Oil pressure sending unit

Does the 4.0L oil pressure sending unit sense/read oil pressure before the oil enters the oil filter or after the oil has exited the filter?

after
I don't know of an engine that reads it before.

So can oil filter internal construction affect the oil pressure reading?......or does only bearing clearances affect oil pressure reading?

What's your gauge reading? On my 99, one day it just started reading about 40psi higher than it previously had (even with the ignition on, but engine not started). Bought a new sensor from the dealer and the issue went away.

Don't have a gauge......just trying to find out if the oil filter (as in changing brands) can affect oil pressure, presuming the filter is not clogged and everything else is all equal.

yes they can effect oil pressure.

So, a filter that traps smaller particles will also have more resistance to flow, thus reducing oil pressure????

djb383 said:

So, a filter that traps smaller particles will also have more resistance to flow, thus reducing oil pressure????

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There is a bypass built into the filter, if it gets plugged with particles it bypasses. There is also a pressure relief valve built into the pump. The early 4.2 engine (or maybe it was the four banger, I forget) had a bypass built into the filter adapter and not one in the filter.

Logic tells me the pressure is measured after the filter, darned if I know for sure. The oil diagrams I've looked at, lack detail in the filter area.

The first thing to check with oil pressure issues is the sending unit. I usually use a mechanical gauge screwed into the sender outlet. Pretty wide range of oil pressure gauge readings using different senders, even new senders.

8Mud said:

Logic tells me the pressure is measured after the filter, darned if I know for sure. The oil diagrams I've looked at, lack detail in the filter area.

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it is

As already stated, it's after, and yes, filter can make a detectable difference in oil pressure. I noticed a change of as much as 5 PSI overall when I changed from cheap oil filters to the premium. Wix, Bosch and K&N seemed the best, with Wix the winner on price and availability.

Which way did your 5 psi oil pressure change?........up or down?

djb383 said:

Which way did your 5 psi oil pressure change?........up or down?

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Up with the good ones.

That's good to know, thanks. It seems like the "good" filters advertise they will filter out smaller particles, which seems like a finer filter media would be used to trap the smaller particles, which seems like a finer filter media would reduce reduce pressure somewhat on the media backside (where the pressure sending unit is located), thus lowering oil pressure somewhat. Oh well.

This is academic. Get the best filter that filters the best. If it changes your oil pressure enough to cause a problem you have deeper issues.

Really? There's a best filter......that sounds like a highly subjective opinion, no?

If everything else remained the same, only difference is changed the brand of oil filter, why would there be deeper issues if the pressure changed?

My advice, stay away from Fram, and keep the oil changed regularly. I have had excellent luck with Wix, Napa Gold (Wix makes them for Napa), Purolator, K&N, etc.

djb383 said:

Really? There's a best filter......that sounds like a highly subjective opinion, no?

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You really need it spelled out for you?

If everything else remained the same, only difference is changed the brand of oil filter, why would there be deeper issues if the pressure changed?

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If it changes your oil pressure enough to cause a problem you have deeper issues.

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That was also in reference to one of the better filters, not the shit Fram passes off as a filter and the like.

Let me rephrase the question......why/how would 2 new but different brands of oil filters change oil pressure? Let's presume the motor is in top notch condition, same brand/weight oil used, oil at operating temp, no bypasses open, everything remains the same except for the 2 different oil filters. Valid data would be preferable over opinions.

Different filter construction, different media, etc. What aren't you grasping? If you want data on such as the flow rates of the different medias used, find it yourself.

I cannot say for sure why a better filter gives better oil pressure, but a couple of possible reasons come to mind. One is that when a filter clogs and bypasses, pressure drops, so obviously a filter that does not clog will work better longer. The better filters have many more pleats, meaning much more working area, than the cheap ones, and thus do not clog so soon.

I suspect there also is simply a different, and better, medium of filtration in some, and that fineness is not the whole story. I have not studied the filter media, though I did cut open a bunch of filters some years ago. Most looked similar, but the media themselves were clearly different, with the best not only having more pleats but what appeared to be a more solid material. The one exception was K&N, whose filter medium was entirely different from the others, harder and less papery.

Whatever the science, when I was putting a lot of miles on my 87 I went through a lot of filters, and analyzed several. I settled on Wix as the best bargain in good filters, and have almost never bought anything else in the half million or so miles that have followed,, except when they could not be found, and I got Bosch or NAPA gold instead.

Who knows what might have occurred otherwise, but I have taken several XJ's well over the 250 thousand mile mark with utterly no internal engine problems, and in the process maybe spent 40 bucks more on Wix filters than I would have on cheaper ones.