Sorry to beat a dead horse, but........I have been reading up on this FOREVER, so maybe I'm just paranoid. But i have very low oil pressure at hot idle (sometimes almost maxes out with acceleration but usually just goes to 35-50). No milky residue on filler cap, no steam from tailpipe, but coolant is slowly being lost, and when i flushed the radiator it looked horrible. No visible cracks under the oil fill opening, or weird sounds from the engine. I am going to get an oil sample test sent out this week, but am i missing something? oh,yeah and the xj has 121,300 on it

 

You're worrying too much, fix it WHEN or IF it breaks. Until then, just drive it and enjoy. Replace the oil sending unit and see if the gauge acts any differently.

 

I will do that today.......God, i hope this works.

 

alright, no milky oil, pressure is low with mechanical gauge installed, and a mechanic said (after "listening to the engine") that i should just start using 20w50 for my low oil pressure. Im not buying it! Am i really paranoid or is my head cracked?

 

Sounds a little over worried. What is the worse that could happen. You will start to see white smoke if you have a crack and the crack gets bigger. Or you will see water/coolant contamination in the oil. Keep a regular check on these and drive a little less worried. Your XJ will get you home. As for the pressure. Stick with the 10W30. Try different brands. I have 490000 km, just under 304000 miles on the 4.0L. Still runs great. I leak more oil than burned, but do burn some. Change oil every 5000 to 6000 km (monthly for me). Uses about 1 litre every 3500 km. New filter every time. For me it's the Fram filter. Now for oil brands I like Valvoline, hard to get around here, holds good pressure through it;s life. Next is Quaker State, hi mi type when on sale.
Next choice is a toss up between Penzoil and Castrol GTX. Both good also but not my 1st choice. Why? Pressure when hot and at mid and late life in my oil change cycle.
When runnin long trips (2+ hours) the pressure at idle can drop to about 15psi with the Penz and Castrol, the Valv and QState hold about 25psi in same conditions.
Now at idle these pressures (even the 15psi) won't hurt your engine. It is at idle and no load of significance. What is your pressure when you are running at speed, or say 1500 to 2000 rpm range. I often see 45+ with the Penz and Castrol and in the 55+psi range with the Valv and QState. All 10w30. Keep your oil fresh. Have you seen other signs to concern you? PS my ZJ also mysteriously looses about 1-2 litres every 30-40 days, all new systems, I look repeatedly, just a mystery. Any chance you may have any leakage at the heater core?

 

That is incorrect on a 0331 head 99% of the time........the 0331 flaw crack does not happen in the combustion chamber.
It cracks in the oil galley and slowly seep coolant into the oil eventually wiping out the bearings resulting in low oil pressure.

I have had 2 with heads that were cracked and neither ended up with milky oil or ever showed over full. You are on the right track with a oil analysis.
13 psi is the lowest recommended oil pressure per the factory service manual.
If you have 13 plus psi + get a new updated casting 0331 from Alabama cylinder head or clearwater cylinder head asap and get it on before the bearing damage gets any worse and looses more oil pressure.

 

I conceed to the experience. Follow the good advice of an oil analysis (can't find that around here). Sorry you had to loose a head and bearing in the past. Thanks for sharing.

 

Rule of thumb is 10psi of oil pressure for every 1000RPM, and 13PSI is the lowest.

 

The above is excellent advice. Take it from someone who has been there.

Tolerate no loss of coolant. You are either leaking it or you are burning it.

If a comprehensive search for a leak (have you done this?) shows nothing, I would pull the valve cover and with a good light, look very closely between cylinders 3-4. Many times (not all), if there is a crack, it can be seen with the naked eye.

Other tests include a cooling system pressure test, cylinder leakdown test.

 

I suck @ posting pics, but today i looked inside the oil fill opening and saw what appeared to be a crack, and i definitely saw bubbles and (pause for dramatic effect) antifreeze so now my question is, since my oil pressue is about 15-20 @ idle, should i just get a new engine or will a new head suffice? I am keeping this vehicle as long as possible, so what is a better long term investment? FYI I am a working college student, so money IS an object, but i am willing to sacrifice to do this right....

 

This is a tricky questions now. If you confirm the head is cracked it doesn't necessarily mean the bearings are bad (yet). But the more you dirve it the worse your chances are that you will do damage to the rest of the motor. Stop dirving it and start sourcing a new head ASAP.

 

Why not do a compression test? Or have somebody do one for you.
It would be much cheaper to replace the head then to just drop a new engine in your jeep.A compression test can tell you the condition of your valves,cylinders and pistons,rings,head and head gasket if done properly.
If the compression test turns out bad then I would start looking for a good re builder company or a new engine.The prices i have seen online for the 4.0L online range from around $1500 to $2000 for a long block.There is always the junk yard option.
I would do the compression test first.I purchased my own gauge to do the compression test and it was only around $20-25.
Your low oil pressure could be unrelated (bad pump or something) or it could be related to the loss of coolant ( maybe a cracked head could cause a low oil pressure reading??I dont know)but it does not necessarily mean that the rings and or other internals are bad.
Keep us posted

Edit
You seen the crack in the head so this explains the coolant loss but I still recommend the compression test be done before you replace the head to get an idea of the condition of piston ring condition etc.Valve condition really don't matter at this point since you are in for a new head any ways.

 

i will attempt to have a compression test today, and i have already sourced a head ($425 to my door)

 

Unfortuantely it probably wont tell you the condition of the cam and crank bearings, which is the common failure when coolant gets into the oil. Compression test will help determine other problems so it's still a good idea, but it's not a certainty when it comes to all parts of your engine that will be effected by the crack.

 

what would be better then? Or is there no sure-fire method?