Low oil pressure....sometimes

[iamdub]

Really? I did not know that. The aluminum castings do appear to have come out of the same tooling, but the Melling assembly is somewhat different. The Melling front cover is CNC cast iron, GM is stamped steel. The Melling housing is hardcoat anodized, GM does not appear to be, but not sure. Don't know about differences with the internals.

GM on the left, Melling on the right. This is the standard Melling pump, not HP or HV.

View attachment 228672

View attachment 228673

Interesting. I've never compared them side-by-side. I can see the differences in the front cover and clean-up after machining being expendable for manufacturing costs. But a comparison of the internals and performance of each would bear more significance. It's really no big deal as they're both around the same price and, after seeing the finishes, I'd spend a little more on the Melling just for that.

Melling, AFAIK, is the OE supplier but their pumps that you and I buy are improvements over the original according to this article:

https://www.enginebuildermag.com/wp...es/03_01_2012/986163348GMGENI_00000054947.pdf

Lots of good info in here even for you Gen III LS guys.

@swathdiver, any idea what the improvements are?

I just looked up the pics from my brother's truck and realized the front cover differences between the pumps. I put the M295HV on his but I swapped in the low pressure spring. I'm glad I did cuz it holds ~30+ at hot idle and pegs out the pressure gauge near redline.

 

[swathdiver]

Interesting. I've never compared them side-by-side. I can see the differences in the front cover and clean-up after machining being expendable for manufacturing costs. But a comparison of the internals and performance of each would bear more significance. It's really no big deal as they're both around the same price and, after seeing the finishes, I'd spend a little more on the Melling just for that.



@swathdiver, any idea what the improvements are?

I just looked up the pics from my brother's truck and realized the front cover differences between the pumps. I put the M295HV on his but I swapped in the low pressure spring. I'm glad I did cuz it holds ~30+ at hot idle and pegs out the pressure gauge near redline.

I thought they were spelled out in that article.

 

[DougAMiller]

OK, took it to a shop today to get their assessment. Even though the overfill test didn't kick the pressure back up, I am still thinking o-ring just based on the symptoms. Anyway, they say everything is fine, that it is above the minimum spec, which they state is 6 psi at 1000 rpm. That didn't sound right to me, so I looked it up. Yep, GM specs on the Vortex 5.3's are minimum oil pressure is 6 psi @ 1000, 18 psi @ 2000, and 24 psi @ 4000. So, I'm certainly above the minimums. The only time it gets close is after a cold start, right after it goes from high idle to low idle, but before the oil warms up, it will sometimes get down around 7-10 psi.

I guess as far as risking engine damage, I have sufficient pressure, it's just that it was only 2-3 months ago I don't think I ever saw it under 30 psi. Still feeling like the o-ring should be changed.

 

[iamdub]

I thought they were spelled out in that article.

I blew right past that part every time. It jumped out once I slowed my scrolling and included reading the bottom of the pages.

 

[iamdub]

OK, took it to a shop today to get their assessment. Even though the overfill test didn't kick the pressure back up, I am still thinking o-ring just based on the symptoms. Anyway, they say everything is fine, that it is above the minimum spec, which they state is 6 psi at 1000 rpm. That didn't sound right to me, so I looked it up. Yep, GM specs on the Vortex 5.3's are minimum oil pressure is 6 psi @ 1000, 18 psi @ 2000, and 24 psi @ 4000. So, I'm certainly above the minimums. The only time it gets close is after a cold start, right after it goes from high idle to low idle, but before the oil warms up, it will sometimes get down around 7-10 psi.

I guess as far as risking engine damage, I have sufficient pressure, it's just that it was only 2-3 months ago I don't think I ever saw it under 30 psi. Still feeling like the o-ring should be changed.

If the average pressures are getting lower and lower, I wouldn't blow it off just cuz they're still "above minimum spec". If they continue to get worse, it may be too late by the time they start averaging below the minimum.

If you did a proper O-ring test and it didn't make any difference, I'd suspect at least the pump. I say "at least" because there are other causes for increasingly dropping oil pressures that you won't find until you open it up. The O-ring will be replaced along with the pump.

 

[DougAMiller]

If the average pressures are getting lower and lower, I wouldn't blow it off just cuz they're still "above minimum spec". If they continue to get worse, it may be too late by the time they start averaging below the minimum.

If you did a proper O-ring test and it didn't make any difference, I'd suspect at least the pump. I say "at least" because there are other causes for increasingly dropping oil pressures that you won't find until you open it up. The O-ring will be replaced along with the pump.

No, not going to ignore it even if the shop doesn't think anything is wrong. Because the lowest pressure is at low idle before the oil is fully warmed up, I'm still thinking it's sucking air past the o-ring. Since dropping the pan is about half of the hours to replace the pump, I am probably going to take the risk that it is just the o-ring and tell them to go ahead and replace it.

 

[SnowDrifter]

Those specs are ******* junk and should be thrown out. 6psi at 1k would toast your engine quite quickly. These rigs really need 25+ at idle

 

[DougAMiller]

Those specs are ******* junk and should be thrown out. 6psi at idle would toast your engine quite quickly. These rigs really need 25+ at idle

Have to agree. I am not at all comfortable with 6 psi. I may not need 40 psi at idle, but I'd like it to never go under 20, and 30 minimum would be preferred. We'll see what a new o-ring does. Given that I have had this truck since new and have always done the maintenance much sooner than recommended and have always put MobilOne synthetic in it, I would be surprised if there is much wear yet. Hell, it's hardly broke in, it's only got 179,000 miles on it!

 

[Erickk120]

Mine sits around 45+ on a cold start, after it has gotten to operating temp it sits at 35+ stable. That engine has 226k miles. I would look into the pump if nothing changes with a simple Oring, might as well just do it all at once. Now if that does nothing, then I would be concerned. Any sludge build up when taking the pan off keep us posted.

 

[DougAMiller]

Based on careful consideration of all of the symptoms and what I have already ruled out, I'm pretty well convinced this is the o-ring. Shouldn't be any sludge, it's been fed nothing but MobilOne synthetic since new, and frequently. The shop that I had look at it didn't want to say anything was wrong since they never saw it go below spec, but I'm going to have them replace it next week. I had to get it back from them because I'm moving my daughter into her dorm this weekend and need it.

 

[Reebok59]

I am very interested in what you find changing out the o-ring. Mine is basically doing the same thing. Repeatable low oil pressure on low engine temps. Just the opposite of what it should be. Been watching for about 6 months now. The oil press will drop below 20 psi until it hits operating temp at 210 F. Once at operating temp, no problems, always 30 to 40 psi depending on rpm. Which makes me think an o-ring problem. It may be swelling up with increased temperature. Until it warms up, I am always putting it in nutural at red lights just to keep it above 20 psi. Once at 210, no problems. Also no problems if it cools between daily uses (hours between uses). But if it's days or maybe even 24 hours, back to low pressure issues. Very repeatable issue and would like to get it fixed. The Chevy place is clueless !

 

[DougAMiller]

I am very interested in what you find changing out the o-ring. Mine is basically doing the same thing. Repeatable low oil pressure on low engine temps. Just the opposite of what it should be. Been watching for about 6 months now. The oil press will drop below 20 psi until it hits operating temp at 210 F. Once at operating temp, no problems, always 30 to 40 psi depending on rpm. Which makes me think an o-ring problem. It may be swelling up with increased temperature. Until it warms up, I am always putting it in nutural at red lights just to keep it above 20 psi. Once at 210, no problems. Also no problems if it cools between daily uses (hours between uses). But if it's days or maybe even 24 hours, back to low pressure issues. Very repeatable issue and would like to get it fixed. The Chevy place is clueless !

I was also a little disappointed that the shop I took it to didn't have a more definitive answer than "well, it is above the minimum spec", but I decided to go ahead and have them replace the o-ring. They are doing the work today, so hopefully I will see normal oil pressures when I get it back.

Hated to let someone else work on it, I prefer doing my own repairs, but I just don't have the time right now. Since they are going to have to drop the front axle, I gave them the axle seals to replace that I have had sitting around for a couple of months too.

 

[swathdiver]

I am very interested in what you find changing out the o-ring. Mine is basically doing the same thing. Repeatable low oil pressure on low engine temps. Just the opposite of what it should be. Been watching for about 6 months now. The oil press will drop below 20 psi until it hits operating temp at 210 F. Once at operating temp, no problems, always 30 to 40 psi depending on rpm. Which makes me think an o-ring problem. It may be swelling up with increased temperature. Until it warms up, I am always putting it in nutural at red lights just to keep it above 20 psi. Once at 210, no problems. Also no problems if it cools between daily uses (hours between uses). But if it's days or maybe even 24 hours, back to low pressure issues. Very repeatable issue and would like to get it fixed. The Chevy place is clueless !

This sounds more like a tired motor liking higher viscosity oil. So when she heats up, the oil is now at 30 weight and the pressure goes up then cold at 5 weight.

 

[SnowDrifter]

This sounds more like a tired motor liking higher viscosity oil. So when she heats up, the oil is now at 30 weight and the pressure goes up then cold at 5 weight.

Not sure that's fair to say. Oil doesn't change from a "thin" viscosity when cold only to thicken at operating temp.

5w-30 just means it's a 30w at operating temp, but the oil when cold, will not thicken at the same rate as an SAE30. It'll maintain some better flow characteristics between a 5 weight and a 30 weight, but it WILL be many many times thicker when cold.

See this graph here

Fun point of note; see how the viscosity vs. temperature change is shallower with the 5w-30 as opposed to the 10w-30? If you were to continue to plot this function above 100c and look at it closer, the 5w30 would actually maintain a thicker viscosity than the 10w30 as temps rise. Essentially the higher the viscosity index of the oil, the shallower that temp vs. viscosity slope is. Not as thick when below 100c, not as thin above, with 100c being the intercept point. It's why I always recommend oils with a good viscosity index, and never subscribed to the "you need to run a 10w30 for hot weather because it's thicker" ideology - it's just wrong. There are some cases where you might want that, but trying to argue temps and viscosity isn't it! I'd run 0w30 year round. From -10 to over 110 degrees. Just switched to a 0w40 due to availability and pricing but that's a different discussion

 

[SnowDrifter]

If I graph up some theoretical oils:

Note that the 0w30 doesn't get as as thick when cold, but still quite thick

If we go really cold, you run into some pretty substantial differences. Note the viscosity numbers at the bottom, knowing that both oils are supposed to be 11cst@100c

The 0w30 is nearly 500 times that value, whereas the 10w30 is over 1000 times that!

If we change gears and look at higher temps, you get some rather interesting results. The oil that's thinner when cold is now... Thicker when hot? That goes back to the "slope" description I was using above. That 'thin' 0w30 actually does a better job at resisting thinning as temps go up.

Realistically, you're talking about 0.1 to 0.2cst differences. In the engine, that's not enough for it to care or really notice. BUT it is there. And I think provides enough into to frame the argument I'm making.

 

[DougAMiller]

So....got the Tahoe back last night from the shop, and the o-ring solved my low oil pressure problem. However, next time I think I'll bring the pan gasket with me. $141 for a pan gasket??? That's ridiculous, shouldn't have been more than about $35. Also, my backup camera is no longer working, although I really can't see any way to blame them for that. Just a coincidence I think.

 

[madbuilder]

What a relief. It makes sense that it was the O ring because when the oil was cold and thick, the pressure was low. I think the bad rubber became pliable after it warmed up.

Good thing you didn't wait for idle pressure to go "out of spec". Now she's good for another 200 k.

 

[iamdub]

So....got the Tahoe back last night from the shop, and the o-ring solved my low oil pressure problem. However, next time I think I'll bring the pan gasket with me. $141 for a pan gasket??? That's ridiculous, shouldn't have been more than about $35. Also, my backup camera is no longer working, although I really can't see any way to blame them for that. Just a coincidence I think.

Now I'm wondering why your O-ring testing didn't reveal this. If the testing methods aren't reliable, it'd be helpful for others to know why and what can be done.

 

[Reebok59]

I was wondering the same thing about the o-ring test. I highly suspect my low oil pressure issues are due to my O-ring but if possible, I would like to confirm with a test before dropping the oil pan. Is the recommended procedure 2 qt overfill plus incline if possible? And has it been used reliability by others? Sounds like it really didn't work for Doug.

 

[DougAMiller]

Well, the backup camera was just a bad ground, all better now.

Now I'm wondering why your O-ring testing didn't reveal this. If the testing methods aren't reliable, it'd be helpful for others to know why and what can be done.

Good question. From what I've read either the overfill method OR a steep incline is recommended to test this. However, when the overfill didn't change anything I put it nose down as much as I could and still nothing. So, I would have thought that even if I didn't do either one of them quite enough, combining them should have submerged the o-ring.

I guess someone needs to fill one with the timing cover off so they can visually confirm how many quarts it takes to submerge the o-ring.