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no oil pressure after warmed up

unrestricted817

NAXJA Forum User
Location
fort worth tx.
oil pressure is about 40psi until it warms up and when it's warmed up pressure drops when idling but at about 1500 rpms it rises back up to 15 - 20 psi. has anyone run across this problem?
 
year? engine? oil changed recently? Hard to help when you give us nothing to work with...
 
Number one question is......are you running a Fram oil filter?
 
unrestricted817 said:
oil pressure is about 40psi until it warms up and when it's warmed up pressure drops when idling but at about 1500 rpms it rises back up to 15 - 20 psi. has anyone run across this problem?
Uhhh, YES!

Lots of threads on this here already if you search for them. Causes run from defective oil filters, to defective oil pressure sensors, which sit close to the oil filter, easy to bump and damage them changing the filter, which leads me to the sensor contact wire, also gets loose, dirty and causes funky oil pressure readings, even gets knocked off the sensor, even the dash gauges can act up, lastly wear in the engine can cause it.

Try the easy stuff like check the sender contact wire, change the oil filter, try a different oil (which gets to the questions already asked, what oil are you and have you been using???).

On the older Renix jeeps (87-90) I have even seen a partial oil pressure drop (as high as 5 psi) at the gauge that is caused by poor ground wires, and having all the electrical stuff running like head lights AC blower on Max,....etc.

Jeep says the minimum hot oil pressure at idle in drive is 13 PSI.

IF you have some engine wear you can switch to higher viscosity oils, like 20W50, I did. But find out what the real problem is first. Some here recommend getting a mechanical gauge and testing the oil pressure directly where the sensor goes.

Give us some history on this beast, year, miles, service record....as it is relevant to the question and answers.
 
I would physically remove the sending unit, and verify that you actually have a pressure loss. Start off with changing the oil and filter (use anything but a fram)

If after you've done that and the pressure loss is real and you actually have 0 psi at idle, then you have to start looking at mechanical causes ex: excessively worn bearings, oil pump

And to answer your question, yes I have run across this problem: on Gas engines with 200+k However, the low oil pressure would be accompanied by knocking.
 
Ditch the Fram filter, and unplug the sender from the pigtail (located just above the filter). Blast both sides of the connector with contact cleaner.

Your "problem" will be gone.
 
unrestricted817 said:
it's an 01 hooked it up to a computer gauge is fine running fram and 10w40

By running fine do you mean the computer gauge matched what the dash gauge was reading, or did the computer gauge read higher than the dash? If they matched, you have a problem, in which case it could be a couple of things, but more than likely a cracked head....this is if the two gauges matched.

If the computer one read higher, replace the oil pressure sensor. They're easy to replace, cheap enough, and very easy to damage when changing the oil.
 
UNCC_99XJ said:
By running fine do you mean the computer gauge matched what the dash gauge was reading, or did the computer gauge read higher than the dash? If they matched, you have a problem, in which case it could be a couple of things, but more than likely a cracked head....this is if the two gauges matched.

If the computer one read higher, replace the oil pressure sensor. They're easy to replace, cheap enough, and very easy to damage when changing the oil.

How can you diagnose a cracked head from a single symptom, namely low oil pressure when hot at idle?
 
The cracked head is a potential culprit. This allows water to enter the crankcase. Pick up a slug of water in the oil pump and no pressure! It happened to me. The head cracked between the no 2 and no 3 cylinder and you could see it through the oil fill hole. If this were the case you should see:

Unexplained rise in oil level.
Intermittent low oil pressure - sometimes its ok
Loss of coolant.

You could have the cooling system checked if you suspect.
If this is happened to you, search for an engine- I wouldn't bother fixing the head for fear that the main & rod bearings have been wiped out.
 
My vote is a poor connection at the sender or improper grounds. Try the cheap easy stuff before we get to things like engine wear or cracked heads. Geezz guys you are going to scare this guy into selling his XJ instead of fixing it.
Unrestricted don't worry we will get you through this little hurdle.
 
I was getting erratic oil pressure readings, ultra high and times and it would bounce around alot. Cleaned the contacts on the plug and the sending unit and all was well. Apparently the water proof connection is not oil and grime proof.
 
Ecomike said:
How can you diagnose a cracked head from a single symptom, namely low oil pressure when hot at idle?
Not saying for sure that's the problem. Given the information: a 2001, low oil pressure at idle, only 15-20psi. at 1500 rpm. Not saying that's 100% the problem, just trying suggest other areas to look at given that the 2000+'s are more prone to cracked heads, in addition to the common things like faulty sensors that could happen to any year, that's all.

Not trying to scare him or anybody, just trying to lay out all the possibilities, given the little info we have.
 
He stated that the computer gave back the same information as the dash (I wasn't aware the PCM paid attention to oil pressure?)

I think a few of us told him to check it with a mechanical guage, no point in speculating until he checks it with a mechanical guage.
 
I would proceed as follows:

1. Inspect sending unit (aka: switch) wiring, clean sending unit connector
2. Replace sending unit with a new factory Jeep OEM for best accuracy. Sending units are a VERY common failure on the 4.0 XJ
3. If still unsure, install mechanical gauge where sending unit goes. It is futile to speculate on a cause until you verify if you have an actual problem or not.
 
I had this problem also, cleaned the connector with electrical connection cleaner and put dielectric grease in the holes and gasket of the connector and have not had a issue since. Hot idle is 20 psi with Rotella T 15-40 and Wix oversize filter(NAPA Gold).
 
old_man said:
Number one question is......are you running a Fram oil filter?
Please explain.
 
Oh, I can do that.

Fram commonly uses a phenolic paper end cap on the filter element that can lose adhesion and cause internal bypassing. In the worst case, the oil pressure goes to zero, followed by a spun bearing, possibly a thrown rod, and complete denial by the maker for any liability.

It's happened to me twice with the standard "ford" type filter - the first time cost me $1,800 when I threw the rod into a passing car on the interstate, the second on the replacement motor. I cured the problem of intermittent oil pressure and engine loss by replacing the filter as recommended by a knowledgeable parts guy. It saved the second motor.

I don't buy Fram - because I don't buy cheap filters at all now. You cannot ever save enough money on filters in your entire life to make up the cost of a rebuilt motor. If you can balance a checkbook, you can do the math.

Saving $1,800 is a green enough reason to protect my financial environment.
 
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