qwain76

I have a 98 XJ 4.0 with 213k miles

In the previous 1-2 years have always noticed that when I get to a stop light, the oil pressure sometimes dropped from the normal 40 psi. It usually never went below 25.

Lately this phenomenon started being more importand and happening more often, I noticed that in several cases the oil pressure drops to low values, around 10 psi. The check gauges light never came on, so far.

It is very difficult to identify a rule of thumb for this to happen, but I would say that this mostly happens when the engine is at low rpms and it is getting warm (i.e. it is not cold, but not fully warm). Sometimes reving up the rpms brings back the pressure to 40 psi, but sometimes it does not work and the pressure stays low. Then suddenly it goes high.

My car has no leaks, however the oil level was low (below the "add" mark). I added 2 qts of oil but it did not change the behavior.

Is this normal? Or do I have a problem? What should I look for?

FYI I use 10W-30 high mileage oil and mopar filters.

Thanks!

landude

Oil sending unit is probably on it's way out.
However 2 quarts is quite a bit, when was your last oil change?
How does you're coolant look with you take off the cap?
Does it smoke like a *****?

Roler

fluctuation is normal, will increase when you drive and drop when you stop.
the sending unit is not expensive and if you feel it 'sticks', swap in a new one.

If you want you can take pressures with manual gauge to verify, but I wouldn't botter in your case.

qwain76

Thanks guys, even if I am a mechanical eng I have to admit my ignorance about the oil sending unit. Never heard about its existence before tonight.

Can you guys tell me what does this part do and where I can find it?

I tried at some autopart online sites but did not find straight answers.

Thanks and sorry again..............

bigbadon

I agree Roler. Due to many variables pressure at hot idle means little.
If it goes to zero then you have a problem. My 4.0 has 4psi and it has been running fine for over 3 years now.

Glockmeister

It's located right above and in front of oil filter. It takes a deep well 1 1/16 socket or a crescent wrench to get it off. ( socket is faster) about 50.00 at O'reilleys with a lifetime warranty.

qwain76

Landude,

i changed the oil 5k miles ago approx.
i will check the coolant color tomorrow.

thanks

bigbadon

Your overdue. What is your idle speed? Is idle speed steady or is it loping. What is pressure at highway speeds...that is the one to pay attention to.

qwain76

idle speed is steady below 1000 rpm.
at high engine speeds pressure is normally good around 45 psi

Xcessive

For what it is worth, after driving about 15 minutes in my 96, if I pulled up to a light the oil pressure would drop to 5psi. After messing with a bunch of stuff and replacing the pressure sending unit I finally just ran a piece of wire into the port where the sending unit threads in and worked it around a bit. Haven't had a fuel pressure issue since. Guess it was a chunk of sludge/carbon stuck in the port. Might as well start with the free stuff first.

qwain76

Xcessive I seem to have the same issue! The pressure drops to 10 psi after 10-15 mins I turned the car on and when I get to a stoplight. Stays there for a while and then it goes away. Sometimes it comes back, sometimes not. I guess in these cases, when there is not a clear behavioral path, contamination can be the reason.

Regarding one of the questions landude had, I checked my coolant and it is black. So I might have another issue to fix here. What do you suggest to do in these cases (loss of 2 qts of oil in probably 3-4 months, without any spot under the car and black coolant)?

Thanks

RTorrez1

Just curious, What brand of oil filter do you have on it? A crappy brand such as fram can give you low oil pressure reading too. Plus it can cause dry starts too.

Roler

And frequent cold and/or dry starts is what's probably the worst for longevity of your 4.0...

landude

When was the last coolant change?
If the answer is I can't remember, do it. Like tomorrow.
If the answer was within the last 12 months I would say you need
to do a compression test sooner rather then later...
That's too much oil in that short of time, how full is your reservoir and what color is it?

Roler

what!?