This only happens when it's hot? How long has it been since you changed your oil? You are using the recommended viscosity? (should be 10W-30). Other than that, the pump could be getting tired or your pickup screen could be partially clogged.

 

Replace the sending unit. Both of the Cherokee's we have did the same thing. My wife's 01 Grand started doing it out of the blue. Pressure would drop to zero at stop lights randomly and then go right back to 40+ when moving. New sending unit from the dealer, all good. My son's 96 GC started doing the same thing then it read 0 all the time. New sending unit, all good.
I asked the parts guy at the dealership if they go thru alot of them and he said they have a hard time keeping them in stock....

 

Replace the sending unit.

Thats what I thought too but that doesnt explain the stalling

 

My xj would stall when the pressure dropped too low and once the sending unit was replace it ran perfect

 

What carb are you running? What is your idle in D? You may have a vacuum leak or clogged idle tubes causing the idle to drop to low. It should idle at 600 rpm's in drive and it should be idling smooth. Oil pressure should be at least 20#'s at 600 rpm. How's your trans fluid look?

 

When is the last time it was tuned up?

 

What carb are you running

It has a 4.0 fuel injected
It was tuned up, plugs,wires,cap,fuel filter,timing chain etc.etc. less than a year ago.
It only runs rough when the pressure drops at a stop light and not all the time usually that is when it stalls.

How's your trans fluid look

What does trans fluid have to do with oil pressure?

 

What does trans fluid have to do with oil pressure?

It has an auto? I was going off of you're specs you have under your avatar.. If it's still and auto and the coverter is "dragging" it could stall the engine out. Kinda like trying to stop a manual trans without pushing in the clutch. If the trans fluid is burnt or discolored you may have a trans problem showing it's head.
My wifes Grand Cherokee ran fine with no oil pressure reading from the sender but it did set off a warning chime and light the Check Gauges light on the dash. That said it must've gone thru the ECM. The engine never stalled or stumbled so I doubt low oil pressure has anything to do with the stall.
To me it sounds like low idle just before a stall is causing the low oil pressure, not low oil pressure causing the stall. That's just where I'd start looking.

 

Probably just excessive bearing wear.

 

I'm gonna go with this,try replacing the sending unit just incase it is it since there cheap.I'm gonna go with excessive bearing wear too.Just cause it's low mileage doesn't mean it was put together right or mistreated previously. Run a thicker oil to help keep oil press. up.It's a band-aid but it will help keep your bearings alive if that's what it is.

 

At low oil pressure will the oil press. switch shut down the engine ?

 

my cherokee started having the oil pressure drop to zero and then back to 40 intermittently. i just replaced the oil pressure switch and still havnt test drove it yet. when i was in the part store the guy ask me if it had one ,two, or three wires. he said if it had three then the third wire is for a fuel pump cut out to protect the motor if the oil pressure went away. i didnt think this was done by jeep tho.

 

Oil pressure problems

I have been researching this topic for months and reading everything that I could find on the internet to solve my oil pressure issue with no success. I change the oil every 3000 miles and take care of my equipment.

Jeep: 2001 TJ 4.0L

Problem: When starting the jeep when the engine is cold the oil pressure is perfect at 40psi. After the engine reaches operating temperature, the oil pressure drops to less than 10psi and sometimes to 0psi! If I increase the rpms to around 1500 or 2000, the oil pressure begins to increase to around 10-15 psi.

Things I have done to TRY to fix the problem: Changed the oil and oil filter (10w-30). Changed oil pressure sending unit and checked with a manual gauge. Performed a compression check on all cylinders (checked out good). Changed oil pump, pick-up tube, and screen.

Anybody been this far and not have any success? I'm guessing the next step would be main/rod bearings. Any help would be appreciated.

 

You said you put a manual gauge on it. Did you use a mechanical gauge or just attach a different gauge to the sending unit? How many miles are on it? if the oil pressure really is that low you have a blockage in an oil passage or worn bearings or both.

 

I attached a mechanical gauge to the block to bypass the sending unit. I just hit 110,000 miles on the jeep. How would you troubleshoot an oil blockage?

 

I'm having the same problems as you. I just rebuilt my oil pump as it was 0.001" over specs for gear to body and gear end clearances. I got slightly better pressure but its still not right. Did you ever fix yours?? I'm guessing bearings too but dont want to keep throwing money at it. Right now I get 45pis cold, then about 7 hot idle 10-13psi driving. Oh and I'm running 10-w40.

 

If the new sending unit didn't bring the pressure back to normal,

Go to a parts store and buy some Plastigage. Drop the oil pan and measure the clearance on the main and rod bearings.

Check the maint manual, but IIRC, .003 is in-service max clearance on the bearings. Until you measure, you aren't diagnosing the problem, just spending money.

 

On high mileage motors go to a straight 30W. It will help a lot to bring the oil pressure back up.

 

I recently measured clearances on the main and rod bearings. All checked out to .001 - .002".

 

try running 20w50 i have a 92 yj 2.5 with 163,000 miles my pressure does same thing i have been running 20w50 for about 20,000 miles and it has helped im going to put new pump on next week ill post rather it helps or not.

 

I haven't read of many instances where a pump has helped. I replaced my pump with the same result. I'm guessing the last thing on my list it to r&r the cam and bearings.

 

I recently rebuilt the bottom end of my son's 94 4.0 FI. Rods, mains, pump, screen with little effect on the oil pressure. I'm affraid the next step will be cam bearings as they are on the same galley as the mains. He is also having the stalling issue at an idle in gear (auto). I'v seen mention of possible torque converter, transmission issues. Is that a common problem.

 

I recently bought a 2000 Cherokee (4.0) with this same problem. 40 PSI cold, drops off to 0-5 PSI warm at idle. The guy I got it from just put a new oil pump in it and a new sending unit before that. My next step is to pop out the motor and replace rods, mains and cam bearings.

For what it's worth, Ford straight six motors with low oil pressure problems are usually caused by worn cam bearings (unless you have 0 pressure, then it's the pump). I'm wondering if Jeep 4.0 motors could have the same thing???

 

both of our 4.0s (wife's 93 and my 87) run 40+ while driving and idle at 15ish

as long as your getting oil everywhere, i wouldn't be overly concerned with pulling hte motor and replacing bearings. if your not getting any knock and everythings good with it being like that, it might be just like the chevy 4.3l v6. they just get low low oil pressure and it never breaks anything. just make sure oil is coming out of hte sprayer bars or whatever there is at the highest point in the system and keep going.

 

It's not knocking, or making any unusual noises that indicate dangerously low oil pressure. But the Red "Check Gauges" light that glares you in the face is annoying. I happen to be unemployed currently so the time to address this problem is probably now before I don't have any free time.

I wouldn't mind low pressure (like 5-10 PSI at idle) but this engine drops to "0" when it's hot. That's when the check gauges light comes on.

 

i think if you really had zero oil pressure then you would very quickly develop a bearing knock. it sounds more like the gauge or sending units are bad.

are the o-rings between the pump and block good? or does the oil pump for this engine have a split line? i ask because i have never messed with a 4.0 oil pump but have done lots of oil pump related things on bmw motors. if you have the wrong type of o-ring or it gets pinched on those motors, then you will have the same symptoms. also, the pick up is removable on the bmw pumps so sometimes is comes loose so the pump is trying to pick up oil and air, like when a soda straw gets a hole. maybe this is your issue?

 

i think if you really had zero oil pressure then you would very quickly develop a bearing knock. it sounds more like the gauge or sending units are bad.

are the o-rings between the pump and block good? or does the oil pump for this engine have a split line? i ask because i have never messed with a 4.0 oil pump but have done lots of oil pump related things on bmw motors. if you have the wrong type of o-ring or it gets pinched on those motors, then you will have the same symptoms. also, the pick up is removable on the bmw pumps so sometimes is comes loose so the pump is trying to pick up oil and air, like when a soda straw gets a hole. maybe this is your issue?

MY neighbor was actually the guy who put in the new pump and sender. Then I bought the truck from him. When I worked for Volvo, blown out oil line "o" rings was a common problem also. I don't know what that 4.0 pump looks like either, I didn't see it while he had the oil pan off.

This motor looks very easy to get out. And it's much nicer to have the engine flipped upside down in a clean shop for doing bearing work. But whatever problem I find I will be sure to pass it along.

Worth noting, The guy I know who worked for Jeep/Chrysler said make sure I don't use Fram oil filters on the 4.0 motor. Apparently there is too much resistance through them for oil flow. He said it's worth the couple of extra bucks to use factory filters.

 

As an Auto mechanic by trade I've worked with a lot of guys over the years who worked at various dealers as well. I remembered a guy named Doug who used to work for Jeep/Chrysler. I called him and asked his opinion. He said I should have called before wasting my time with the oil pump and sending unit (not that is was a terrible idea to replace them anyway). He said to go right to the cam bearings. I guess I'll get started on that tomorrow AM.

 

oh man, that sucks. at least your pump is new though

 

cool