Overheated - What to Check

To make a long story short, my XJ's temps started acting wierd day before yesterday. Within a mile in a matter of seconds it would go from cold to ~220. Then after a few miles it would make the same type of jump to ~260. Everything else seemed/looked fine, so I chalked it up to the guage being screwy. Before I left work that night I topped off the coolant, 1/3 gallon low in the rad. Strange readings because of air, Im thinking at this point. Othe way home does the same ~220 jump. But there it sat the whole trip. No biggie I thought, get me to my day off and Ill check out the water pump/tstat.
So I drove it to work yesterday, did the same thing as the previous morning but actually started boiling over. Couldnt see why/where and had to get in to work. After work I made sure the coolant was full and tried to make the ~25 mile trip home. I got about 14 miles and things started happening again. I pulled over, couldnt see anything. Jumped in and tried to make it to the store a mile down the road. It stalled as I was pulling in, and the steam started billowing out once I coasted to a stop. After things cleared the only leaking I could see was around the water pump.

I got a tow home and just got to looking at it now. Other than coolant everywhere, everything looks normal. But if I start it, it sounds like its only running on 4-5 cylinders. I pulled the plugs off the first couple injectors, no difference. I pulled plug 3 and swapped it with 1 to see if it changed anything. Nope. Pulling the plug on injector 4 caused it to run worse, which is good. Once I thaw out I plan on grabbing plug 4 and moving it up to see if I somehow fouled a plug. They all look fine, though.

What else should I look for or could I have damaged? Obviously most of the cooling system, but could I have wrecked my engine? This is about where my wrenching experience is at its limit. Keep in mine Im in an apartment parking lot working in the snow with limited tools. Fun Fun.

I had a similar problem. Went through all the normal checks (hoses, t-stat, coolant bottle/cap, etc.) to no avail. Ended up finding a pin-hole leak in the radiator. It vaporized the coolant on the way out, so there was no drip to see. I heard it more than I saw it. It's amazing how much noise a hot 4.0 makes after shutdown. It was just a little hissing noise that I hadn't heard before. A little JB Weld solved it.

I think I have faintly smelled coolant around the engine bay in the past with no sign of a leak, now that you mention it. I am thinking new rad, tstat, and pump.

Any thoughts on the running like crap now?

I'd get it hot, but not 260, and spend a little time looking/listening for escaping coolant prior to throwing money at it. Make sure the system is full first. You might check to see if anything melted during one of the overheats. Sensors, fuel hoses/fitting and the like.

I'm not a mechanic by trade, just necessity. The vast majority of my time under the hood is usually spent just standing there and staring/listening/smelling at the damn thing, looking for what might be causing a given issue. Often going off of a tip from . It's not very efficient, but usually pretty effective. I found my leak by accident. It was running real hot when I got home and I happened to hear a new noise as I was walking around the front of the jeep to go inside.

Sorry I can't be of more help, but that was my experience with it.

Although XJ's before 99 are pretty robust, you could have blown the head gasket and/or warped the head. Keep your eye on the condition and level of your oil. Rise in level or foamy is sign of coolant in the oil. Looking at the spark plugs may tell you something. A compression test would be a very good thing to do. Look at the spark plugs and reinstall if they all look OK. Disconnect on spark plug at a time and test run for a minute or less. If disconnecting a plug doesn't make it run rougher, that cylinder is not functioning. Then you could swap the non-working plug with a working one to make sure it's not just the spark plug. If the spark plug is good and that cylinder is not firing, the head gasket there could be blown. Then you really need the compression test to verify. I wouldn't start fixing the cooling system until I knew the engine was right. It must be cold where you are, so I doubt that your problem is the radiator.

Definately cold lol. I did check the oil for that exact reason, a little low if anything. Most every fliud leaks on this thing. I will probably go buy a pair of spark plugs and try to pin down non firing cylinders. If I cant get it to fire with new plugs, would it be safe to assume that the injectors are good and I should just jump to a compresion test? I think the parts store has a loaner...

Say worst case and I have no compression on a few cylinders. Is it likely that the head warped, or odds are it is the gasket? How could a freezing backyard mechanic tell?

open or closed system ??just went through this on a closed system 87 xj .....cracked head in 2 places and the head gasket was borderline .when a cooling system works properly it won't boil when it is under pressure , so for it to boil over you have a failure in either the head or the head gasket ;on a closed system ; on an open system the first check would be the cap but i'm sure you checked and the cap is good either way that is to hot to be just lucky and not have anything happen especially with these motors they have a very small temp range to operate in .......sorry to tell ya !!!!

If it won't fire with the plugs, I'd just jump to the compression test, given that you have got it hot. You would probably have to take it in to a shop to check the head for flatness. You need a surface plate or at least a straitedge to check flatness. I would hazard a guess that it would be more than 50% chance that if the head gasket blew, the head is warped. That inline 6 head is pretty long. It needs to be flat within 0.008" over the full length. A human hair is 0.003".

Its a 98, open system. The rad cap was new this fall. I also forgot to mention it wouldnt blow heat when this happened, which is why I figured pump/tstat involvement.

Well crap, Ill see if I can grab a tester while grabbing a couple plugs. Cant hurt to test. What should the compression be?

The compression values can vary engine to engine, and within the same engine over time(higher when new vs. now). I don't know the 4.0 specs, but iirc compression should be within 5-10% of each other across all cylinders.

according to mr. Haynes, 120psi min, 150psi max.

Well a new plug didnt help. Damn.
So I took the hoses off and pulled the thermostat. And started it. And the wisps of smoking coming out from the thermostat hole looks an awful lot like exhaust. Damn.

Going to see if I can get my hands on a compression tester although it may be a moot point.


Damn.

Cylinder 1 - 20 psi
Cyinder 2 - 10 psi

Running out of daylight so I jumped to cylinder 4 which was firing. ~90 psi. Engine was cold, 4.0 with ~190K on it.

So it definately screwed the front. The question now is,head gasket or whoe head? How can I tell? Is a full head replacement a job I can do in the driveway with basic tools or do I need anything special? Or should I be getting it to a garage before I get in over my head?

do it yourself if you feel comfortable but if not take it to a shop and have it done hopefully the hjead is OK !!!! just be prepared to buy a head or you could have it pinned, i've run pinned heads before with no problems just get a GOOD machine shop to do the pinn/head work !!!! i just went through all of this with my 87xj that my wife could have cooked an egg on the valve cover she got so hot .......cheers!!!!
**if you do it you will need a torque wrench that goes to 125 ft.lbs. IIRC
BTW the 4.0 heads always crack between #1 and #2

Very few shops around here. Ill have to check around. I can turn wrenches all day, but I have only basic tools and am in a snow filled parking lot. Im willing to try, just no experience with it. Thats what worries me more, not knowing what Im getting into. Time to get my hands on a FSM and read up.

Best of luck to you on this project. Anything is possible, but you're going to need a little equipment. You need a torque wrench, gasket scrapers (paint scraper will do), hopefully a friend to help you lift the head on and off with the manifolds still attached. Please don't put that head back on without checking it for flatness. Maybe you should just look for a replacement head on craigslist. I got one for $50. Lap the valves with some valve grinding compound. Hopefully you have a ride to go get parts. Don't get a 0331 casting number head from a '99-'01. They are the ones prone to cracking like mine did.

Yeah fortunately this isnt my DD. If it was May this would be a fun project and learning experience.

Perfect world its leave everything intact as much as possible and lift the whole thing out?

I know I also have a cracked exhaust manifold(who doesnt), might be a good time to take care of that, too.

I have to do mine soon too. Waiting for the weather. Hope to take pictues and post a "how not too". Seems like the manifold bolts (especially in the back can be a pain, so the plan is to lift the whole thing off. Hopefully better access will mean less broken bolts/studs. I got a $10 ex. manifold with my $50 head.

Well Im going to start searching for a head. Unless I can find a machine shop that will check for warp, I think I will just be better off replacing it while it's out.

Does it matter much which year I look for for a replacemement?

It's all here. http://www.naxja.org/forum/showthread.php?t=935856&highlight=0331

Basically, you want a 0630-07120 head casting. 1991-1999 are the years. Some '99s were 0331 though and you DON'T want those. THe number is cast in pretty big on the top surface on the manifold side.