OK so here's the whole deal... Just had the 4.0L in our 99 Cherokee Sport rebuilt. Cooling system was looking pretty sorry when we pulled the motor out, so we bought a new radiator, hoses, water pump, thermostat. Basically replaced everything but the heater core and overflow bottle (which is in decent shape).

So new motor was installed, along with all the new parts, topped it full of fluids, burped the coolant, and on test drive/shakedown it overheats. Pushed water into the overflow, and blew the radiator cap.

Electric fan and Manual fan are both working. The oil looks brand new (only 5 miles on it) And the spring was put back in the lower hose to prevent collapsing (I assume that's what it's for)

So here's what we've done trouble shoot:
Flushed the system (it was muddy)
Pulled the thermostat out (still overheating)
Checked water pump (looks like it's spinning the same way as old one)
Bypassed the heatercore

There is hot water moving through the upper radiator hose, so I'm fairly certain the pump is doing it's job. One weird this was once the thermostat was pulled, we refilled it and started it up, and the temp gauge hovered a little past 210 for a while, suddenly the "Check Gauges" light came on and the temp spiked.

I'm running out of ideas here, and my searches are offering no help, so here I am pleading for assistance!

PS. I might have forgotten something else we've tried/done...been messing with this problem for a over a week now.

 

Almost sounds like a bad Head Gasket.
I would do a leak down Compression test on all Cylinders and check the Spark plugs for signs of coolant.

 

Blocked coolant passages in the block?

 

I'll borrow a comp test kit from a buddy and see what it has to say. And XJ?2010, I've considered this as a possibility as well. I'm not really feeling pulling the head off at this time, so I'll see if the same friend has a bore scope i can borrow, and take a peek of the water jacket through the water pump mount.

 

pull your oil dipstick out and see if theres coolant or a muddy water look to your oil.. im about to do a head gasket replacement on an xj i got for 350 bucks.

Mark

 

The oil looks like it just came outta the bottle, not a hint of water or milkiness to it.

 

im not really sure but how does coolant get on a spark plug

 

bad head gasket or cracked head leaking coolant into the cylinders.......

 

How are you "burping" the system?

 

It's a funnel with an adapter on the bottom that resembles a radiator cap. You remove the radiator cap, place this funnel on it, fill the funnel up with water, start motor and let it idle until all the air is pushed out of the coolant system. We jacked up the front end of the rig before doing all this, since I've read that getting the radiator above the head can help push any bubbles out towards the radiator.

I'm wondering if this new radiator might be clogged/damaged. Is there any way I could check, without replacing it with another radiator?

 

Got a bizarre little update. After running some more checks, we filled the coolant back up and tried again. Let the jeep get up to temp, walking the RPMs up down because of the rebuild. When it first got up to about 210 degrees, the heater worked. I let it run for about 15 more mins to see if the water temp would climb, it didn't. This was looking good.

Left it running while poking around to make sure everything was working the way it was supposed to, then climbed back into the cab to check the water temp again. Temp was steady at 210 still, but when i went to turn the heater back on there was nada. Got out and checked the in/out lines ot the heater core. The "in" line was nice and warm, the "out" line not so much.

Block in the heater core now? Perhaps my problem all along was a blockage somewhere, and like emobolis it broke lose to wreck havoc further down the line? Thoughts, ideas, critisims? I'm about ready to say f it, haul it down to a radiator shop and hand em a blank check.

Thanks to everyone so far who has helped with brainstorming and troubleshooting.

 

I thank it still has an air pocket in it and the gauge is reading air temp and not coolent.also check your timing if it is to far advanced its leaning out causing engine it over heat.

 

did the block and head get hot tanked to be sure they where clean before it was reassembled?

 

If your correct, how would I go about getting this bubble out? I cannot jack my rig up any higher. Timing is a good idea, I'll dust off the light tomorrow. One question though, the only part of my Haynes manual that covers timing is for older carb'd motors. How does one go about adjusting the timing of a injected 4.0, OBD2?

No. My builder has a smaller shop, based out of his house. He doesn't have the room for a dipping tank, so he uses media blasting to clean the jackets out on blocks.

 

I am really interested in what you discover. I am having an almost identical problem.
My 2000 Jeep Cherokee Sport (4.0L Six) started this thing where it operates at normal temp for 5-10 min. Then all of a sudden it spikes to 250 degrees, then to 260 degrees. If you rev the engine, it will sometimes abruptly return to normal.
I know it is overheating because it is pushing coolant into the bottle.
I replaced the water pump and the thermostat, along with hoses. No change.
The oil looks clean, so I am not leaking coolant into the oil.
The heat works inside the vehicle, so I assume it is not a blocked heater core.
Someone suggested the radiator, but I am hesitant to keep throwing parts at the problem.
I have 240,000 miles on this engine, so it owes me nothing, and perhaps it is just worn out?