bigbadon

I am new here so this is my first post and my first experience with Jeeps. I just bought a 04 JGC with the 4.0 and 95k miles. It runs like a top and purrs like a kitten but at idle the oil pressure drops down low enough to trigger the gage light. I confirmed with a mechanical gage that the pressure at 210 degrees was 45-50 at 2k and 6-9 at 600rpm
No water in oil. Looked in oil fill cap hole and oil is circulating. I hear a VERY faint knock but it sounds more like piston slap and if I put it in drive and hold my foot on the brake it is a little louder.....runs strong and uses no oil. I am not sure if this motor is original because the build date doesn't match the body build date.
The motor # is 009MX27 and under that number is a laser etching that looks like this; CCCCCCC??. I plan on pulling the pan for an inspection but would like to put it off till warmer weather but I don't want to grenade the engine either. Has anyone ran their 4.0 with these type of pressures?

jeepsterpop67

My XJ did it until I rebuilt the engine. I also had a piston slap, and a sticky lifter. I drove it for a year like that with no problems the jeep was a beast. I eventually ended up rebuilding the engine and replacing the top of it and swapping out the pistons so I could get a cleaner start. But not because i had a peoblem with oil. Obviously the slap stopped and so did the weird oil pressure. Still not 100% sure why it used to do it, but I still trusted it while it was riding like that. Never had any other problems with it. I now have a WJ with the same engine. It's possible that the gauge is too sensitive especially if the engine isn't the original and they didn't bother to a) pay attention to the gauges or b) change them if it came out of something like an XJ or a ZJ.

Bustedback

How accurate is your manual gauge? Hard to believe the oil pressure is at the high end of the spec at 2000rpm but drops to almost nothing at idle. I would replace the sender and make sure there is not a Fram filter on there and see if there is any change.

Devin706

mine had no pressure when warm at idle until i got around 1200-ish RPM rthen it would go back up and the 'check guages' light would go off. replaced the sending unit and that didnt fix anything. checked it with a manual and the same deal was happening. turns out it had blow/popped the seal of the fram oil filter and as i was on the way to work the oil pressure suddenly dropped to 0 at 2k rpms and i heard a tapping and my heat started rising. turned right around and went to the house and popped the hood and saw oil all over the place. and the oil filter had a split in it and was loose also. after work flushed motor, threw a K&N oil filter on and filled er back up and never had the issue again. Fram is S**T. lol. hope yours is this simple

bigbadon

Wow,some good info. The low oil pressure is unnerving but maybe it will hold up until I get a chance to pull the pan...hope to find something in there;worn pump,main bearing,sludge,SOMETHING!! I can see oil flowing under rocker cover, it slows down some at idle but it still flows. The rockers,springs and top of head look brand new and shiny...makes me wonder if head has been replaced?
Thanks to all that replied...this is a great forum.

dejectedangle

Thicker oil will help I use 10w 50 weight it raised mine from 20 to 40 at idol but I got 200k on my jeep so it needs thicker oil due to possible bearing wear

bigbadon

I switched from 10-30 to 15-40 diesel fleet oil (walmart brand) and changed filter from AC to mopar,helped just a little.