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.

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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.