Oil Pressure question

Xj 99 4.0. When I start it up after being parked the oil pressure will rise to around 40 at the highest when idling. When I drive Pressure is fine, But when I stop at a red light or to park Check guages light come on and I notice my oil pressure is at( 0 ). This happens every time like clock work.

When i'm driving oil pressure is fine. When I idle at a stop it drops to (0).

What is this from and how serious is it ?

its probably the pressure sending unit. Like 8 bucks at autozone. Im assuming you have oil pressure because your car is still running. Without it you would throw a rod ect. probably just a faulty reading. The sending unit is located by your oil filter super easy to change

-mike

If in doubt ,have the oil pressure check,just in case,thats an expensive fix if you really are losing pressure.However,without any noise I have to agree,sending unit:compwork:

Sport cherokee said:

Xj 99 4.0. When I start it up after being parked the oil pressure will rise to around 40 at the highest when idling. When I drive Pressure is fine, But when I stop at a red light or to park Check guages light come on and I notice my oil pressure is at( 0 ). This happens every time like clock work.

When i'm driving oil pressure is fine. When I idle at a stop it drops to (0).

What is this from and how serious is it ?

Click to expand...

Change the CLOCK, LOL:roll:

All joking aside, Several of us have experienced similar problems including me. I doupt it is the sending unit in your case.

How long has it been since you changed the oil and filter?

What brand and viscosity oil and filter do you have?

How many miles on the engine?

(1) I would not drive it the way it is. Way too risky.

(2) First get a new oil filter (use a good quality one like Mobil one or K & N, anything but Fram) , change the oil filter.

Test the hot oil, idle oil pressure with the new filter.

Then add a quart of Lucas 100% petroleum based Oil Additive.

Restart the engine and retest the oil pressure. Then test drive it to see if the hot idle pressure has come up any.

The minimum hot idle oil pressure is 13 psi for Jeep 4.0's.

If the hot oil, idle, oil pressure comes up, my next suggestion will likely be to change the oil to 20W-50 oil with a quart of Lucas oil additive.

Let us know the results of your filter and oil test.

To read a recent, thourough discussion of your problem follow the link below:

http://www.naxja.org/forum/showthread.php?t=901210

If you've driven it this long, and the engine is not making any weird noises, my money is on the sending unit. Pretty common to go out on the 4.0. Also really easy to damage when changing the oil filter.

mnaemark said:

its probably the pressure sending unit. Like 8 bucks at autozone. Im assuming you have oil pressure because your car is still running. Without it you would throw a rod ect. probably just a faulty reading. The sending unit is located by your oil filter super easy to change

-mike

Click to expand...

I have seen this before in my own jeep and JeepinAaron has too (see the link I just posted). I think the problem is that some hot spots start developing somewhere that over heat the oil and the viscosity drops enough to where there is still oil flow (evidenced by the gauge pressure at start up idle, 40, and operating rpms reported as OK, but the hot idle later on drops to 0) but not enough pressure.

The fact that the gauge is changing values like clockwork, reproducably, and registering a range from 0 to 40 psi from a warmed up engine to a cold engine tells me the sender is working. The calibration could be off and he could still have some pressure, but a 40 psi drop seems to me to be an indication of some bearing/oil trouble developing.

I got to mine when it hit 10 psi at hot idle. JeepinAaron was already down to something like 12 psi max and 3 psi at idle. A new sending unit did not fix his problem or mine. Lucas oil additive and 20w-50 got him up to 14 psi at hot idle and lowered his water temperature by 10 degrees too!

I have put about 8,000 miles on mine since switching to 20w-50 with the Lucas oil additive, and the pressure is holding so far at 50 cold/20 hot idle, and 55 cold/40 hot at 2000 rpm.

In my case I also changed the main bearings (like JeepinAaron) which had some wear, but mine were not toasted, just worn enough to warrant new bearings and the journal looked new.

I installed a high flow oil pump which might explain why I reached 20 psi and JeepinAaron only reached 14 psi hot idle presure.

Two question

Anyone with a link to the pressure sending unit?

and does anyone have an on-line manual?

Can someone provide an on-line source for the high flow oil pump? I checked my usual sources like Quadratech...no luck...at least not in the way Quadratech describes their replacement oil pumps.

Replace the sending unit, trust me it's $20, and the easiest place to start.

The sending units are notorious for giving false readings. If you want to change the oil pump, or rebuild your engine thats up to you. Theres a 90% chance that is totally unnecessary

I had the same problem a couple months back, but mine would only go up to 30 PSI when driving, and then hit 0 once it was warmed up and at idle.

The sending unit is just above the oil filter, and can be removed with a cresent wrench. Takes 10 mins to replace.

Question one, what brand oil filter are you using.

I just came back from the oil changer got 20-50 and new filter now won't drop below 10. didn't add lucas just yet wanted to see what the basic oil chage and filter would do.

I just really need it to pass smog and it would have failed with that check guages light on.

Next I want to change the sending unit and when I get some more bucks change the bearings.

Thanks.

Sport cherokee said:

I just came back from the oil changer got 20-50 and new filter now won't drop below 10. didn't add lucas just yet wanted to see what the basic oil chage and filter would do.

I just really need it to pass smog and it would have failed with that check guages light on.

Next I want to change the sending unit and when I get some more bucks change the bearings.

Thanks.

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10 PSI Minimum now, after seeing 0 PSI at idle before with just an oil (20W-50) and filter change! Impressive! Most Impressive!

So please let us know what the Lucas additive does to the pressures, as well as the other changes you are planning!!!

I am planning some filter changes to test oil gauge pressures with different filters next.

Have you noticed any change in the upper coolant gauge temperature after a full warm up? Has your peak coolant temperature dropped any???

Sand-Dog!

I bought my highflow oil pump from the local Autozone store. They stock it! Therefore, they should have it online at autozone.com as well.

Sport cherokee said:

I just came back from the oil changer got 20-50 and new filter now won't drop below 10. didn't add lucas just yet wanted to see what the basic oil chage and filter would do.

I just really need it to pass smog and it would have failed with that check guages light on.

Next I want to change the sending unit and when I get some more bucks change the bearings.

Thanks.

Click to expand...

I hope it ends up being your sending unit. Always, always change your oil and filter, at least every 5 k miles. That is the life blood of your engine. Sure you can go longer, but things like what you are experiencing now will show up later.

shit,,,,,I am running 6 quarts straight Lucas in my 4.0!!!! I run about 42 while driving and then it drops to like 20 when at hot idle.

Rebel XJ Monster said:

shit,,,,,I am running 6 quarts straight Lucas in my 4.0!!!! I run about 42 while driving and then it drops to like 20 when at hot idle.

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Are you saying you already use straight Lucas, or that you are going to use straight Lucas? Details Man, please!!!!!

I didn't know you can use that much lucas at one time. Napa has the best price around 36.00 for the sending unit. I coudn't find any cheaper. I don't If I will use lucas yet probaly will decide after I install the sending switch. By the way I'm using 20-50 quaker state.

I was shocked:shocked: at the pure Lucas myself.

I changed my pressure sensor unit and no change in my pressure. Might drain a quart or two and replace it with lucas and see what that does.

Well, well, well, now you have a spare pressure sensor. Too late to use it for a christmas ornament this season? LOL.

But I will be REAL curious what the Lucas does to your pressure and coolant temperature???????

One more question on this subject. Is Extemely low oil pressure a sign of a bad head or head gasket leak?