So my my radiator has been leaking from where the overflow hose meets the cap due to a poor connection but I just opened the cap to check the fluid and I see this... It seems like a the coolant has bubbled up somehow and I was wondering what this means.

 

That is horrible! What year is your jeep?

 

It means the radiator was probably never flushed and corrosion took over along with old antifreeze and gave you that nice brown sludge.

Since you can get radiators cheap for our jeeps, I personally would just replace it rather than try and clean that out. Once you have it off, you can flush/replace the lines of the system. And if your feeling peppy, you can change the water pump and thermostat out as preventive maintenance. Also be sure to flush the heater core and lines as well.

 

Always post year/engine.

What is the history of routine coolant system service on this XJ? IE: coolant changes?

At best, this is an abused cooling system.........

I would be looking at a flush and a COMPLETE cooling system replacement, unless you have proof that it's been done recently. Radiator, water pump, thermostat, hoses, cap. Parts are cheap if you can do the work yourself.

 

X2!!! You need to seriously overhaul your cooling system.

 

That sludge is what's left behind when water is boiled out of coolant. Once that stuff gets started, it creates all kinds of havoc, cakes up in the water jackets in the engine and wrecks the water pump. At a minimum you need a complete overhaul of the cooling system. Replace it all and flush flush flush.

People here tend to be quick to assume that an XJ with sludge has not been maintained properly, but a cracked head or blown head gasket can cause a reasonably well-maintained system to sludge up very quickly if exhaust gases are getting into the coolant and boiling it. We're talking about a clean system sludging up in a matter of a few months.

What year is this Jeep?

 

That is from an unmaintained radiator, lack of coolant and using tap or river water in it instead of distilled water as should be in all systems.


First thing is to start cleaning and flushing the entire system including the heater core. Do it as many times as necessary till the water runs clear (you will need serious cleaner) to clear that block and heater core out.


Once that's done then replace radiator (cheap at AutoZone $100), water pump, belts, T-Stat and housing, "ALL hoses, radiator cap, coolant year around to lube the pump and help with the AC.


Then every year or 2 at the most, drain and flush the system installing new T-Stat and coolant while checking over everything else.

 

It is a 1999. Thanks that was all super informative and it looks like I've got my work cut out! This was the first time I really checked it since I bought it a few weeks ago because I hadn't had any problems with it till now. And the PO didn't have any record of what was done so yall are probably right that is has never been flushed.