Hey guys. I dont mean to be repeating past threads but Im yet to find one of the same exact nature.

FYI, 89 Cherokee 4.0L

Symptoms:

- Slight engine knock, not yet sure if its the motor, or the flex plate.
- at warmup, good oil pressure
- after warmup, pressure drops to 25% on the guage while driving, and drops to ZERO at idle. knock never dissapears, but is only present at rpms above 1500ish

Diagnosis:

- No coolant/oil mixing
- oil smells fine and looks normal
- not burning anything, smoke is normal color and smells fine
- minor oil leakage near the front, probably a front main seal.
- engine does not run hot. 200 deg. all the time.
- idles and runs fine, plenty of power.
- tends to backfire if its high rpm and let off the gass completely. always done it.
- no blow by problems. recently had one, but repaired it.

- oil is 15w40.
- fram crap filter, changing it tomorrow to see what happens.
- recent oil change
- swapped the sending unit.


any ideas?

 

My first thought before I even read the entire post was "I wonder if he's running a fram filter?"

That would be my very first thing to check and replace. I've read lots of stuff where fram filters have caused oil pressure problems. I'd first change the oil and filter with a quality filter.

 

yea i did read up on that, but didnt think its bad enough to let the pressure get THAT low! lol!

 

You swapped the sending unit, but haven't completely ruled out an accuracy issue with the gauge yet.

Because confirmation is so important, get a "mechanical" oil pressure gauge. They can be rented by many parts store (call around) for a few bucks.

Temporarily install it where the oil pressure sending unit resides. Then you'll know for sure.

Good data = good decisions

The spec. for oil pressure for the Jeep 4.0 is as follows:

*Not less than 13 psi at HOT idle
*Between 37-75 psi over 1600 rpms

I've seem more than a few 4.0's last a darn long time with 10 psi confirmed pressure, but it gets dicy when you get into the single digits for pressure.
Confirmed low oil pressure (below spec) indicates a "clearance" problem, usually either the engine bearings and/or the oil pump.

Good luck!

 

Hmm, also saw that it could be jammed with sludge etc. especially in older ones, 200,000km on it.

will check the pressure and go from there!