How does low oil pressure indicate bad bearings?

Oil pumps have to have resistance to build up pressure or the oil just flows free at 0 psi.

The oil channels lead to the bearings have a 'little' restriction, but not much. The big restriction is at the bearing faces.

The biggest bearings are on the crank so when the bearing faces get worn, the restriction goes away and the oil flows past faster and under lower psi.

This makes for a lower pressure reading on a gauge in the system.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

William Oliveri wrote:

Think of the sprinklers out in your yard.

There is probably about 10 of them connected to a single valve, and you have perhaps 6 valves to water the front and rear lawns, and the planter boxes. There is something like 60 psi on the system, and if there are too many sprinkler heads on a single line, then none of the sprinklers will spray very well because the pressure at any one head will drop relative to the pressure there would be if there were fewer heads. If one head blows off, then the water flow through the remaining heads will drop.

Your bearings are kinda similar. There are a number of particular sized holes (gallies) that the oil will flow through. If a bearing wears, then the oil pressure will drop because the amount of oil that can flow there will increase. If the crank and bearings are in good shape, then the oil galley is a known size. but, if the bearing or crank journal wears, then the galley effectively becomes larger because oil can not fit in the space between the crank and the bearing face. When this happens, there is less oil for some location further down the line. Basically, the oil pressure is dependent upon having specific sizes of oriffices to be pushed through, if one or more of those oriffices can accomodate more oil that it is designed to, then the remaining oriffices will get less oil because the amount of oil available is fixed due to the capacity of the oil pump.

If the bearings wore a little bit, then theoritically one could install a higher capacity pump to raise the amount of oil available. the problem with that theory is that the worn parts will continue to wear, requiring increasingly larger capacity pumps. At some point, the ability of the pump to do its job would exceed the ability of the oil passages to carry oil, and the pump would cavatate, and you would still have the worn bearings that allowed the oil to bypass all of the other places that it ought to be going. If you connected a fire hose to your sprinkler system that had lost a head, the other heads might get some water, but the one that was broken would still be broken and lots of water would be wasted. (there is a significant difference in the oil pump and the fire hose, the pump is in a closed system so there wouldn't be the waste, but what these have in common is that there would be lots of oil in a place that couldn't benefit from it, and minimal oil in a place that needs it badly.)

So, in my 98 TJ, my oil pressure at idle is pretty low compared to what it used to be (I think). It seems to be down around 30psi at idle, but at speed it is up around 50 or so. Is this the type of indicator we are talking about for worn crank bearings, assuming of course my pump and pressure sending unit are good. It only has 66k miles on it (around there, would have to double check.)

hmmmmmmm...........................similar situation with your blood pressure

But you can't lower your blood pressure by poking a hole in an artery!! makes sense to an irishman....but makes a mess too!

sb

Yes, the oil pressure will get slowly lower over time as parts wear.

There is a minimum safe pressure. Once that is reached, wear starts happening fast.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Joseph P wrote:

But cholesterol, decreasing the size of the vessel, will increase BP (it will also give a false high reading because it causes the vessel to be harder to compress - but that's another topic)

*stares with a blank look* --blink--blink--

We can't chisel it off cause it isn't there...who uses a stove, let alone knows what one is anyway???? I got a grill!

George Foreman grill!! SWEEET!!!

Here, I speak medicalese, I can translate.

Picture engine oil passages nearly choked by sludge and tar. Even though your crank bearings are worn enough that you could shim them with popsicle sticks and your oil pump has more free-play than a Volkswagen driving through an airplane hanger, your oil pressure will read high, because the oil is being forced through a restriction.