old_man said:
Before you spend a couple bills on a radiator, take it to your local mechanic and have him do a sniff test on the radiator. That will tell you if you have a leaky head gasket. You can buy a tester for around $40-50 IIRC or have him use his emissions sniffer to test for HC's.
You haven't said how it runs. Have you pulled the plugs to see if it is running way lean?
X2
A couple of things, that may help.
Often the junk and deposits are worse just at the outlet. With a dental mirror and a flashlight you can see much of the clog, if present, looking in the outlet. Another tip off is a really hard and swollen intlet hose. You should be able to squeeze it some and have some give, with the motor running and at operating temp. If it's drum tight and swollen, you may have a clogged radiator.
A universal gasket for the thermostat will cause you all sorts of grief, either use an OEM style, make your own (being sure to make the cutout for the heater surge tank hose) or just RTV.
Sounds screwy, but I've seen it too many times to think it that odd, make sure the feeler end of the thermostat is pointed towards the block, most say right on the rim which way to install it. Also the thermostat for the XJ is a low profile type, a common Chev will fit, but if installed just a little wrong, will crush some.
Nothing I can pin down, but if I'd been through all that, I'd sure test for a head gasket leak to see if I'm passing exhaust gasses into the coolant system. I keep a hollowed out spark plug with a tire valve silver soldered, into what used to be the ceramic end. You can also often find compression gauges (not all will, many have a check valve built into the base) that allows you to push some air into a cylinder. Move each cylinder up to the top of the compression stroke and push some air into them, one at a time. If the pressure gets to high the motor will try to turn over, I usually jam a breaker bar and socket onto the harmonic balancer nut (which can come loose) or jam the starter ring on the flex plate. A leaky head gasket and air will bubble up into the surge tank with the cap off, if there is much of a haead gasket leak into the coolant system. Most auto parts places sell the adaptor to push some air into the cylinders, which is a common valve seating test, which also works for head gaskets. If you use a compressor don't overdo the pressure, I often use a bicycle tire pump (foot pump type).
You might also try blowing through the surge tank heater hose hoses and see if they aren't clogged. The thermostat needs the surge tank/heater flow to function properly and be responsive in a timely manner (see proper themrostat gasket mentioned above).
I've seen a leaky head gasket pass enough coolant into a cylinder to actually push some coolant out the exhaust pipe (drops at start up) that stained a white papaer towel the same color as the coolant (green on mine, sometimes red or even yellow or blue). The spark plug often looks rusty on that cylinder and may have deposits on it /which may also be stained the same color as your coolant), depending on the severity of the leak.
Way down on my list of things to check, but something I'll never forget, had a FSJ 360 that would constantly overheat, drove me nuts, just on a whim I hammered on the side of the block, both sides with a hammer and flushed the block out with a garden hose, got a double handfull of casting sand out of the block. never overheated again.
