Hi, everyone. Recently I've been having some trouble with my D100 dropping lots of oil pressure at hot idle. Lately it's been in the 90s and extremely humid and I recently installed an oil pressure gauge.

On the original pump, I got about 45 PSI cold (high) idle. When it got hot, it settled down to about 8-10 PSI at 750 RPM. On the highway after it got hot, it would run 35 PSI or so, at 70-80 mph.

I pulled the old pump out today and installed a new Melling standard volume M-66C pump. Started it up and now getting about 60ish PSI cold and once it gets nice and hot (20+ mile drive), I see about 12-14 PSI at 750ish RPM idle but jumps quickly to 40-45 at around 2,000 RPMs and will get up to 48+ PSI at 70 MPH. I'm still thinking that might be too low, but what is everyone elses Slant 6 engines oil pressure run at a really hot idle? I'm currently running Rotella 15W-40. The truck only has 66k on it and not really sure if I may have some worn main bearing issues. Or maybe I just have a really severe case of OCD. I've done an oil analysis with Blackstone and all of my wear PPM (Iron, Copper, Lead) numbers are quite below average. Thanks in advance.

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2007 F-150 5.4L

1985 D100 225 Slant 6

15W40 is much too needlessly thick/heavy for a low-miles engine in good condition. Change to 10W30. Your oil pressure is fine—it was fine before. Factory spec is at least 8 pounds at idle, 30–70 pounds at 2,000 rpm.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks for your reply, Dan.

If I drop it into drive, the idle slows to about 500 or maybe a tad less, and my pressure is about roughly 5-6 pounds. Is this still considered normal? If I put it in park, it goes right back to around 12 pounds.

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2007 F-150 5.4L

1985 D100 225 Slant 6

500 rpm is too slow.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I agree with Dan- 500 RPM is too slow of an idle speed.

A 1985 truck was built with a hydraulic lifter engine. The lifters need at least 4 psi of oil pressure to "pump up" or pressurize. Try increasing the idle speed to about 650 RPM in drive and see what your hot idle oil PSI is.

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Casually looking for a Clifford hyperpak intake for cheap.

Thanks for the replies. I set the idle up in drive to about 650. I took it for a good long drive in the heat of the day, came back and I'm getting about 8-9 pounds of pressure. I would have thought it should make more than that, but seems to make good pressure going down the road. In park the idle is just under 1,000 now and makes about 15-16 pounds in park.

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2007 F-150 5.4L

1985 D100 225 Slant 6

As a general rule of thumb, you should have 10 lbs. for every 1000 RPM's. My old van with god knows how many miles on it sits at about 15 at idle, and buzzes up quick once I get on the gas to around 45 lbs. at cruze. I think your good.

Yes, it definitely jumps quick as I step on the gas. I used to not be able to get above 35 PSI hot on the highway with the old pump. Now I get about 46-50. At a 50 mph cruise at around 2k RPMs, I'm getting 43-45. So hopefully everything is sound. It's a very quiet and smooth engine.

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2007 F-150 5.4L

1985 D100 225 Slant 6

My oil pressure is 25 lbs cold or hot Moving or not Never moves Kinda weird