When I start the jeep, the pressure is about 40 or in the middle. After I have been driving for about 25 minutes, the pressure was around 15. Is this normal? I checked the oil level, and it was full.

 

you have 15 at idle but it climbs while you rev the engine, correct? you are ok as long as it climbs up when the engine rpms climb. it won't climb exactly as quick as the tach but it should go up and down with the rpms.

you just don't want zero at idle. that can causes knocking and other noises that you shouldn't be having. if it drops lower, you might want to consider a new pump

 

A general rule of thumb is a minimum of 10 psi per thousand rpm at full operating temperature.

Keep in mind that oil viscosity plays a rule, as does operating temperature. Consult owners book for correct viscosity.

Of course before declaring the engine bad, or good oil pressure wise, make a confirmation reading with a known good gage.

7 psi at low idle, fully warmed up is ok. On freeway at 2500 rpm you want at least 25 psi. These are for when fully warmed up. If owners book allows a thicker oil, and you are at the low end of pressure, then that allowed thicker oil maybe helpful in restoring to a higher pressure.

If your fuel system is running rich, say a leaky injector, or what ever, excess fuel can leak past the piston rings and mix with the oil, this thins the oil, which drops the oil pressure, and is very harmful to the moving bits of the motor.

 

Using a quality oil filter there, too?

 

Can we please start the Holy War of oil pressure again? I'll throw in my rule of thumb. I've heard (a rumor) that a solid(what ever that is) warmed up engine should idle at 15 psi and it should go up 10 psi for every 1000 rpm. No mention in the rumor of what kind of engine, what design era, what type of oil, what climate, etc.

I've heard much less solid info on when an engine reaches broken.

(Unrelated supporting evidence of nothing related to Jeeps) I've been driving my Subaru for about 5 years now with hot idle at about 8 psi going up to about 65 psi at high rpms. (Cold idle is about 85 psi) (I don't think a football is a very good Christmas present.) Except for the low pressure at idle it fits the rule of thumb and the factory spec. Also, it was probably always like that, but 5 years ago is when I added a pressure gauge. (The Factory Service Manual for that engine calls for 14 psi at idle and 44 psi at 4000 rpm.) I'm still driving it and having no troubles, and the pressure is not, so far, dropping slowly with miles on the clock.

(Therefore I am the ultimate expert on all things oil)

Anyway, There is my two cents for what that's worth. A couple laughs, I hope.

 

OP, what pressure at what RPM?