Brand Used 1st time Owner questions

[MrTwinkles]

Hi folks,

1st time Jeep owner, recently acquired a 98 XJ Limited with 241k mi after looking for a few months, good value ones are very scarce these days so pulled the trigger when I found a clean-ish one.

I have only had it for literally 3 days thus far, runs and drives great and got to play with it in the recent snow we had.

Questions on the most glaring things i noticed:
1. Oil pressure: Been doing a lot of reading on this with varying conclusions on what is deemed normal/expected. 40psi cold idle/driving drops to 12 ish at idle (or closed throttle) when warm. Safe to assume the gauge is good at relaying oil pressure as it is ambient air temp at its age but the only noise from the engine is the sweet 6cyl sound.

2. Vacuum leak: Seems like there is a leak somewhere underneath the intake manifold, there is an audible high pitch blowing sound under acceleration and suction sound on decel. Also audible at idle when you stick your head in the engine bay. Should be an easy fix but would like to know what is all controlled by vacuum reference.

 

[Tim_MN]

Typical Jeep 4.0L oil pressure is about 40 psi at 55 mph and about 12-18 psi at warm idle. 13 psi is the specified minimum, but s lots of Jeep have driven lots of miles with less than perfect oil pressure. All 7 of my XJ's have/had similar pressure readings.

Are you sure that is an intake leak, and not an exhaust leak ? Most 4.0L Cherokee have a crack in exhaust manifold next to a weld. Any intake whistles are likely at the throttle body, but cracked vacuum tubes and dry rotted rubber fittings are also common.


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[Unclewolverine]

Factory oil pressure gauges are notoriously inaccurate. I have a mech gauge mounted in dash along with factory and they rarely sync up. That said, your oil pressure is pretty average and acceptable. Try running 15w-40 rotella, it may come up some. Noise under the hood is almost 100% a cracked exhaust manifold. Almost every exhaust I've come across was cracked to some degree. If it was vacuum it probably wouldn't run right and have a cel on.

 

[RCP Phx]

As far as the vacuum side goes, it's used for the CC, heating/cooling system, and emissions. Easy check for leaks is to spray Sea Foam around all the areas while the engine is running.

 

[MrTwinkles]

Thank you for the responses, good to have some comfort in knowing low pressure at idle is somewhat normal, it is just frightening to see the needle drop when coasting downhill.

Highly doubt is it an exhaust leak, i have had many of those and it also does not sound like it has the stock manifold anymore. The pitch and tone changes while coasting and modulating the throttle from closed to current engine speed.
i.e. it changes with throttle position and not engine rpm. highest pitch on closed throttle at +3k rpm aka vacuum leak.

 

[75SV1]

Also, there is a Vacuum revivor. Its a black bulb tucked inside the front bumper on the passenger side.