So I just picked up this suuuuper clean XJ with 165k. This is my second jeep bone stock Cherokee sport 4.0 AW4 trans np231J yada yada yada. Well, I just did my usual 4.0 oil change 5 qt gtx 10-30 high mileage 1 qt lucas oil stabilizer a K&N filter and a bottle of CSR Engine restore after I changed the oil psi started to sit around 10-20 psi at idle (before I changed it 30-40) Now a week after the oil changed and some pretty crazy low temps here in TN 3 times now it has dropped to 0 when sitting at idle. I was just wanting to know if anyone has experienced this issue and could point me in the right direction to getting this sorted out. Does anyone else out there have a secret oil combo mix like mine that may help or should I start thinking about looking into internals? Also I added a regular 10-40 gtx to it to try to get the pressure up and it did seem to help temporarily but after my 30 minute drive to work I pulled in and parked and it was at 0 again. I have been running that oil change on my 91 XJ laredo for years and it loved it. this one not so much.

Any input helps I just want to make sure I don't damage the poor thing, It's not like a good low mileage 4.0s grow on trees.

 

Just curious what problems you were having with the new '91 that led you to adding the Lucas and CSR to the oil?

Adding junk to the oil doesn't help with pressure problems.

 

To start with do another oil change without the Restore or Lucas, that stuff is garbage, at best it is "snake-oil" that will do no benefit, at worst it causes problems and possible engine damage.


The Restore can clog oil passages, especially the one to the sending unit, the Lucas is known to cause oil "foaming" which could also affect the reading at the sending unit.


What filter are you running? If it is a Fram replace it with something else (Wix/Napa Gold, Purolator, etc.), Fram filters are poorly made and known to cause pressure issues. If you want to run an "oversized" filter you can use a filter for an "old" Ford 302 (Motorcraft Fl-1A, Purolator Pl30001, Wix 51515/ Napa 1515, etc.) this will add aprox. a half quart of oil capacity.


Aside from that the 4.0L I6 isn't too particular with oil. Shell Rotella T6 5w40 is often viewed as the "best" oil for the 4.0L I6 (based on wear numbers and used oil analysis) but just about any 5/10w30/40 will do, a good filter is more important than the oil you use.


Bad oil pressure sending units are also common so you may want to replace it and/or verify your pressure with a mechanical gauge if an oil/filter change with a quality filter and no additives doesn't fix it.

 

Pull the drain plug and dump all that garbage out. Then replace the plug and add 6 qts conventional 5w-30 & wix filter. Interesting, 165k is low mileage compared to what?

 

Also after you drain all of that garbage you put in your oil pull off the oil sending unit and see if all that stuff didn't plug it up.

 

1. I agree with the others. Drain this mixture out. Refill with appropriate weight oil for your location. No additives.

2. See where you are at for oil pressure. You mentioned you were at 10-20 psi at idle. Did you know that the spec. for oil pressure at hot idle is 13 psi? My 99 has been running at 18 psi at hot idle (verified) every day since the day I drove it off the lot. So you may be concerned for no reason. VERIFY your oil pressure with a mechanical gauge if you really want to know. Many parts stores will rent you this gauge inexpensively; call around.

3. If you want a good snapshot in time of the internal condition of this "new-to-you" XJ, perform a simple compression test on all cylinders. The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

 

x2 Good advice. Oil additives are for the gullible. I have rebuilt engines years ago when STP was popular and they were a mess. STP,Lucas, Motor honey are nothing but viscosity improvers. These polymers do nothing but gum things up and stick rings. The best additives you can find are already blended into most every brand. IMO Rotella t and the 4.0 go together like peas and carrots.