Have a new-to-me 98' 4.0L 6cyl Cherokee, 168k - know a few of engine basics here, but please forgive any ignorance...still have a lot to learn.
Few different variables here, going to lay them all out in case something is more important than I think it is....

Car ran perfectly for a while offroad and on highway...then overheated while stuck in a rut a few weeks back.
Temp gauge spiked, slight steam cam from engine...shut her down, then larger steam plum came out.
Small amount of smoke also came out of passenger side of radiator.
Let cool, checked/added fluids (needed a little of oil and a full gallon of coolant) - no drips noticeable (confirmed after getting back to garage - no leaks/drippage at all).

Started up the next day, let run for 25-30min. Has a new whine, but nothing terrible (sounded like something related to fan, I hope).
All gauges look normal - HOWEVER, after 30min of running, back of engine block (ONLY from end closest to firewall) started to smoke slightly and knocking began.
No smoke from radiator this time.
No correlation with increased RPMs, just started to knock/smoke after running/idling for 30min.

Few potential issues:
- oil seems thin to me, but is NOT cloudy/milky nor is low
- coolant was very low, not anymore - no more smoke from radiator anymore
- area around oil filter seems moist - not a major leak and could just be condensation, but does look brownish
- no white smoke coming from exhaust, nor is smoke off engine blue-ish in tone

Plan (any thoughts/suggestions would be greatly appreciated):
- will replace thermostat, gasket, and radiator cap (hoping it's an easy fix)
- if none of those solve the issue, will do pressure test on engine to see if it's a head gasket issue after the original overheating
- drain/measure amt of coolant in radiator, make sure isn't low or has bubbles
- I've read plenty about pot'l flexplate or flywheel cracks and bad harmonic balancers....given the knock/smoke comes only after 30min of running and has no correlation with RPMs, can I eliminate any of these potential causes?

THANKS for any advice here. Less than 30miles on the rig (for me), want to get her back on the trail!

 

I would do all the things you mentioned,mainly the pressure test on your cooling system,I had a headgasket leak for about a month until it went out. The knock could be your lower end or flexplate,lifters make more of a ticking sound than a knock.

 

Thanks much. Is there a way to see if it's the lower end vs head gasket, etc?

 

If it sounds like someone inside with a hammer trying to get out,it is a lower end bearing or flexplate,The knock only starts after 30 min of driving,I doubt it is the flexplate,I would say it is a bearing if in fact it is coming from the lower end,after your oil warms up it has less ability to maintain a good film on wore bearingsndows the slapping sound due to exccesive clearance. the head gasket just do the pressure test.

 

Just heard radiator is definitely cracked - mechanic said it was spewing like a fountain as soon as he put pressure on the system.
So, now need to replace this before testing to see if the head gaskets are ok....
If this is the best route to go... what better (i.e. not the OEM) radiator should I put in that might cool her down a little more in the future?
While in there replacing the radiator, are there any other mods that should be done to deal with future overheating issues, given that the majority of my driving is low-speed offroarding at med-to-high RPMs with little ventilation/air getting to engine?
Thx

 

I would look at getting a 3 core radiator,I have never looked them up but have heard of them on here,look at quadrtec for more on that,as far as mods,put a switch on your electric fan and run it all the time,it does not sound like this is your daily driver,if so remove your ac cond,unless you use it,air will be able to get to your radiator easier,there is also better water pumps than stock. Quadratec has alot of products for the xj and jeeps. I am sure there are others just familiar with them.

 

Thx again.
Correct - not a daily driver...but AC needed occasionally in the summer in PA.

Was reading this on coolant system mods, seems pretty solid/reasonably priced:
http://jeephorizons.com/tech/xj_cooling.html

 

Its too late now, but you probably should have not shut down the engine on temp spike. By doing that, all coolant circulation was lost. The hot block cooked the oil and excessive pressure built up with no circulation, busting the radiator. The transmission probably overheated too.

Your knock is probably a scored bearing(s), either rod or crankshaft, or both, resulting in excessive play when fully warmed up. Could be more than one.

You might have scored the piston walls, resulting in piston slap when warmed and clearances have opened up. But you said the knocking and smoke are coming from back of the block. This is the hottest part of the engine, so you probably have a piston trying to seize in the bore. The smoke is from excessive heat and friction.

Teardown, inspect, and rebuild the engine. Its not a head gasket problem.

 

So much for the easy fix.... Thx

 

I'm sorry. I really, really, hope I'm wrong.

BTW, that "new whine" you mention is a squealing bearing. Either main. rod, or camshaft.

 

It is not all that bad even if is bearings,I bought mine from the jy,did a compression check and put it in,the thrust was bad and knock bad,took it back out,took out the bearings and crank and had the crank polished and put in new bearings,it cost $150,and two days of time,runs like a top now.

 

Haha, no worries re: the hard truth! Advice is much appreciated, fellas. Very helpful. Will let you know how it works out.

 

Replaced the radiator with the all metal, 3-core from CSF. Also upgraded the electric fan, setting it to run off the ignition, not the thermostat.
Is it worth changing the theromstat to the 180 version vs the 195?


After these changes, engine is running fine now, no major issues (happily surprised given the commentary above). However, want to make sure nothing was missed...


Given the previous knocking/smoking/whining, is there anything in particular that I should keep an eye on pre/post-taking it out in the woods during the next few trips?
(other than knocking and smoking...haha)

Again, apologies for such a broad question - still have a ton to learn.
Thanks much.

 

Drive it,if it going to break not much you can do,watch water and oil.

 

I would get a crate motor and swap the motor... Diagnosing everything that is wrong and fixing everything you'll prolly spend around the same as buying a crate motor and just dropping a new one in