Low oil pressure at hot idle

I have a 2001 xj with 116,000 miles on it. The other day when pulling into work the check guages light came on and I noticed that the oil pressure was reading 0. The motor was not making any unusual noises and when I reved it up the oil pressure came up with it and then return to 0 when I let it idle down. When I drove home after work the oil pressure was at 40 psi when at cold idle and stayed around 40psi when driving on the freeway. When I got home I let it idle and the pressure then stayed at around 10 psi. The fallowing day the after driving the oil pressure was again going to 0 at warm idle and holding 40 psi while driving. These readings seem to bee good one day and not the next day. I'm thinking it's the sending unit, what do you think?

You're probably right about the sending unit... Search around here and I'm sure something will pop up.

Also what brand oil filter are you using? I had the same problem using Fram filters.... I switched to Napa Premium brand (Wix??) and the problem went away. It stays above 30 psi all the time now.....

Edit:

I see you have been looking around here.... Good job... Again, try switching to a better brand oil filter... I'm not bashing Fram... It's just that the Napa filter seems to hold pressure better in these tractor motors we have....

Thanks for the info, I cant remember what kind of oil filter is in there now but I have a K&N that I will put in when I do the oil change. Have you heard any good or bad about the K&N oil filter?

It's got to be the sending unit, the inline 6 are great motors and this one has way to little miles on it for it to be bearing problems.

Jonesy51 said:

Thanks for the info, I cant remember what kind of oil filter is in there now but I have a K&N that I will put in when I do the oil change. Have you heard any good or bad about the K&N oil filter?

It's got to be the sending unit, the inline 6 are great motors and this one has way to little miles on it for it to be bearing problems.

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Like I said, keep searching, you will find the answer.... I'm not trying to be an ass, I just don't have much experience with your problem.... Once I switched from the Fram to the Wix my problem went away... Sorry, I have zero experience with the K&N oil filter..... Hope you solve your problem soon....

Original_MudButt said:

Like I said, keep searching, you will find the answer.... I'm not trying to be an ass, I just don't have much experience with your problem.... Once I switched from the Fram to the Wix my problem went away... Sorry, I have zero experience with the K&N oil filter..... Hope you solve your problem soon....

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Again thanks for your input, I didnt think about the oil filter being a problem.

From what I gather it has something to do with the relief valve.... And WTH are you doing up so late???

Original_MudButt said:

From what I gather it has something to do with the relief valve.... And WTH are you doing up so late???

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working a night shift... And WTH are you doing up so late???

LOL!... Hyper kinetic.....

That said.... I'm in bed......

K&N is a quality filter.

Sounds like a fluky sender. But you need to get a mechanical gauge and verify. You should have 13 psi minimum at hot idle...and with only 110k miles you should probably have a bit more than that. Should go up to something like 40 psi when you're on the throttle (hot engine).

Robs92XJ said:

Sounds like a fluky sender. But you need to get a mechanical gauge and verify. You should have 13 psi minimum at hot idle...and with only 110k miles you should probably have a bit more than that. Should go up to something like 40 psi when you're on the throttle (hot engine).

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That plus change the oil to a better quality??? and or higher grade. Grade 5-??? what ever is the norm in the winter may not be good enough in the summer. I use 15-40 all year round and the oil pressure never falls below 30 PSI when hot and at idle.

1. Run a good quality filter. Mobil 1, K&N, Napa Gold (Wix) are all good choices. (Napa gold will be the cheapest of them).
2. Run good oil. I personally run a 10W30 in the summer months and a 5W30 in the winter months.
3. If you're still having these readings, get a mechanical gauge and verify what numbers you're actually getting. The gauges in XJ's aren't known for being the most accurate in the world. If you see higher readings with a mechanical gauge than you are on the dash, pick up a new oil pressure sender unit.
4. It does stand to be said that a 2001 does have the 0331 head, which is prone (but not 100% troublesome across the board) to cracking. You'll notice loss of coolant first usually, followed by a drop in oil pressure. A mechanical gauge reading will verify if it's the sender unit or a more serious problem.

How does your oil look? Milky at all? Coolant level still holding good?

techno1154 said:

That plus change the oil to a better quality??? and or higher grade. Grade 5-??? what ever is the norm in the winter may not be good enough in the summer. I use 15-40 all year round and the oil pressure never falls below 30 PSI when hot and at idle.

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If he can confirm that he has 0 psi with a mechanical gauge then I think his problems are beyond what can be helped by running thicker or more expensive oil. It would either be an issue with the pump or bearings, maybe the oil filter if its a fram. He does have a 2001, so it is possible the head is cracked and coolant has been getting into the oil. Over time that would wipe out the bearings and cause low oil pressure.

But he says it will have 10 psi one day and 0 the next. To me that sounds like an electrical problem in the sender/gauge circuit, or possibly sludge buildup within the engine blocking oil passages intermittently. If he truly had 0 psi he'd probably have knocking lifters and connecting rods.

Things I would verify:
-Adequate oil level in pan.
-Oil filter is not fram. Something like Wix, Mopar, K&N, etc.
-Actual oil pressure w/ mechanical gauge
-No excess sludge buildup (pull valve cover). If there is much sludge, try flushing the engine.

If low pressure is confirmed by the mechanical gauge and there is not a sludge problem, the next step is to check the bottom end.

Also, I would not drive it until I had confirmed it had acceptable oil pressure. If for some reason it has actually lost pressure, it might still be possible to fix the problem by replacing the pump and/or bearings, without doing a full rebuild. If driven to the point the rods are knocking, it gets more complicated.

Only one way to go here. VERIFY with a mechanical oil pressure gauge. It installs temporarily where the oil pressure sending unit resides. This tool can be rented from many big box parts stores for a few bucks.

Good data = good decisions. TEST

Low oil pressure makes me nervous. Are you using any coolant? The #1 symptom of a cracked 0331 cylinder head is "unexplained coolant loss".

I changed the oil and oil filter and installed a new sending unit. This fixed my problem. I am now showing 13-15 psi at hot idle and about 50 psi when driving.Thanks for eveyones input.

While 13-15 psi is not "impending death" low, it is basically at the absolute minimum acceptable pressure. I would definetly keep a close eye out for coolant loss and milkshake oil, and you might run a thicker oil (like 15w-40 instead of 10w-30) at least during the summer. Lucas Oil Stabilizer, while somewhat controversial, does help raise oil pressure. I run it in my '92 with 220,000 miles on it, and I have 20 psi at hot idle.

I didn't see it posted... but did you ever pull the code when your Check Engine Light came on? What was the code?

My '01 sees the oil pressure fall (on my gauge) a bit at idle when it gets hot out. I'm running a good Ford filter and 5W-40 Synthetic oil... I suspect our sending units are suspect at best. As mentioned, get a good mechanical pressure reading and go f/there. If you don't have any of the other mentioned symptoms (coolant missing, etc...) then you're likely ok.

Why would anyone want to run a 5-?? grade oil in a 4.0L engine I never did understand.

Why would anyone run a 5-?? grade oil in a high mileage 4.0L :confused1 .

Why would anyone run 5-?? oil in any internal combustion engine when it is this hot is beyond my mediocre comprehension.

Redsnake said:

I didn't see it posted... but did you ever pull the code when your Check Engine Light came on? What was the code?

My '01 sees the oil pressure fall (on my gauge) a bit at idle when it gets hot out. I'm running a good Ford filter and 5W-40 Synthetic oil... I suspect our sending units are suspect at best. As mentioned, get a good mechanical pressure reading and go f/there. If you don't have any of the other mentioned symptoms (coolant missing, etc...) then you're likely ok.

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I didnt have any other symptoms, no knocking, coolant missing or milky oil. I installed a new sending unit and it seems to have fixed the problem. The check engine light never came on, it was the check gauges light.

techno1154 said:

Why would anyone want to run a 5-?? grade oil in a 4.0L engine I never did understand.

Why would anyone run a 5-?? grade oil in a high mileage 4.0L :confused1 .

Why would anyone run 5-?? oil in any internal combustion engine when it is this hot is beyond my mediocre comprehension.

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If that's a reference for me... to each their own.

The oil I've started running in my 4.0's is Rotella T6 5W-40 and so far, so good.

If the oil was always hot then running 40W would be fine, however here in Oklahoma it get's quite cold in the winter and quite hot in the summer. I don't want just a 40W oil in my motor on cold days during start-up...