98 fluctuating oil pressure

I just picked up a 98 xj today and noticed that my oil pressure is varying between 10psi to 40psi. It usually is lowest when idling or slowing down. even when i would give it gas on the freeway itd drop to 20-30 psi What do you think it can be???

Check the grounds. Do an oil change and use a quality filter. When I picked up my 98, it would idle around 15psi, adter the oil/filter change, it idles at 20+psi (both recorded with a hot engine) and the running pressure went up over 40.

Just my .02

Make sure you have a quality oil filter--Wix/Napa Gold, that you are using the correct oil viscosity, and that you have no oil leaks at the OFA--oil filter adapter or the sending unit.

10 psi at idle, 10 psi per 1000 rpm is "normal". I think the FSM says 13 psi at idle. You can rent a mechanical oil pressure gauge to get a more accurate reading, if there is a substantial difference then consider a new sending unit.

Oil pressure spec for the 4.0

*Not less than 13 psi at hot idle
*Between 37-75 psi over 1600 rpms

1. If you are in the above specs, there is no issue.

2. If you are NOT in the above specs, you should get a mechanical oil pressure gauge and verify your real oil pressure. As mentioned, the oil pressure sending unit is a common failure on the 4.0

If you purchase a sending unit, I recommend buying it directly from Jeep for best reliability/accuracy/mating with your factory gauge.

Fluctuation is NORMAL. You should have less oil pressure at idle than you do on the highway......

Also put diolectric grease in the connector that fixed my flux problem

I'm gunna start off with an oil change and nice filter.

My 98 XJ does the same thing. Reads just over 10psi at hot idle then around 40psi at speed. The psi basically fluctuates with the rpm. Asked my mechanic (I trust him) and he laughed. Said that just about every Jeep with a high mileage 4.0 he's ever seen does something similar. He also said that as long as it runs fine and isn't making strange engine noises it is good to go. I usually drop in a quart of Lucas Oil Treatment with every oil change too.

jdougn said:

My 98 XJ does the same thing. Reads just over 10psi at hot idle then around 40psi at speed. The psi basically fluctuates with the rpm. Asked my mechanic (I trust him) and he laughed. Said that just about every Jeep with a high mileage 4.0 he's ever seen does something similar. He also said that as long as it runs fine and isn't making strange engine noises it is good to go. I usually drop in a quart of Lucas Oil Treatment with every oil change too.

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Yeah I just noticed it happens after its nice n warmed up. I have a sunpro mechanical oil pressure gauge I just need to find a t fitting to make it work and I don't where or what size I need to get.....

I changed my oil with Pennzoil high mileage 10/30w a Mobil 1 oil filter and a bottle of that Lucas oil additive and now it stays at 30psi-40psi.
I'm stoked