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Low oil pressure at hot idle


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Hey I'm new to the form so if I post in the wrong place I apologize. I recently purchased a 92 hd with a 599 block . I was told that it was parked do to a fuel issue ( found out it was the ops). It had Sat for years and had rust in the cylinders so wouldn't turn over a complete revolution. Tore it down did an overhaul,rods, mains, cam bearings, hv oil pump . lifters and lifter bores looked good. Truck runs great now. My issue is that the oil pressure starts out at 65-70 psi at cold idle and drops to 15 once warm. Pressure verified with high quality gauge teed into sending unit fitting. Oil clearance is .0025 on rods and mains ,checked every journal with mic and bore gauge twice. Oil galley plugs are installed and oil bypass valves under the filter were not removed . I even used a hydraulic hose to jump my cooler ports to eliminate a possible plugged cooler. Running 15/40 rotella and wix filter . I'm out of ideas , I'm new to 6.5s but build a lot of 4 and 6cyl diesels for equipment. Is there some plugs that could have been missing before I got to this or a bolt or passage that might need sealed ? I know these engines have low pressure at idle but over 50 psi drop and only 5 psi at the turbo at hot idle doesn't seem right. Any suggestions would be appreciated.

Cam bearings are the most common place these drop oil pressure when hot.
With rust in the engine, did you check or replace the cam bearings?
The front two are always the worst on the older blocks that ran the mechanical lift pumps.

As to the pressure that low- gm says the acceptable pressure at hot idle is 8psi. Thats not a typo: 8psi. Ya might try just running it a while and see if you like it enough to keep messing with it. A 599 block shouldn’t need the HV pump, so your options are limited. Regardless of name on the oil pump Melling actually makes them all, they rebrand the boxes for all the resellers.
There is the option of going to a tighter spring in the oil pump, or custom fitment work on the oil pump to try bringing it up a bit- but chances are you need to find the passage loosing the pressure.

It could be there is another place you are loosing the oil, but like mentioned, my money would be on the cam bearings.

Oh, wait- one thing to try. Removing turbo so it doesn’t spin dry. Put a pressure gauge on the oil line that feeds the turbo if you have one. A bad turbo dumping oil can drop pressure when hot.
Yeah replaced cam bearings , I though about doing what you're saying with the turbo just hadn't convinced myself it could be an issue sense
The turbo has very little shaft play

Have you set or verified the warm engine idle speed?

Offhand a stuck relief valve in the "new" oil pump.

Because I deal with extreme heat: when the oil pressure is down measure the pan temp with an IR gun. It may be overheating it's oil.

I would sample the engine oil to make sure it's still in grade. Then replace and cut the oil filter open looking for problems. Like a bunch of coolant or diesel fuel leaked into the oil. Rust or debris missed by the cleaning process clogging the filter up.

Did you replace the engine oil cooler? IDK the condition of the "bad" engine, but, if it spun a bearing the oil cooler is a glitter bomb waiting to go off. Engine failures that create a lot of debris require an new engine oil cooler as this design can't be flushed clean.

Yes the turbo and why not the turbo? It shares the same oil that rusted up and locked up the engine so why would it be ok exactly? It would be a blessing to put it out of it's misery and get a better turbo anyway.
It idles around 900 , is that normal? I actually changed the oil pump trying to solve the issue it was a standard pump originally, the hv pump really only raised my cold oil press 10% so the first pump is probably fine. I will definitely take a closer look at the turbo now . The rust was just in the cylinder above the piston, the bottom end was spotless and no debris in the pan or filter . Only reason I did new bearings is because I had to tear the whole thing down to clean up the cylinders and your there so why not . The highest oil temp I've seen is 175 f . An oil cooler may be in my future though just to get rid of the junk factory line if nothing else. Thanks for suggestions.
Out of curiosity what oil pressure are y'all seeing on a fresh overhaul?
Crank has been ground at some point .25mm undersized on both rods and mains. I went with a set from Mahle. Checked every journal and it was pretty uniform across the board at that .0025 +or - .0002 maybe . I'm starting to wonder if I have a crack in the block that's opening up when it gets warm. Anyone experienced this?
Would a lifter cause a leak big enough to give these symptoms? I've got a couple that bleed off overnight causing it to run rough on start up but pump up in a few seconds and it's smooth after that.
This might sound elementary, but...

Have you tried other oil brands to find the same situation? It's a different animal but I ran Castrol brand oil in a relatively fresh Vortec 350 many moons ago and at idle, the oil pressure would drop to nearly zero when warm. As soon as I kicked the idle up, the pressure climbed. I probably had under 50 miles on it but I drained it, replaced with the same weight oil of a different brand and the pressure never offered to be close to zero again. I ran it for years afterward without the issue ever coming back.
Yes you can have a crack in the casting that dumps it. Yes the lifter can.
The lifter clearance in its guide can too.

Sometimes using an external pump and pressurizing the galleys while in a stand is the only way to find the issue. Not just for these engines but any one. Ive had engines on a stand: oil pan, valley pan, heads off chasing unusual oil pressure drops.