RocketMouse

Oil pressure fluctuating.

99 XJ 4.0.
approx 105k
Running Rotella T6.....
I'm at 7,000 ft. altitude...
I've noticed here in the winter that after a colder night... it fires right up, but after I'm up to full operating temp, my oil pressure starts to seriously fall.
Normally... in warmer weather, when cold, she runs about 65-70 psi at startup. Once at full op. temp. it drops to around 50psi at idle.
Recently with a few cold snaps here, At cold startup she'll be at around 60psi....but when when warmed up, I've seen it drop to where it reads on the gauge as low as 15. but usually running around 30psi when warm.
Any ideas? Is this normal? or does anyone with experience think this could be because of running T6 in the winter? I don't want to cause any long term damage.
Also, for reference, my XJ does not leak any oil.
Thanks for any input to assist with this anomaly.

ericjw1991

Viscosity of your oil, a slightly clogged/ dirty oil pump pickup screen, a failing/ dirty oil pressure sending unit, or the gauge itself.. There could be some sludge running around in there. T6 has a good detergent package. Or in my book, a 4.0 that had bearing problems from a cracked head lol. All bearings replaced on mine, but pressure is still much lower than yours.
Mine starts up and I get to 40psi, then drops to 13-15 and I get 10psi per 1,000 RPMs. As for your oil choice, T6 is great! That synthetic should flow well in any temperature, better than anything.
Hook up a pressure guage where your Oil Pressure sensor is, and get a true reading to be sure. In the meantime, use some pipe cleaners to clean out that passageway where the oil filter/pressure unit is. Clean your unit too.

my .02

P.S. Wish I had your oil pressure regardless lol.

tjwalker

What you are seeing may or may not be accurate. But I don't see any correlation to the oil you are using.

The only true way to determine what your true oil pressure is would be to temporarily install a mechanical oil pressure gauge where your oil pressure sending unit resides, directly above your oil filter.

The primary symptom of a failing oil pressure sending unit is erratic oil pressure readings so keep that in mind. If you do replace it, do yourself a favor and replace it with a genuine Mopar/Jeep sending unit. It's your best bet for reliability/accuracy/mating with your factory gauge. Worth the few extra bucks.

Below is the oil pressure spec for the 4.0

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

If you are in this range and from your notes it appears that you are, I don't think that oil pressure is an issue with your engine.

Good luck and keep us posted!

RocketMouse

I appreciate the responses.... and as I mentioned... the only time I've seen this fluctuation in the oil pressure is after she's been sitting out after a pretty cold night... meaning dropping down into the 20's (F)... other than that, she stays pretty consistent.
I have a friend that had a Renix vintage XJ.... and said that his used to do something very similar in the wintertime, but I just wanted to check to see if it was normal or something I should actually take a look at. She may have done this last winter too without my knowledge...as it wasn't until this past Summer that I did a gauge cluster swap out for all the gauges as mine came with the basic idiot light gauge cluster from the factory...

And yes... Not to launch a debate or war as it tends to get on this site at times...but I personally run the T6 in mine for the detergent package and the extra goodies that it has/does for our old 4.0's.

RocketMouse

warm, normally running about 40-50 at hot idle.... and around 60ish over 1600rpm.
This issue I'm only having it after running her after a cold night where the low is down to at least the 20s.