I have a 99 xj 4.0 L I6 and my oil pressure has been acting weird. It first dropped about 2 weeks ago, when I blew a head gasket. So I spent a few days fixing that, and now I'm not losing coolant anymore but my pressure is still low. When I cold start, it'll jump up to 40-45 psi, then as it warms up, it drops to about 20 psi idling. Then as I drive longer it'll drop to 20 psi during acceleration, and about 7-9 psi at idle. I do occasionally hear rattling, but it's not loud enough to make me think rod bearing, plus it stops when my rpms go up. I'm thinking the oil pump is shot, but I'd like another opinion. I'm not noticing any performance issues, but I'm taking it easy on the driving right now just in case it's something more serious.

 

Have you confirmed it with a mechanical gauge? My girlfriend has this same issue. If your head gasket went, your bearings may be washed, and your cam bearings also need to be replaced as well as the crank ones.

 

I mean I just replaced the sender, and it's sounding like all the symptoms of a shot oil pump, bearing wear should be constant noise and heavier sounding, but this goes away, and is light clicking instead of thumping.

 

You could get a bad sender new in the box or the gauge in the dash is acting up,Always double check it with a mechanical gauge first to know for sure.

 

In my girlfriend's Jeep, her rod and main bearings are brand new, but her cam bearings are not, and those don't make any noise. I'm just saying it could be a possibility.

 

I used to have that problem, the oil would fluctuate more than my girlfriend's moodiness. One day I started it up & the pressure was @ 0, so I shut it off, put new oil in it, and by some miracle it hasn't given me any **** since. Hopefully everything goes well for you.

 

What brand of oil filter are you using?

 

stp

 

VERY poor quality filters. Wix is the best to use.

 

That's where the Champion Labs filters go when they aren't good enough to label "Fram".

The noise you're hearing is likely the valvetrain when the oil pressure drops too low. Verify your oil pressure (or lack of) with a mechanical gauge before driving much more.
Since you had a head gasket failure, I'm betting it isn't the oil pump.

 

What do you think it is then?

 

How badly did it overheat (assuming it did) when the head gasket blew? What weight oil are you using?

If a different oil filter doesn't fix it, I'd suspect worn bearings.
You can drop the pan and inspect the pump to rule it out, or perhaps you'll find it has failed.
Here's the stock clearances for the pump:
.002-.004 between the gears and case.
.002-.006 gear to cover plate. (use plastigage)
If the pump looks good, then you can check the main bearings while you're in there. If those are within spec, it might be cam bearings.

 

It's never overheated and I use 10W 30. Worn bearings should make a loud, constant clicking noise that increases with RPMs. Mine is soft sounding, and stops when you hit the gas, and then doesn't come back. I replaced the head gasket, thermostat, and water pump already. I'm replacing the pump and filter soon. I really think it's just valvetrain noise.

 

Ditch the STP filter in favor of a Wix and see what happens.

Have you ever shone a light into the oil filler hole and assessed sludge buildup?

 

Not necessarily. Bearings can be worn enough to affect oil pressure without making any noise at all.
Only one way to tell. Pull the pan, check the pump and go from there.

That's likely valvetrain noise. Possibly a symptom of low oil pressure. Can't say for sure without hearing it.