Oil pressure is a function of rpm, since the oil pump turns proportional to rpm.
There is a bypass valve that above a certain pressure, ports extra oil directly back to the oil pan.
On dead cold start, you should see high oil pressure (40# or higher) even at idle. As the oil heats up, the pressure will drop. You should see at least 15# on a tight engine at idle fully hot. 7or8# is the absolute minimum.
With the engine at "room" temperature (50-90 degrees or so; don't do this at -40 degrees in other words), slowly increase the engine speed from idle to 2500rpm. You should see the pressure increase from a minimum of 15# (more likely 30#) at idle up to 45# or so (usually that's about the bypass setting) by 1500rpm, then stay constant above that.
When the engine is fully hot, you may see 15# at idle, and it will increase and hit 45# at maybe 2000rpm indicating it takes more rpm to produce the same pressure with hot (thin) oil.
Unless an engine is really worn, at some reasonable rpm (2500 or so) you will achieve maximum pressure when the bypass valve is open hot or cold. Note that with VERY thick or very cold oil, it IS possible to exceed the bypass valve oil pressure since the oil is just too thick to bypass enough through the return passages to lower the pressure all the way to its calibrated pressure.
This also depends upon what type (dinosaur or synthetic) and viscosity (10W30, 20W50, or straight 30 weight, for instance). A 20W50 will not thin out as much at high temperature, so you will experience less pressure drop with high temperatures.
The oil pressure in a TJ is "smoothed" meaning time averaged, so the readings are damped, or delayed somewhat. If you install a mechanical gauge, oil pressure changes instataneously with rpm, not smoothly.
Originally Posted by TJeffro
Thanks for the replies guys. I do have another question... Does the TJ usually run with a higher oil pressure? I am always between 45 and 60 on the guage, and sometimes it sweeps along with acceleration. Is this normal?