Oil Pressure Acting Weird

[Rooster71]

I have a 1997 with the 4.0. It has 196000 miles on it. I am using 10w30 synthetic oil.
When I first start it, the oil pressure is about 50 psi. When I start driving, it is about 45 psi. After about 15 minutes, it will drop to 20 psi and fluctuate between 15 and 30 psi.This is at about 1900 rpm (65 mph). If I drop to 55 mph, it will go up to 40 psi. At idle, it is 35 to 40 psi. At the higher rpm's, it will do the 15 to 30 psi fluctuating. But, it doesn't do this every time I drive it. Other times it will stay at 35 to 40 psi. I did a search on here, but I did not find anything like this. I did see some posts about the oil pressure range, and my pressures are within the range. I just wonder why the pressures drop and fluctuate at higher rpm's some times. Anyone have any ideas?

[Runnin'OnEmpty]

The first thing that comes to mind is a defective oil filter.
If a new filter doesn't help, then it might be something obstructing
the oil pickup...

[devildog0]

Could be a bad sensor or a chafed wire. Going to have to do some more troubleshooting. Inspect the wiring harness for any bare spots. If the wire grounds out it will max the gauge out or cause wide fluctuations in the gauge readings as the wire grounds on the block or other metal parts as wind blows it around at speed.

[Ralph77]

I am with devildog0 on this.
Also where the oil pressure sensor goes in sometimes gets gunked up.
Guy report low oil pressure just to find out that cleaning that fixed everything right up.

[old_man]

Pressure that drops after sustained higher rpms is indicative of oil starvation.

Verify your dip stick is the correct one, and not having you have too little oil in the pan.

I have seen this when oil is not draining from the head back down to the pan quickly enough. This is normally caused by a gunked up head.

[Black1990jeep]

Put a temporary known good gage on the oil system to assure your dash gage is or is not at fault. That should be first stop, to confirm you have a real oil pressure problem or a gage problem. Proceed with repair after gaining that knowledge.

good luck

[asymptonic]

Originally Posted by Black1990jeep

Put a temporary known good gage on the oil system to assure your dash gage is or is not at fault. That should be first stop, to confirm you have a real oil pressure problem or a gage problem. Proceed with repair after gaining that knowledge.

good luck

Can confirm. I had a "low pressure" situation that I started treating with additives that may or may not have caused other engine issues. After a rebuilt engine I still had low pressure, and finally changed my sender after getting a mechanical gauge on it. No way to prove it but that may have been a costly mistake on my part.


Your problem sounds more complex but rule out the sender first.

[Rooster71]

Thanks for the tips. I will check these things this weekend. I check the oil level about every other day and it is always good. Is there a way to verify that I have the correct dip stick? What is strange is that it stayed around 40 psi on the way to work and on the way home today. No fluctuating or low pressure.

[old_man]

Without pulling the pan and measuring, or comparing to another known good dipstick, the other way is to drain the oil and see how much you get. Allow for a quart or so for the filter. IIRC, the stock 4.0L specs 4.5 or 5 quarts.

[lazyxj]

Originally Posted by Rooster71

What is strange is that it stayed around 40 psi on the way to work and on the way home today. No fluctuating or low pressure.

With the intermittent problem, I suspect the oil pressure sensor is failing or there is a wiring issue. I have had two sensors go bad.
I suggest getting an oil pressure tester and seeing what the actual pressure is.
https://www.harborfreight.com/engine...kit-62621.html

[Driver86]

The first problem is the 10w30 oil. A common recommendation is Rotella T6 5w40. The 4.0 holds 6 quarts of oil. I would recommed installing a new oil pressure sender and change oil and a Wix filter.

[devildog0]

Originally Posted by Driver86

The first problem is the 10w30 oil. A common recommendation is Rotella T6 5w40. The 4.0 holds 6 quarts of oil. I would recommed installing a new oil pressure sender and change oil and a Wix filter.

10w30 is not a problem.

Sent from my SM-G892A using Tapatalk

[lazyxj]

Originally Posted by Driver86

The first problem is the 10w30 oil. A common recommendation is Rotella T6 5w40. .

The most common recommendation is 10w30, as specified in the Owner's Manual and the Factory Service Manual.
So, what do you know that the Factory engineers don't?

The OP has not established if an actual oil pressure problem exist or if it is a wiring and/or sensor issue.
It's a best practice to determine what, if any, problems exist and the cause before throwing parts at it, IMO.

[old_man]

Originally Posted by lazyxj

It's a best practice to determine what, if any, problems exist and the cause before throwing parts at it, IMO.

Bless you my son.

[Black1990jeep]

Originally Posted by Rooster71

Thanks for the tips. I will check these things this weekend. I check the oil level about every other day and it is always good. Is there a way to verify that I have the correct dip stick? What is strange is that it stayed around 40 psi on the way to work and on the way home today. No fluctuating or low pressure.

not only can the dip stick be wrong, the other thing is the dipstick tube. I had to replace ky tube, it broke off. new tube was a tad longer, so that the original dip stick read wrong until I altered the tub length. the error in my case was about a quart. red chinese made dip stick tube!

but you got to be a good deal low, probably near half low to even get near sucking oil into the pump.

Also some of us have installed larger than stock filters and even external,oil coolers which may or maynot fully drain at oil change time, another consideration for amount of oil to install.

if low oil problem, often the pressure drops at higher rpms worse than at low rpms do to the increased pumping speed and more splashing at high rpms.

I still suspect you might have a gage/sender/wire issue as I assume you have been putting in enough oil. get a known good gage on the motor to see if pressure problem is real,or not.