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Oil Pressure normal (40) cold drops when Warm (20)

T

thezfunk

NAXJA Forum User
Location
WI
So,

1998 4.0L. Maybe this is normal but I just wanted to bounce this around because my buddy with a Wrangler thought it was weird because his 4.0 doesn't do it.

I start the Jeep my oil pressure is ~40. Running down the road or accelerating it's ~40. After the engine warms up and idle comes down the idle warm oil pressure will be ~20. Should I be worried?
 
What oil are you using? I'm running rotella t6 5w40. My oil pressure is around 30 at idle and 50 at cruising speeds. Check your oil and make sure you are not low or overfilled.
 
The factory spec on oil pressure is 13 psi minimum at warm idle, and 37 to 75 psi above 1600 RPM.

The 6 Jeep 4.0L I have owned ran about 40-45 psi at 60 mph.
 
If you think your oil pressure is too low, use a thicker viscosity oil. I use whatever oil is on sale.
 
running 10w30 my '00 4.0 starts at ~60psi cold and warms up to 15psi at idle and 35-40 psi at 2000rpm

now I run 5w40 Syn and it starts at ~60psi cold and warms up to 20+ psi idle and 45psi at 2000rpm and makes less noise than it used to.

I don't expect my guage to be accurate in any sense ... and it's metric so take my psi conversions with a grain of salt ...

My 88 MJ runs more or less the same ...

in short ... it's normal ... does the wrangler have the same mileage as you? when we had our '06 TJ 4.0 it ran steady pressure and never wavered ... leading me to beleive the guage in those later years was just a dummy guage anyway.
 
Try to go with a quality oil filter that has good filter media and internal parts.
There are several manufacturers that produce quality filters (ie. Wix, Mobil 1, and factory Mopar).
 
When I do the change it gets either Bosch, Mopar or Wix . Since I got it done at the quick change pennzoil place it's a Pennzoil branded filter. I didn't have one with me at the time.
 
My original problem was my gauge was always pegged at 80, Used my manual gauge from HF today for the first time. 96 XJ, 200K plus on the clock. 4.0 w/AWG. I put Rotella 15/40 in it with a NAPA GOLD 1515 filter in it 4 weeks ago when I got it Cold I idle at 55 and rev up to 75. Hot I idle at 45 but still rev up to 75. Is this too high?

Also, when I pulled the sender out I gave it and the connecter a thorough cleaning with some electrical cleaner. Then I coated the connecter with electric grease and reinstalled. Now it mirrors my manual gauge within 5 pounds
 
How did you determine "hot"? I am using the same oil and filter in my 87(120,500) and it takes a while for the oil temp to stabilize.

Today (73 degrees) cold start up was 75 idle. After a trip of 20 minutes idle was 40 and cruise@2750 rpm was 65. On the return trip, the cruise pressure was 60, idle 25. On a recent 250 mile trip to TN, with temps in the mid 80's, she cruised at 60 psi all the way. Low 20's at idle.

You are fine. Congratulations on your efforts. You have a great oil and filter.
 
I'm sorry, I was half asleep when I put that last post up. I don't even know what "AWG" was for. I don't have an oil temperature gauge, yet, I was going by the dash coolant gauge. I let it idle in the driveway for about 15-20 minutes. When it got to its normal temp of 200-210, I checked the presure at different RPMs. That's how I got my "hot" temp. Not the most scientific method.

South West Florida. 11 feet above sea level, 93 degrees F air temperature, 100% humidity, 29.99 BAR
 
Just take it for an hour drive, and your pressures will come down. It takes oil longer to come up to temp than water, and it takes longer for oil to cool down. My trans temp was sill 150 degrees after an hour at the vet. When I got my lost pressure readings, the trans was up to 170 degrees.
 
4.0 isn't fussy about motor oil or oil pressure. 10 psi at idle and 40 at higher rpm's is enough for long term reliability.
If you drive foot to the floor bouncing off 5000 rpm then a little more oil pressure at speed is nice..
 
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