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Discussion Starter · #1 ·
Find Melling makes a replacement OEM pump M208. Some of you have them. What are the differences between these and the pump Navistar specs/installs?
In the winter time with temps near 0 and 5w40 weight synthetic oil I can still occasionally hit 80 psi if I apply too much skinny pedal before the oil is warmed up. This I am told is the pressure the filter will bypass the filtration element. Really don't want to do that on a regular basis. So what are the advantages of the Melling unit. I know, I know move to a warmer climate. If only I could get the University of Michigan to move to Kentucky.
 

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I don't know the exact numbers for specs, however the Melling has fewer lobes on the rotors and the rotors are a bit narrower, at least the one I have is this way.

The pressure-regulating valve is set at around 80 psi and will dump more pressure back to the pan. The filter bypass operates on a pressure differential. If the pressure before and after the filter is more than x psi (not sure what the number is, probably 12-20psi) then some oil will bypass the filter and go directly into the engine.

You can run 80psi of oil pressure and not hurt anything that is why there are valves in the system to control the pressure. The advantages of the Melling pump are around 10% more volume. In my case, that means I wasn't pulling the HPOP reservoir down to 5 psi on a hard pull.
 

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I don't see what a "hard pull" has to do with dropping lube oil pressure?

The high pressure pump(s) move a given volume of oil per rpm no matter what the throttle. WOT or no throttle the lube oil pressure should never change for any given rpm except for changes in oil temp or aeration. And at any rpm off idle different rpms really shouldn't change the pressure anyway. A change in ICP doesn't "take more oil" it simply doesn't return as much to the front cover through the IPR.

You might want to check for oil aeration issues. Or if you've already swapped the pump and all is well, I think it was possible that the OEM pump was scored and causing aeration.

My OEM lube oil pump maintains around 20-30 or so psi at anything off idle with both pumps no matter what the throttle position or rpm. The pressure doesn't change as rpm increase I just can't remember the exact number of hand. And throttle has zero effect.

I also chose the OEM pump over the Melling because of the size of the Geroter and the whole OEM replacement for a part that wasn't a problem anyway.

.02
 

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Discussion Starter · #4 ·
You can run 80psi of oil pressure and not hurt anything that is why there are valves in the system to control the pressure. The advantages of the Melling pump are around 10% more volume. In my case, that means I wasn't pulling the HPOP reservoir down to 5 psi on a hard pull.
Do you mean the LPO would fall to 5 psi as measured at the HPOP reservoir?
 

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Yes, with a mechanical gauge on the HPOP reservoir the pressure would fall to 5psi +/- 2psi at WOT. This has been reported by several other owners of BO, so it's not an isolated incident. The factory pump was in excellent shape, actually was in slight better shape than the melling I put in. I believe the reason maybe the way the IRP is put in the system possibly. Doesn't matter, between the Melling and the aux feed line I don't have that issue any more.
 

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Ah, yes, if you were expecting all of the oil to get to the reservoir through that little port in the front cover, then yes. I can absolutely see why it was dropping pressure. But that would still be rpm related, not throttle related. In that case the lube oil pump capacity was coming up with the volume usage of the high pressure system, it just couldn't all get through the feed port and was going elsewhere instead.

The additional feed line is why the issue was resolved, not the lube oil pump change. I have never run the truck without the additional -6 feeding the reservoir.

.02
 

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Discussion Starter · #7 ·
Ah, yes, if you were expecting all of the oil to get to the reservoir through that little port in the front cover, then yes. I can absolutely see why it was dropping pressure. But that would still be rpm related, not throttle related. In that case the lube oil pump capacity was coming up with the volume usage of the high pressure system, it just couldn't all get through the feed port and was going elsewhere instead.

The additional feed line is why the issue was resolved, not the lube oil pump change. I have never run the truck without the additional -6 feeding the reservoir.

.02
How/where did you feed the HPOP reservoir from. I have noted my LPO pressure warning light blink in my Spa tech gauge (set at 9 psi) occasionally at WOT. I initially thought it was due to low fuel pressure but that LED is a different color. I could not explain the OP finding as the reservoir since the transducer looks are the rear main bearing through the port on the driver's side near the tranny housing. I have heard of such rapid oil flow that the flow through the pickup cannot keep up. Modifying the pickup seems to fix that but I will not get to that until the engine is pulled.
 

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After installing the the LPOP before I put the extra feed line on the issue was MUCH better, but not totally solved. After the aux feed line I have no issues at any throttle setting for supply to the HPOP. I have developed a no engine oil pressure problem at hot idle, but that is a separate issue.

I ran a 1/4" stainless outer braid-teflon liner line from the filtered aux port on the filter housing to the #5 (I think it's #5) oring boss plug on the top of the reservoir.
 

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Now with the addition of the second turbocharger drinking oil I have a hot idle pressure of about 12-15 psi. And I do not see the pickup ever being an issue if you mean lube oil pickup in the pan. The only issue I had was when mine had a hole in the side where a tack weld fell out and went through the lube oil pump. But that pickup is HUGE and the screen looks the same. The most oil usage will be at maximum engine rpm. Throttle doesn't matter. The only thing that calling for more or less ICP effects is the ratio of oil going through the injectors to that going across the IPR back down the front cover as the high pressure side always displaces the exact same volume flow for any given engine rpm. And as they flow more at higher rpm the lube oil pump delivers more at the higher rpm it's also spinning at. They really should run even from just off idle (where pump efficiencies are changing rapidly) until redline unless there is a restriction somewhere in the system.

JMHO
 

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I understand that the swash plate pump displaces the same amount of oil per revolution. All I can tell you is, as far as the reservoir pressure, the new pump did make a difference. I don't know why, it shouldn't. I also shouldn't have a no lube oil pressure situation at hot idle, but I do.

The lube oil pressure at hot idle is 0-4psi. When the gauge (reading main oil gallery left rear of block) reads 0 that's what it means. You can spin the oil filter loose and get just a little splash. Just off idle then there is plenty of oil pressure. I also know that there is a lube oil issue because lead has gone up considerably in the oil.

After replacing the LPOP with a high volume unit I RnR'd the pressure regulating valve and found all to be well. I replaced the sleeve, spool and spring with a new one out of another oil cooler I had waiting for another truck. Still have the same issue.
 

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That was the reason I was saying it shouldn't be doing that, to alert you that something has gone astray with your setup.

That's why I was thinking aerated oil possibly, or a chewed up front cover and such. I personally had a cracked pickup tube and DID pull the engine to fix it. I know the pains of oil issues on a 7.3.

Something is causing your issues, and the OEM pump isn't the something. Just wanted to help alert you to the fact that it isn't normal as well as clarify in a thread entitled Melling vs OEM LPOP, that the OEM pump is not the issue here.

JMHO

And Good Luck.
 

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No the OEM pump was not the issue, nor is the front cover as it doesn't have a single mark in it. The Melling pump does put out more oil as it helped the condition.

I have tested and looked at just about everything I can think of that can be accessed without pulling the engine out. At this point I believe the pickup is cracked, although it really doesn't act like it. As far a aeration goes, as quickly as is practical after a hard pull I've stopped and checked the dipstick and filter for evidence of aeration and haven't found anything to take note of.

The engine is coming out this winter anyway, it'll live until them.
 

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After installing the the LPOP before I put the extra feed line on the issue was MUCH better, but not totally solved. After the aux feed line I have no issues at any throttle setting for supply to the HPOP. I have developed a no engine oil pressure problem at hot idle, but that is a separate issue.

I ran a 1/4" stainless outer braid-teflon liner line from the filtered aux port on the filter housing to the #5 (I think it's #5) oring boss plug on the top of the reservoir.
I don't mean to hyjack, but can you elaberate on your aux line to the hpop res., pics and or p/n's would be great too. This might be a good topic to post as a supporting mod for BO (if it's not already). You're right, there has been several people with BO that have found this issue and added the aux. line.
I've seen pressure drops in my LPOS since BO too. It first showed up while live tuning with Jody last year. I'd like to try the aux line as a fix before I dig any deeper in my mill.
Thanks for your help.
 

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FWIW, my truck had HORRABLE oil aeration, to the point of blue smoking with a cloud trailing behind me when I drove, and at times actually killing the engine because the injectors wouldn't even fire on the froth.

Through all of that, the dipstick showed nothing, the oilchanges showed nothing all that strange.

What finally clinched it, was when I ran the truck at high rpm until it started running like total ass (usually only took around 40 seconds) and then imidiately drained the oil and caught some in a small styrofoam cup. It looked moderately foamy on the surface, but nothing crazy. Then When I poured it out the ENTIRE surface area of the cup was nothing but a frothy ooze of huge air bubbles. Wipe it out, catch another sample, same thing. If I sat and watched it bubbles would come to the top and pop on a very regular basis, like probably 3 or 4 per second.

But watching the oil fall from the pan and hit in the drain pan you couldn't really see anything. I mean once it slams into the pan it's going to be bubbly anyway you might think.

Long story short, my pickup tube had eaten one of it's own spot welds and sent it through the pump. The hole was above the oil level so it was pulling in air constantly and keeping a nice ratio of oil with the air it was pumping.

Truck made more power on the dyno and just generally drove like a dream after killing the aeration problem.

Good Luck.
 

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I ran a -4 line from by the oil filter to the inspection hole on the top of the HPOP reservoir.
 

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Not much to look at, it's just an oil line. -4 Stainless outer-braid with a Teflon liner, 37* female swivel field serviceable fittings on the hose, one 90* ORFS/-4 JIC male on the HPOP reservoir and a 45* 1/8" pip to -4 male JIC on the oil cooler housing. Can't give you PN's as I just buy this stuff at the hose supply place and this was all stuff I already had.
 

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