Really low oil pressure

[cp2295]

I got a 98 4.0 ohv with 155k. I hooked up a mechanical gauge because I'm getting a ton of upper engine noise. At hot idle with 5w30 I have only 10 psi, even lower maybe (hard to read the gauge) and when I rev it up I can get it to read 35 psi. My question is can I run diesel oil or racing oil until I can afford to put new rod and main bearings in (which I'm thinking is the issue I hear no knocks just valve train clatter). I live in the pnw and its 75 today.

Also I have very clean plugs and don't lose oil. Would a re ring be in order or can I just do the bearings?

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[RonD]

10psi is OK at idle.

General rule of thumb for oil pressure is 10psi per 1,000rpm
At idle, 800rpm 6 to 10 psi would be fine
3,000rpm 25psi to 35psi

Ford oil pressure switch is set for 5psi

Upper rocker noise could be dirty push rods and lifters, quart of ATF added to oil for a 100 miles before an oil change could clean system out a bit.

My 4.0l OHV upper end noise is the fuel injectors, lol, click, click, click........like a darn choir crickets under the hood

 

[cp2295]

I also have a weird noise around 2500+ rpm I've written another thread on which is what prompted me to call a machinist since no one had an answer and he said check the oil pressure because he said the rods or Mains could be shot and cause not enough oil pressure to make the lifters work good enough. Reason I agree with him is because it doesn't do it cold.

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[cp2295]

I just changed it to 15-40, idle is still about the same brought highest psi I can attain up to about 40. It also changed the weird rattle noise from 2500 to 3000 RPMs and my tap tap tap at idle has been significantly reduced. Gonna take my oil filter into the machinist and have him tell me what's up, if there is any bearing material in the filter

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[adsm08]

A somewhat invasive test, but one that is worth checking on a 4.0 is to pull the valve covers and rocker arms, check the push rods. See if they are foot-balled. Foot-balled push rods can make a heck of a racket and make you think there is an oil pressure problem.


If you want to do a non-invasive test for bottom end bearing problems pull the oil filter and cut it in half, check the filter media for metal particulate.

If you do have genuine oil pressure induced noise but the main pressure is good, and it sounds like it is, the cam bearings may be the issue. I understand budget building, but I would not have installed a used engine without at least doing those.

 

[cp2295]

Yeah for sure, learn while I'm young at least. Oh well I like projects so pulling the motor ain't to big of a deal, I got the tap noise to go away before by replacing the rods with new ones and getting some nice looking junkyard rocker arm assemblies. (made sure there was no galling) but they have withered away slowly as expected, soon I will go with the delta cams rockers and rods.

However the rattle noise at light load and 2500+ rpm has always been there.

And I still have low oil pressure I think, and worn out pushrods wouldn't cause this, worn rod and Mains would cause that right?

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[RonD]

I don't think it is that low but, yes oil pressure is Back Pressure from the main, rod and cam bearings.

The oil pump creates the flow of oil to the oil filter and then to the main passage, passage splits off to the different areas, this flow hits the small gaps in the bearings, all of it can't flow out so it builds up a back pressure in the main passage, where oil pressure sensor is.
So oil pressure measures the amount of oil that CAN NOT be used by the bearings.
When RPMs goes up the oil pump flow goes up but bearing gaps don't change, so back pressure goes up.

At idle you have 10psi of oil that can not be used
At 3,000rpm you have 35psi of oil that can not be used

With smaller bearing gaps you might have
25psi of oil that can not be used at idle
And 45psi of oil that can not be used at 3,000rpm

Because oil passages are a common system as far as back pressure, if one bearing gets worn out oil pressure drops in the whole system.

 

[cp2295]

That makes good sense. I'll keep running this engine awhile until it decides to take a crap on me (which should be a long time considering its an ohv 4.0).

I found this on Ebay:
http://m.ebay.com/itm/251486114822?nav=SEARCH

He says its OEM Ford and I messages him, 1 year warranty... Seems too good to be true, but if its legit that's a great price.

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