Oil Pressure Woes

[hogseye]

My 87 Cherokee only has 250,000 miles on it with no internal work done on the engine. Although the oil light has never flickered, I am getting nervous about my oil pressure so I installed a mechanical oil pressure gauge. When cold it holds decent pressure at idle. 35 to 40 lbs. Road speed it holds 30 to 40 lbs at all times even when hot EXCEPT....I live in the mountains and after pulling 3000 ft in elevation and is really hot it drops to 25 to 30 lbs. The problem is hot idle the engine is only holding 7 to 10 lbs pressure. This is not really enough. Do you think the problem is only a worn out pump and maybe a dirty strainer and pickup? I hate to crack a bearing cap lose if the bearings will still run ok, cause that just opens up for more trouble in your not really careful doing it. If the bearings were bad I would think I would have more pressure issues at speed. Anyone else have any thoughts?

 

[SteelHeadz]

A 4.0 liter can operate at 9 PSI of oil pressure at idle without catastrophic failure however, if that pressure stays at 9 PSI at 1500-2000 RPM , that would be a reason for concern. At 250,000k ,i would start with the oil pump replacement and check the strainer while your at it. If you have a rear main seal leak, this would be the time to replace that too. With that said, the rear crankshaft main cap would have to be removed, from there, you can inspect that bearing and see if it's severely worn, gouged etc. The wear on the bearings would contribute to a low oil pressure at idle and at higher RPM's depending on the condition. The clearance on the camshaft and crankshaft bearings is a restriction for the oil passage thus creating pressure, as the restriction reduces, the pressure drops. Sometimes, a heavier oil can compensate for the poor oil pressure but, if the oil pump is severely worn, it might not make much of a difference.

 

[hogseye]

Yeah, I'm not real worried about the low pressure right now, but summertimes coming and when it really gets hot I am not sure that it will be ok then. The pressure goes up above dead idle so I don't think it is critical. However with 250K I am sure the pump is getting weak. I may have the shop check pull a rear cap just to look. If I go with a high volume pump it should make up for some wear in the bearings if they are not too bad. I run a fairly thick oil for all but dead winter here. I run 15w-40 high detergent diesel truck oil in it for all summertime running. thanks

 

[hogseye]

Just to update....as the warmer weather started early, when my high mileage XJ 4.0 was really warm and sitting at a stoplight my oil pressure dropped to 5 psi or less!!! OOPS!! I would keep the engine at high idle until the lights changed. Tore it down as soon as was possible. Main bearings looked good but the rod bearings were wore out! On four of the six it was all copper showing. But believe it or not the crank still looked good. Replaced the oil pump with a high volume pump and replaced all rod bearings. Would have replaced the mains but...had ordered std mains and rods believing this engine has never been down as I have owned the jeep myself for about 14 yrs or so and have put the last 150K on it. Upon removal of the bearing caps to my surprise the mains are .010 undersized with the AMC stamp on the back of them! Rods are standard. As the mains themselves looked good I reinstalled them rather than wait a couple days for the other set to arrive.
The result of all of this is 70 psi oil when cold. 30psi oil now when warm and have never seen the pressure under 50 psi at road speed. Some of this I attribute to the high volume pump I installed though. Will see how she holds up.