Low oil pressure....sometimes

[DougAMiller]

Well, at 180k and zero problems of any significance, I guess it was due. I've been having intermittent low oil pressure since my last oil change, or at least I first noticed it when I started it to check for pressure and leaks after changing the oil a couple of months ago. My first thought was oil filter and I was going to replace the filter, but it seemed like it was getting better before I bought another one, so I kept driving it to see if it went away. This last weekend it was time for another oil change and the intermittent low pressure had seemed to be getting worse, but I talked myself into believing it was the sensor due to the intermittency. So, I changed the sensor, and....no difference, pressure looked exactly the same so I thought OK, must be the filter. I changed the oil and filter next and this time put an AC Delco filter on it (I usually use MobilOne). For reference, on a cold start the pressure is around 20-25psi, but when the engine idles down to around 600 RPM the pressure will drop, sometimes as low as 5-10psi. As long as the engine is running above 1000 RPM the pressure stays in the range of 25-40psi, and usually after running a while it will be more normal and get up to 50-60psi at higher RPMs.

I've owned this vehicle since new and it has had the oil and filter changed religiously well before the required service interval. I've always used MobilOne synthetic and filters. No oil leaks or burning oil, in fact, there is very little oil loss between changes. I usually get about 5 1/2 quarts when I drain it. So now the question is what is the problem? I know that the pickup tube o-ring is known to be a problem on these engines and another possibility is the pump is going bad, or a sticking bypass valve which again is the pump. However, neither of those is a fun job, so is there anything else I should be thinking about before tearing into it? I was planning to do the overfill test to see if it's the o-ring, but haven't done that yet.

 

[SnowDrifter]

Mechanical gauge is king. Verify with that to rule out any and all electrical errors


If mechanical gauge reflects the same pressure drop, I'd order an oil pump, pickup tube o-ring, and all other needed parts. Tear down, replace as needed, then you can return unused parts. Pickup tube o ring is common, but worst case would be mechanical damage such as a cam bearing issue.

Oil pressure from your description sounds low across the board TBH

 

[iamdub]

Pop off a rocker cover to see what the inside looks like. As Chris said, your pressure is low across the board.

 

[DougAMiller]

Agreed that pressure is low across the board and verifying with a mechanical gauge would be ideal, but I don't have one and accessing the port to connect it is a real pain. So I was inclined to believe the pressure since two different sensors gave the same numbers.

There have been no unusual engine noises, it runs fine, and there are no metal flakes on the drain plug magnet, so I don't think there is any engine damage. I was primarily asking if anyone thought there was any other potential cause i was overlooking.

Thanks!

 

[iamdub]

Agreed that pressure is low across the board and verifying with a mechanical gauge would be ideal, but I don't have one and accessing the port to connect it is a real pain. So I was inclined to believe the pressure since two different sensors gave the same numbers.

There have been no unusual engine noises, it runs fine, and there are no metal flakes on the drain plug magnet, so I don't think there is any engine damage. I was primarily asking if anyone thought there was any other potential cause i was overlooking.

Thanks!

There are a few potential causes and it's not something you wait until it IS making noises or producing metal flakes before investigating further.

Not to jump immediately to far left field, but my brother's truck ('05 Sierra 5.3) had similar symptoms progressing over the course of a few weeks. He didn't call me until it was tapping loudly and had 0 oil pressure. The heads had a factory defect (cracked 706 casting flaw- look it up) and it had slowly been getting coolant in the oil. Not enough to make a milkshake or show any obvious symptoms. But, over time, it gunked up the entire inside of the engine and slowly choked off the oil pickup tube until it couldn't pull any more and lost all pressure.

 

[swathdiver]

I've been having intermittent low oil pressure since my last oil change

...For reference, on a cold start the pressure is around 20-25psi, but when the engine idles down to around 600 RPM the pressure will drop, sometimes as low as 5-10psi. As long as the engine is running above 1000 RPM the pressure stays in the range of 25-40psi, and usually after running a while it will be more normal and get up to 50-60psi at higher RPMs.

M1 filters are more restrictive than others, I've noticed about a 3 psi decrease in oil pressure when using M1 EP filter over the K&N or ACDelco. But that is not your problem. It's time to replace that o-ring Doug.

As mentioned before, you can park nose down on a steep ramp and see if the oil pressure comes back up or add 2 more quarts and idle the engine only and see if it comes back up (submerging the o-ring) and then drain that excess out before driving.

 

[SnowDrifter]

Not going to quote but yeah if you wait until there's mechanical noise or seeing metal flakes to investigate the problem, you're wasting your time. At that point, the damage has been done. Engines, and other mechq iCal things do not "heal"

Damage done is permenant unless you actively repair it. No paint chip, scratch, wear and tear will ever resolve itself because one day you wake up and give a damn. It's a manual task, one which is why it's generally better to be proactive rather than reactive with this stuff

 

[DougAMiller]

Not going to quote but yeah if you wait until there's mechanical noise or seeing metal flakes to investigate the problem, you're wasting your time. At that point, the damage has been done. Engines, and other mechq iCal things do not "heal"

Damage done is permenant unless you actively repair it. No paint chip, scratch, wear and tear will ever resolve itself because one day you wake up and give a damn. It's a manual task, one which is why it's generally better to be proactive rather than reactive with this stuff

Of course. I was just not certain I had a real issue until after replacing the sensor and filter this weekend.

 

[DougAMiller]

M1 filters are more restrictive than others, I've noticed about a 3 psi decrease in oil pressure when using M1 EP filter over the K&N or ACDelco. But that is not your problem. It's time to replace that o-ring Doug.

As mentioned before, you can park nose down on a steep ramp and see if the oil pressure comes back up or add 2 more quarts and idle the engine only and see if it comes back up (submerging the o-ring) and then drain that excess out before driving.

You'd think living in Colorado I could find a steep slope to check it on, but haven't found a spot that would work. Just picked up some more oil this evening to try the overfill method. Would prefer to go ahead and replace the pump with a Melling pump, but both jobs are a lot of work and time, which I am short on. I don't really have the time to replace the pump if it's just the o-ring. The fact that the pressure fluctuates a lot at idle suggests to me it's the o-ring. I think a string of bubbles would cause that whereas a worn out pump I would think would be a more consistent loss of pressure.

 

[SnowDrifter]

You'd think living in Colorado I could find a steep slope to check it on, but haven't found a spot that would work. Just picked up some more oil this evening to try the overfill method. Would prefer to go ahead and replace the pump with a Melling pump, but both jobs are a lot of work and time, which I am short on. I don't really have the time to replace the pump if it's just the o-ring. The fact that the pressure fluctuates a lot at idle suggests to me it's the o-ring. I think a string of bubbles would cause that whereas a worn out pump I would think would be a more consistent loss of pressure.

I had some issues with fluctuating pressure on mine but only when hot, and primarily at higher RPMs. Wasn't really low pressure and I'd still be in the 30-33 range at idle

On teardown, found that my pickup tube o ring and screen were both in good shape. Found some minor cavitation wear on the oil pump vanes (I think I still have them laying around if you want a Pic) and some scoring on both the bore and piston where the high pressure relief was. Current theory was it was probably a mix of cavitation and binding of the relief valve that was causing my issues. Didn't notice the cavitation thing at first, only really found it later when I had the gears in hand playing with them which, by the way, is a bit satisfying.

If you want a walk through if the job, let me know. It's annoying but it's not difficult. Honestly the hardest part is re torquing the harmonic balancer, and trying not to curse every time ya get dripped on

 

[swathdiver]

You'd think living in Colorado I could find a steep slope to check it on, but haven't found a spot that would work. Just picked up some more oil this evening to try the overfill method. Would prefer to go ahead and replace the pump with a Melling pump, but both jobs are a lot of work and time, which I am short on. I don't really have the time to replace the pump if it's just the o-ring. The fact that the pressure fluctuates a lot at idle suggests to me it's the o-ring. I think a string of bubbles would cause that whereas a worn out pump I would think would be a more consistent loss of pressure.

Just do the o-ring first and if that doesn't solve it, make time and do as Chris says, replace the pump.

 

[DougAMiller]

I had some issues with fluctuating pressure on mine but only when hot, and primarily at higher RPMs. Wasn't really low pressure and I'd still be in the 30-33 range at idle

On teardown, found that my pickup tube o ring and screen were both in good shape. Found some minor cavitation wear on the oil pump vanes (I think I still have them laying around if you want a Pic) and some scoring on both the bore and piston where the high pressure relief was. Current theory was it was probably a mix of cavitation and binding of the relief valve that was causing my issues. Didn't notice the cavitation thing at first, only really found it later when I had the gears in hand playing with them which, by the way, is a bit satisfying.

If you want a walk through if the job, let me know. It's annoying but it's not difficult. Honestly the hardest part is re torquing the harmonic balancer, and trying not to curse every time ya get dripped on

It's possible that cavitation could cause these kind of symptoms, but onset seems kind of sudden for the kind of metal wear that you're describing. The o-ring losing its seal seems more probable to me. Anyway, I am planning to do the overfill test today to confirm.

I've done jobs like this one before, just not on this engine/vehicle. There's not much I haven't torn into on a car at one time or another, so I'm not too concerned about knowing what to do, but thanks for the offer. I've never run into anything that I couldn't fix, but I'm still debating whether to let a shop do it or not.

 

[DougAMiller]

Just do the o-ring first and if that doesn't solve it, make time and do as Chris says, replace the pump.

Well, it needs to be taken care of right away. It hasn't gone to zero pressure, but I don't like these low episodes and don't want to risk being stranded if it did drop out completely. Problem is that the forecast is very hot here for at least the next week and either job is going to take hours to do. I'm also pretty short on time right now with needing to get two kids off to college and work is rather busy at the moment.

I really don't like trusting shops to do these jobs. They just don't tend to live up to my standards, but this is one that I may have to. However, I do want to go in being able to tell them exactly what I want done. I don't trust them to diagnose things accurately, too many just throw parts at a problem until they finally hit the bulls eye. I also only have one shop that I trust, and that's the Acura dealer that I let work on the wife's MDX, but doubt they would take a Tahoe.

 

[SnowDrifter]

I really don't like trusting shops to do these jobs. They just don't tend to live up to my standards.

Hah you sound like me. Good man

 

[DougAMiller]

Well, did the overfill test, even backed the rear wheels up on ramps to be certain the o-ring was submerged. Didn't change the pressure at all, so I guess its going to have to have a new pump (and o-ring). Going to get an estimate from the local 4-wheel drive repair shop. The service departments at my nearby Chevy and GMC dealerships aren't getting very good reviews these days. Besides, this shop offers the option of a GM or a Melling pump.

 

[iamdub]

Besides, this shop offers the option of a GM or a Melling pump.

Melling makes the pumps for GM. AFAIK, the only difference is the GM pumps have the part numbers engraved or embossed on them.

 

[DougAMiller]

Melling makes the pumps for GM. AFAIK, the only difference is the GM pumps have the part numbers engraved or embossed on them.

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.

 

[swathdiver]

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

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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.

 

[DougAMiller]

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.

Nice article, thanks for the link.

 

[SnowDrifter]

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

Adding to info:

M295 is your drop in replacement

M295hv is high volume. 18 percent more if memory serves. It includes a high pressure spring pre-installed, or a standard pressure spring in the packaging you can put in. Both would provide an increase in pressure however on account of flow, but the high pressure spring would give even more of an increase. I have the high volume with high pressure spring on mine. Hot oil Pressure with a 0w30 was 46ish psi. 12.2cst hot viscosity. Current oil is 0w40 with 13.6cst hot viscosity. That's 50psi hot idle and 68 when cruising at 1530rpm. Hot pressure tops around 115psi at redline. No idea what it would be cold as I don't redline my engine until oil gets up to temp anyway.