk was the most important part of the answer...

Seriously, in the aircraft maintenance business, we have to send everything out annually to a testing service which can trace its accuracy back to the NIST. If the feds come in and can't find a current sticker on my torque wrenches and pressure gauges, I'm busted...

[DennisG01]

Oh, sorry about that, I was actually paraphrasing a bit. I can see how what I wrote might have been a little misleading as to our actual conversation. He did use the word "calibrated" in our conversation, but to be honest, I don't remember everything he said associated with that (at least not well enough to accurately repeat what he said). What I do remember is that based on what he said I was convinced that this is an accurate gauge. If I was more "in the know" on the process involved, I'm sure I would have remembered more or asked better questions. However, I have known these guys for quite a while and have come to trust them - if they tell me something, I believe them. Don't get me wrong, I don't have "blind" trust - I mean I like to hear explanations, but I do trust them.

[Hubert]

Anyone have a picture or remember the view of the engine block oil filter base. In my minds eye I remember the outlet port and I think the bypass valve and knipple but fuzzy on the cup plug with orifice.

I am still wondering about this 1/16" orifice cup plug. I want to say it might be something like this. Oil comes from pump to cooler or bypass then to outer portion on oil filter base. On the outer ring of oil filter base is the cup plug with orifice and the bypass valve both lead to left oil gallery. Oil would go down into filter or adapter and up through cup orifice and if too much resistance up through bypass valve. But normally should go down into outer oil filter can, through media, then up center of filter and back through knipple on block towards the oil galleries.

So does a % of oil always escape filtering through this cup plug orifice? And if so at low flow could that explain an appreciable difference in left oil gallery pressure vs higher flow? comparing remote mount measurement to left oil gallery?

[DennisG01]

I looked through all of my engine rebuild pics, but didn't find one that would help. However, would a pic of either end of the 90* adapter work? It's just laying on my workbench.

[Hubert]

Thanks but I specifically need the engine block view as to understand where oil flows from and to (and or options to flow as in through cup plug and or bypass valve).

If no one has a picture or memory of it to confirm accuracy of what I just posted then I'll try and take a picture.

[ronniejoe]