high oil pressue at start up.

I have a quick question. I have a 93 YJ with the 4.0 and manual trans. I just got the Jeep as a gift for Christmas and don't know the history of the maintenance. My oil pressure guage goes to the max when I first start the Jeep, but when it gets to operating temps, it hovers around 40 lbs of pressure. I changed the oil to 5W30 Mobile 1 with a Mobile 1 filter last weekend thinking that maybe the prior owner used too dense an oil. It runs the same. I have a smooth idle and it runs and drives great. Is this normal or am I going to be looking to buy a new oil pump? Thanks in advance for any assistance.

Jeff

Oil pumps have pressure relief valves located inside. It's comprised of a calibrated spring and a check ball.
If the the valve is sticking untill the oil warms up, it could cause your symptoms.
Normal is ~25 at idle, and 37 at 1600RPM, when at op temp.

Hallo. Also the oil pressure sending unit can fail.
It is very common by Jeeps. LOL

'92 XJ

Running synthetic may help by cleaning up the oil pressure relief valve and freeing it up. If you search more threads, though, you will see that normal Mobil 1 is not good for flat tappet motors (such as XY's have), due to lack of ZDDP (zinc and phosphate). Should be OK for one oil change though.

This is normal on startup. Until the oil warms up, it is has a high viscosity rating (although the oil manufactures use additives so that the oil behaves as though it is a low viscosity oil at startup). Once the fluid warms up, it loses some viscosity and flows much more easily (once again, there are additives in the oil that make the oil behave as though is has a higher viscosity when warm).

In another words, cold oil-high viscosity. Warm oil=low viscosity. The oil manufacturers use additives to make the oil "behave" as though it was exactly the opposite.

Oil pressure gauge maxing out at start isn't normal. Odds are that you may be at the front end of a problem with the oil pressure sending unit. A VERY common hardware fail on the Jeep 4.0 The fact that it is "hovering" around 40 isn't normal either. With engine hot, pressure should drop at idle and increase when accelerating/at constant speed.

A new sending unit will probably restore accuracy to your readings. I highly recommend purchasing it directly from Jeep for best reliability/accuracy/mating with your factory gauge. I have had very poor luck with aftermarket oil pressure sending units on the 4.0.

If you REALLY want to know what your oil pressure is and to confirm a bad sending unit, temporarily install a mechanical oil pressure gauge where the sending unit sits. Can be rented from many big box stores if you're so inclined.

Factory spec. for oil pressure:

13psi MINIMUM at HOT idle
37-75psi on a hot engine running above 1600 rpm

OK, thanks for all the info everyone. A quick note, maybe I mispoke when I said that it hovers around 40. It stays around 40 while on the freeway, but does drop and rise with idle and acceleration. Since I am completely new to the Jeep world, what oil would you recommend. I plan on using the Jeep as a weekend driver and also for trail use. Thanks again for all the info.

Jeff

Turd_Hurdler said:

Since I am completely new to the Jeep world, what oil would you recommend.

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