Oil Pressure drops when idle then goes backup when driving.
 

when i drive the jeep the oil pressure is fine as long as im on the throttle then when im at idle it drops down to nothing

the oil pressure at idle should first be verified with a mechanical gauge ( or even a Ebay digital OP sender)

report your pressures at cold idle, hot idle, etc to the members here can then help you

could be just be an inaccurate sender

be helpful to provide what year, mileage, and any other useful info

Also have a look at your sender and see if it's starting to leak oil.

*Does your engine make noise when the oil pressure "drops to nothing"?

*As mentioned, the problem could be the oil pressure sending unit which is a very common failure on the 4.0 liter engine

*Best strategy with this is always to VERIFY so you really know. Use a mechanical oil pressure gauge. If you don't have access to one, they often can be rented by some parts stores; call around for availability. Then compare your readings to 4.0 liter engine oil pressure spec below.

*If you do replace the sending unit, do yourself a favor and buy a Mopar unit from Jeep for the best accuracy/reliability/mating with your factory oil pressure gauge. Good luck and keep us updated on what you find!

*Not less than 13 psi at HOT idle

*Between 37-75 psi over 1600 rpms

I second the Mopar sensor. It DOES make a difference. Be careful when you buy it. The parts guys almost gave me one for the 2.5L 4-banger!

I know what you're feeling. I get nervous whenever the pressure drops below 40, but the Cherokee 4.0L seems to last with only 15-20 psi at idle. Scary, I know. My WJ 4.0L never drops below 40. As long as the pressure begins to rise above 1000 rpm, you're good to go.

It's completely normal for the pressure to drop at idle because the pump isn't spinning as fast so less oil is moving around.
0 PSI is bad - if it's true. As said above, the gauge on your dash isn't necessarily accurate. But, in alerting you to a potential issue it has done its job.

You don't say how old your oil is. Without getting into arguments about oil spec/brand/magic you do need to keep on top of oil changes and old thin oil will reduce the indicated pressure somewhat. If you change your oil sensor, you may as well do the oil and filter at the same time. (Spin the filter on by hand in case you smack the brand new oil sensor with a filter wrench and have to start again - ask me how I know!)

I believe the low oil pressure in the XJ is totally caused by temperature because of the lack of air circulation in the engine compartment. My idle pressure is around 40 when the engine just reaches full operating temp, but drops off the warmer it gets, the whole block and stuff. I used to think it was caused by the filter adapter restrictions, but I eliminated that by using a slightly smaller filter screwed directly into the block and it didn't change a bit. Running 10W30 oil, my idle pressure when fully hot is 15 psi, but running Rotella T6 5W40, it's 25 psi. Now that the outside air temp is much colder, it idles at 30 psi. In the extreme heat of the Great Southwest, an external oil cooler might help.

Three lifters are not filling with oil
 

Hello, I did a stroker build on a 2001 XJ and started it yesterday for the first time. I had what appeared to be coolant coming from the rear of the valve cover so I freaked a little bit and pulled the valve cover to see if my head cracked. No sign of cracking or coolant in the top of the head, probably just some spilled coolant that ran back to the rear of the cover, but I did discover that 3 of my lifters were not getting oil. Aside from the front two and the very last ones, all were squirting streams of oil. I am using CompCams 10-30 startup oil. All lifters appear to be pumped up. There is no up and down slack whatsoever, very solid. I have a Melling HV pump with the proper length pick up. Does anyone have any advice on how I can get these things squirting oil?

THANKS!

You need to start a new thread. Your issue is different than this one and it gets confusing for others following it and reduces the odds of you getting good replies.

You need to adjust your valve lash to make sure the lifter plungers aren't bottomed. The lifters should be preloaded at between .020" and .060" and is adjustable by using shims under the rocker pivots. If everything is right, you tighten the rocker bolt until you just can't spin the pushrod with your finger, then continue tightening it to 21 ft/lbs. 3/4 turn gives you .042" which is perfect. Each 45* turn of the bolt gives you .007". At only .020", you run the risk of the snap ring in the lifter popping out and anything more that .060" bottoms the lifter plunger and may hold the valve open slightly. Using stock parts and a new head, everything should be withing tolerances, but a milled head or decked block will change things. You can also buy specific length pushrods if it's way out of whack.

IN ADDITION: If you're using a "Clearwater head, make sure the pipe plug for a rear temp sensor is sealed. It's on top of the head next to the valve cover at #5 cylinder.