Low oil pressure....sometimes

[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.