My first Jeep rebuild. '89 XJ 4.0L rebuild w/Melling High-volume pump. Start-up pressure 15psi at idle and at 2k rpm. As it starts warming up (within 3-4 minutes) the oil pressure starts dropping to the point where it reads 0 psi at an idle and only reaches 15psi max at 2k rpm. New camshaft, pistons, rings, bearings, reconditioned rods & crankshaft, visually inspected all oil galleys and bearing alignment... All galley plugs replaced. Could I have missed an internal galley plug inside the block that I'm not aware of? Could the "New" oil pump I installed be faulty? I've switched out two mechanical gauges to the same results so I know it's not a gauge problem. I primed the engine while on the stand by using a drill with a flat-bit to drive the oil pump and verified oil coming up at all rockerarms with #1 cylinder at TDC. #6 cylinder rocker-arms showed most oil and #1 cylinder rocker-arms showed less, but not so much less that it set off any alarm bells. I figured that once the lifters started moving that the oil flow would increase to those rockers. I've tried 10w30, +additive, Straight 40w, +additive, and just put in straight 60w today to see if that made any difference. All the same: 15psi at start-up, drop to 0psi when warm at idle, 15psi when at 2,000 rpm. Any suggestions before I replace the pump?

 

was it the pump where you choose the differant color springs for the differant pressures?

 

I used the same oil pump on my 88 when i rebuilt it and i have 70lbs of oil pressure so i would say bad pump. Did you plasti guage the bearings when you installed them?

 

Thanks for your reply! I did Plastigage the bearings when I put the bottom-end together at the beginning of June but am not finding my notes as to the exact measurements. I do remember that the measurements were within tolerance, or I wouldn't have torqued it all together. As I mentioned, I put 60wt in it today and fired it up with the same results as with all the lower weight oils. Even if the bearing clearance was wide wouldn't the thicker oil compensate a bit and show a bit higher psi? I would think that if the pump was puting out the 70psi that you mentioned on your rebuild that it would show at least a bit higher than the 15psi max/cold that I'm seeing, even if the bearing clearance was open a bit??? I've rebuilt a bunch of Chevy, Ford, Toyota.... but this is my first Jeep. Can you think of anything particular to the 4.0L that I might be unaware of? I'm very heavily leaning to a bad oil pump (even though it was brand new) but am not too excited about dropping the pan now that it's in the vehicle. Any suggestions/comments will be much appreciated!!

 

No optional springs involved. Pump came with the pick-up and that's it. Any comments/suggestions appreciated!

 

I saw no mention of new cam bearings.

 

I think you have a spun bearing. Maybe a cam bearing or a main bearing.

 

All new bearings throughout. Visually/physically checked bearing/oil feed holes for alignment.

 

Are u using a pressure gauge or just reading the indicator of the vehicle?

 

Tried 2 brand new mechanical gauges with the same results.

 

When I've had a "spun" bearing before in a motor I haven't noticed a drop in oil pressure like this. Especially since I'm hearing it should be up over 50psi. No bottom-end knocks of any kind. If a bearing spun why would the pressure drop so dramatically? I would think the pressure would go up as the bearing spinning would block the oil-flow thus restricting the flow and increasing the pressure???? Any thoughts on this??

 

Do you still have the old pump? I would try that before i buy a new pump.

 

I really doubt it's a bad bearing. I have hideous knock coming from the cam bearing in the 360 in my full size Cherokee and it still has great oil pressure. I'd be looking at the pump first.

 

I found my rebuld notes on my 4.0 rebuild: Plastigage readings of .002" clearance for both main and rod bearings. My understanding is this is where it should be for this motor.

I pulled the pan and put the old oil-pump back in, removed the distributor and used my drill to drive the pump and still only get 15psi at the mechanical gauge.

I was looking at an extra block that I have and the thought came to me that there must be an opening/leak somewhere in the oil pressurization system that is causing such low pressure.

Could it be the oil filter adapter that bolts to the block? There are two or three rubber seals on the bolt that goes through the center of the housing adapter to attach it to the block. Perhaps these are the source of the low oil pressure in my motor?

ANY THOUGHTS ON THIS?

Presley

 

I've always relied on the gauge in the heep, so forgive me if this sounds weird. Any chance that there's air in the line that feeds your mechanical gauge? Since air will compress and a liquid won't, could this not cause an improper reading on a mechanical gauge?