cduckle

Background:
Went to start my 1990 Jeep Cherokee XJ 4.0 about a month ago, hadn't been started in about 1 month. It was 14deg and snowing, so I let it idle for a good 5min. Oil pressure read zero on the dash, but it sounded fine and I trusted my ears more than the 30yo sender. Got 2min down the road, it started to sound horrible, and the engine died while I was trying to pull over. Pulled on the median, oil level was fine, and for some reason I decided to start it up again. It started right up and had 30psi oil pressure?! Drove it 20 miles and everything was normal.

Drove it a couple weeks ago, everything normal.

Started it today, sounds fine but oil pressure read zero on the dash. I tried restarting it a couple times, but still reads zero.

What are your thoughts? Bad pump, something stuck in the pickup, something else? What steps should I take to diagnose?

Thanks!
Christian

exasemech

Put a mechanical gauge on it

tlc1976

I would try cleaning and tightening the connector. I used a tiny flat blade screwdriver. I had the same issue and it solved the problem.

cruiser54

Do you have some cheapie oil filter on it like STP or Fram by chance?

cduckle

Yeah I have an STP. Are they that bad?

cduckle

The connector to the sender? What problem were you having exactly?

cruiser54

It's just a good idea to unplug the sender, see if oil is coming from the sender and make sure the connector fits tightly.

Yes, STP filters are junk, but we can cross that bridge later.

tlc1976

Yes the connector to the sender. What happened is after I brushed against the wiring while changing the oil, all of a sudden it showed no oil pressure, yet it didn’t run any differently. So I unplugged the connector and noticed it didn’t look like it would seat tightly on the sender. So I cleaned it and twisted at the terminals a bit to tighten them. Plugged it back in and all was good. That was a few years ago and still no problems.

cruiser54

Extremely common. Not to be overlooked or ignored. takes about a minute.

cduckle

So finally got around to checking it out last week. Put a mechanical gauge on it, no oil pressure. Changed the oil to see if it was sparkly, no sparkles. Put new oil in (and a wix filter), started it up again just to see, and suddenly it had oil pressure! Drove it a few miles, everything was great.

Yesterday went to start it again, and again no oil pressure, on the dash or mechanical gauge. Not sure what could be going wrong. Any ideas?

exasemech

1) when it "has oil pressure" how much does it have on the mechanical gauge?

2)if it's really bouncing from zero pressure on the mechanical gauge to normal pressure on the mechanical gauge (like 40 psi at 2500 when warm) the only things that come to mind are
- the port going to the sender/gauge has a lot of glop in it so the readings are funky.....but the system pressure is actually ok
Or- the pressure really is going to zero. ( Stuck pressure relief valve maybe?? If the pressure really is going to zero you need to fix that.

5-Speed

On the very rare occasion, your oil pump may be on its way out. Super rare for pumps to work then not work. Could have the plunger getting stuck.