Zero oil pressure

Finally got my new Jeep Comanche project on the road. I changed the oil and started driving it for the first time. I put about 60 miles on the truck in the past week but yesterday while driving then oil pressure dropped from its normal 30-40 psi to zero. I drove it a mile back to my house and put a mechanical gauge on it. Gauge would read 3 psi and when I revved it above idle it dropped to zero. Pulled the valve cover and I’m not getting oil to the top end. Engine has 102K on it and is really quiet for a Jeep 4.0L even without oil pressure. When I rev it I now get some chatter from the valve train but it quiets back down at idle. I let the engine cool down and started it again. Pressure was at about 10-15 psi for a few seconds then dropped to zero.

I dropped the oil pan to see if the pick up was clogged but it was perfectly clean. Decided to throw a new oil pump in while I was in there incase the pickup tube was cracked or the pressure relief was getting stuck. So with the new pump and cold engine I get 60 psi but then it drops to zero after 3 minutes. Even with 60 psi I didn’t see oil coming out of the push rods at idle. Not sure what else I can check besides bearings but pressure was fine a day ago and it’s not knocking.

How much oil did you put in and what weight.....otherwise you're way past my knowledge of the 4L

I filled it with 5.5 quarts of non synthetic 10w30 Mobil oil and a STP filter

Bearings shot? If they weren't before, I bet they are now. Or some plug in the oil gallery is loose/leaking/missing.

Those are 2 things I’ve considered but it’s just odd that I’ll have 60 psi then slowly go down to nothing. I would think pressure would always be on the low side.

At this point I’m guessing my best course of action is to just pull the engine.

Normally i'd just say that the sending unit had died (happened to me a few months back), but if you're visually verifying that the head isn't getting oil, then that's a completely different scenario.

Yeah I was really hoping the sending unit died and I’ve had a few fail on my other jeeps but I put a mechanical gauge on and the readings were just as bad.

It probably won't hurt at this point, but I would ATF/kerosene "oil" mix, get rid of that crappy Fram filter, and run it! I know those had a horrible time with sludging up and blocking things up.

Just to cover all the bases before yanking the engine, I would change the oil filter again to make sure it hadn't collapsed internally or failed in some other way.

It's ok, 4.0 liters are just like slant 6's and don't need oil pressure. Should get another easy 100k out of it still.

STP filter could be the problem, throw it in the trash, avoid all that "E core" plastic cage junk. Spin on a Purolator or a Wix and see what happens. Did you look at the cam while you had the pan off? Could have pushed a cam bearing out.

+1 on trying a new filter and ATF/kerosene mix flush before taking the engine apart.

I’ll try a new filter and some ATF/kerosene. When the pan was down I did look up at the cam and bearings because I’ve heard about 4.0’s pushing cam bearings out before. Everything looked normal.

Ranger50 wrote: It probably won't hurt at this point, but I would ATF/kerosene "oil" mix, get rid of that crappy Fram filter, and run it! I know those had a horrible time with sludging up and blocking things up.

STP filter

iceracer wrote:

Ranger50 wrote: It probably won't hurt at this point, but I would ATF/kerosene "oil" mix, get rid of that crappy Fram filter, and run it! I know those had a horrible time with sludging up and blocking things up.

STP filter

And STP doesn't make filters. Fram does it for them. Unless the supplier changed, but I doubt it, STP numbers are exact copies of the Fram numbering system, just replace the letters in front. If it ended in 16, it was a Fram.

I thought STPs were that E core crap now? At least the orange can of death has a steel center tube.

Kenny_McCormic wrote: I thought STPs were that E core crap now? At least the orange can of death has a steel center tube.

Quick look at RA, shows it still has the Fram numbering for the STP, but I was wrong on the ending number, 3985 in this case.

remove and dissect current filter. report back with pix.

i assume it delivers oil to the rockers through the pushrods. if so, pull a non-delivering pushrod and see if it's obstructed.

drain the current oil and see what it looks like. refill with 50/50 kerosene/ATF and run it a while.

The older engines with shaft rockers had an oil passage to the shaft. Seen them plug up.

I thought STP Filters were made by the Champion Filters people? At least they used to...

I've been using either Mobil 1 or K&N oil filters in my cars, though I really don't know anything about their construction. Just two names that I trust.

Anybody have any background on those two brands?

In reply to jstein77:

Those are good, as are Wix, Bosch, Denso, Mahle, Motorcraft, Purolator, AC Delco with numbers NOT ENDING IN "E", and the higher dollar Frams. NAPA and CarQuest house filters are mostly Wix.

Junk are the cheap frams(not really that awful, just overpriced), and anything "E core" where the steel center tube has been replaced by a plastic cage. Generally speaking a silicone ADVB(the rubbber ring under the ring of holes) is a mark of quality.

If the engine's cold, the oil pressure jumps to 60 and then drops to nearly zip in 3 minutes that doesn't sound like bearings to me. If the oil pressure relief valve is not closing all the way that'll do it. Seems to be in the pump on that motor.

Typically on excessive bearing clearance the engine has to get fully warmed up. It'll start out with good oil pressure cold and slowly drop as the engine heats up.

Curmudgeon wrote: If the engine's cold, the oil pressure jumps to 60 and then drops to nearly zip in 3 minutes that doesn't sound like bearings to me. If the oil pressure relief valve is not closing all the way that'll do it. Seems to be in the pump on that motor. Typically on excessive bearing clearance the engine has to get fully warmed up. It'll start out with good oil pressure cold and slowly drop as the engine heats up.

We should make a new forum section here for diagnosing issues called "Ask Curmudgeon".

I just thought of something else: the 258 and the 4.0 use different oil filters. Be sure which engine you have! Also, the oil does go to the top end via the pushrods.