Oil pressure drops at idle; sends engine in limp mode

Maybe you got lucky and your oil pressure sensor died. Have you verified with a manual gauge?

-Mac

^^^ Agreed. If you bought one of the pressure sensor units from AutoZone, Advanced, ORileys... In short, they suck. You want the Mopar one.

BEFORE you buy one though, I'd test with a mechanical pressure gauge to verify the pressure is accurate. If it is still low it might be something internal like bearings.

Dumb question here - but are you sure its a limp mode the Jeep is going into? Running really rough and stalling doesnt sound like a limp mode to me - that sounds like a bearing starting to heat up and seize from oil starvation or possibly a blocked oil journal or similar.

@Jerry Bransford would be good to weigh in on this but 10 psi at idle would be okay.

The rule of thumb for acceptable oil pressure is 10 psi per thousand engine rpms…

Mine does the same thing but never runs bad, ive been through 3 new sensors with the OEM being the worst offender, the Napa sender is doing the best and only trips the gauge warning after driving on the freeway for extended time
13psi at idle is the factory spec (for my '06)
Mine holds at 10psi too, might be just a tad too low

That seems low for an engine that was just overhauled, but I am basing that on older V-8 engines that I am familiar with and not a Jeep 4.0 engine (my 350 has 35 psi at hot idle).
As said above, 10 psi per 1000 RPM is generally acceptable.

Yeah I went from a working oil pressure sensor to an oil change...then the sensor failed...three defective store brands and a Mopar finally did the trick. Would love to know WHY exactly those other ones don't work. Voltage, resistance...you'd think someone could make a little PCB with a potentiometer to dial in the sensor. Maybe even a couple LED numbers to show the pressure.

-Mac

10 seems low for a newly rebuilt engine, but is sufficient.

My 06 trips the warning when it goes below 6 PSI.

I've restored numerous GMC/Chevy trucks. All did same thing as described here ,oil pressure good cold and goes to zero when hot. My trucks only had a direct reading oil pressure gauge. After confirming that the gauge was correct, first thought was oil pump is bad. Took oil pumps apart and found nothing wrong or worn internals. On my 1940 1 ton dual wheel flatbed when I first started it cold oil psi read 30-35 psi. When hot it dropped to zero. Talked to an expert on GMC engines and he said you can run it with no oil press when hot at idle with no issues. He said if you want better oil pressure and the crank is in good shape just replace with standard bearing size. I did that. The oil pressure improved because I reduced the clearance between the bearing and the crank. Most bearing wear is in the cam bearing. In line sixes cam bearings take a beating. Ad dirt and it just speeds up the wear rate. thus lower pressure readings at idle or low rpms. Take the valve cover off and start cold engine and see exactly how much oil is making it to the rockers. Then do the same thing when it's hot. The difference is in the bearing clearances allowing flow of oil but not enough to build pressure. Back then valve covers were open vented which allowed lots of dirt to be sucked into the engine which prematurely wore out the bearings. Today's engines are configured not to suck any dirt into engine when new thus improving bearing/engine longevity however mods, mudding and shear abuse from driving style can add up to low oil pressure at idle when hot. On my 51 GMC oil pressure was low . I spun #2 rod bearing 100 miles from home in 2006. Had motor rebuilt and since then the oil pressure runs 50-60 psi cold and going down road at temp on the hottest day of the year no oil pressure is 30 psi when hot and at idle. Coolant temp 180* constant. Not bad for a 10,000 lb fire truck.
As long as you have oil coming from the peep holes on the rocker at idle your ok. If not it's time for a rebuild. I use napa 10-30 for all my jeeps, 40 GMC, 51 Chevy dump. In my 51gmc fire truck I use 15-40 rotella. This truck is only operated from May to November.

maybe it has to see 13 to be happy.

Running rough like that sounds weird. I've never even seen a 4.0 go into anything I would think of as "limp mode"...at most I suppose it might revert to open loop but all that should do is run like it does on a cold start.

What did you do with the rod and main bearings and journals during your overhaul?

10 lbs at idle would suffice provided pressure increasing with rpm. As others stated 10 lbs per each 1000 of rpm.

That said it’s definitely on the low side. I would be very conscious on oil selection.