Oil pressure dropping to 0 at idle on XJ

ScienceguyXJ said:
It came with a Napa oil filter that looks pretty new, the oil looks pretty clean too. They prob changed both right before they sold it. Regardless I'm throwing on a Mobil 1 oil filter and Mobil 1 syn.
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GOOD MOVE!
If you don't know what oil is in there I would
change it to 10W30 or 10W40 ASAP. before you do any damage.
Most of the dealers & oil change places tend to use a lot of really light weight oil, 5W20-30 or even 0W20-30 is commonly used in new jap cars these days, They stick it in everything that runs because it's what they have on hand & they don't understand or care what weight oil you Jeep needs.
I run (Oil & filter) mobil1 10W40 winter & 15W50 summer in my 01 XJ.
 
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I would start with the pressure sender.... about the same cost as oil change, only less time.

I had similar following a very long freeway run on really hot day. As I pulled into a parking lot the pressure fell to zero and the check engine light went on. It would run normal pressure at speed but at idle it was so low that I was nervous about it. There was no lifter noise or other indication that it was running dry. It always had had low idle pressure but never zero.

About 3 days later I put in a new sender and have not had any false alarms since. It was about 7 minutes to swap out the sender.... That was even faster than an oil change. for about the same coin.

Jeff
 
1. START with the mechanical oil pressure gauge--if the pressure is in fact going to ZERO then you need to work on the engine, wasting all that new oil.

2. AFTER you have good data--actual oil pressure reading, make decisions based on that.

3. YES changing fluids/belts/hoses and a thorough chassis lube on used vehicles is important--put don't waste money that you may need for the rebuild. Wait until you have good data before making expensive decisions.

4. Is the engine idling too low? If it is idling/stalling then the pressure will drop and the check engine light will come on--sound familiar? Manual or auto, idle should be 700~750 RPMs. Check manual in neutral, check auto AFTER 20 minutes of operation when the both the engine and transmission are fully warmed up. Auto idle speed is checked in DRIVE with all accessories turned off, parking brake set, and a helper with their foot on the service brake pedal.
 
Yeah I already changed the oil pressure sensor/sending unit on the filter assembly. I've got a buddy who's going to lend me his gauge and I'll try that to confirm the 0 reading then change the oil.
 
Yeah I have noticed it idling a little low some times...
 
ScienceguyXJ said:
Yeah I have noticed it idling a little low some times...
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Fix the low idle. Textbook is 10 psi per 1000k RPM; I think its in the FSM for 13 psi at idle.
 
Well normally when it's hot or even warming up it seems to be idling fine. It was just one time when it wasn't quite all the way warmed up that it was idling around 500 but haven't seen that since. Probably needs a tune up...
 
Finally got around to checking the oil pressure with a gauge. The gauge confirmed the 0 psi reading of the oil pressure sensor. Next pulled out the oil and there was a little bit of antifreeze looking gunk on the cap on the valve cover but besides that there was nothing in the oil, even this morning no water has separated out of it (however the oil was steaming a little when it came out so there may have been a little water in it). Put in the new oil and filter and took it out and the oil pressure never fell to 0! The oil pressure was still low though, at idle it was at about 10 psi and it never really got above 45 psi at any rpm but at least it never dropped to 0. I did notice the intake, exhaust, and head have been taken off before and the antifreeze looked fairly black so I think they did have a bad head once that leaked antifreeze into the engine but it looks like it was halfa$$ repaired. So I'm thinking get the antifreeze flushed today and probably have to throw on a high volume oil pump and it should be good. The steaming oil does concern me though, hopefully it was just residual water left in the system. Guess I'll have to see what happens to the antifreeze level after they flush it...
 
ScienceguyXJ said:
Finally got around to checking the oil pressure with a gauge. The gauge confirmed the 0 psi reading of the oil pressure sensor. Next pulled out the oil and there was a little bit of antifreeze looking gunk on the cap on the valve cover but besides that there was nothing in the oil, even this morning no water has separated out of it (however the oil was steaming a little when it came out so there may have been a little water in it). Put in the new oil and filter and took it out and the oil pressure never fell to 0! The oil pressure was still low though, at idle it was at about 10 psi and it never really got above 45 psi at any rpm but at least it never dropped to 0. I did notice the intake, exhaust, and head have been taken off before and the antifreeze looked fairly black so I think they did have a bad head once that leaked antifreeze into the engine but it looks like it was halfa$$ repaired. So I'm thinking get the antifreeze flushed today and probably have to throw on a high volume oil pump and it should be good. The steaming oil does concern me though, hopefully it was just residual water left in the system. Guess I'll have to see what happens to the antifreeze level after they flush it...
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There is always going to be some moisture content in the crankcase. Depending on the weather and how the vehicle is used it can be quite noticeable.

Short trips that don't allow the engine and oil to fully warm up can lead to heavy concentrations.

I have issues with "high volume" and "high pressure" oil pumps in engines that really don't require them. A quality new oil pump is probably all you need if the problem is pump related. If the problem is clearances then no pump is going to fix that.

While you are changing the pump re-torque the rod and main caps.
 
Put in Mobil 1 fully syn 10w-30. I take it back it is still falling to 0, just not as much... bought a pump, guess I'll see what that does. Anyone have an idea what the bearings are supposed to measure with a plastigauge? I'll look into trying to re-torque the rod and main caps.
 
ScienceguyXJ said:
Put in Mobil 1 fully syn 10w-30. I take it back it is still falling to 0, just not as much... bought a pump, guess I'll see what that does. Anyone have an idea what the bearings are supposed to measure with a plastigauge? I'll look into trying to re-torque the rod and main caps.
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If you are getting 10Psi Idle & 45psi highway with 10W30 you are OK, I'll bet with 10W40 you will meet the 13Psi at idle spec.

I have seen Jeep 4.0s with oil pressure issues linger on for a couple of hundred thousand miles with good oil & filters.

I know this my be getting old, but I would go M1 oil & filters 10W40 winter & 15W50 summer, I'll bet you get another 100K++ before you have a problem!
 
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But it's still falling to 0 psi sometimes while it's hot, especially when I shut it off and turn it back on...
 
cruiser54 said:
Might be time to drop the pan ind inspect the rod bearings before they get trashed. Put the oil pump in at the same time.
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x2. It sounds like your XJ had a cracked head sometime during its life. Coolant gets in the oil and eats away at the bearings. Since you said the head and manifolds look to have been moved at some point in time, the head was probably replaced after the previous owner drove it around for a bit with coolant in the oil. If you are not losing any coolant, I would say the low oil pressure is probably a result of damage that has already been done by the previous owner. I would replace the bearings and the pump and it should fix the problem. After doing so, I would shorten the oil change interval to maybe 2000 miles for a few changes to get rid of any coolant that may be left in the engine. :)
 
Yup, that's what everyone seems to be thinking. Replace the pump and check the rod and main bearings. Thanks guys!
 
Anyone know what all goes into changing the pump? It seems pretty simple but I don't have a service manual for it yet so I'd like to know before I get into it.
 
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