Okay, I feel so stupid for posting this thread, but I bought my 89 XJ with an overheating problem.
Story goes- The last owner says his wife was driving it one day and the radiator started leaking. She said it did not over heat. He replaced the radiator with a new one. Filled the system, ran it, now its overheating. It sat for a year. Now I have it.
It was leaking oil, so I went ahead and replaced the headgasket, and visually checked the head and block. Still overheating.
I got rid of the plastic bottle completely, and installed one of those fillers in the top radiator hose and bypassed the heater core (for testing purposes).
What happens- From cold start, engine starts to heat up, when thermostat (new thermostat) opens the coolant comes through the top hose boiling and puking out.
Do I need to bleed the block/head?? I have never seen a problem like this before.

 

check if the radiator is the same tempat the top and the bottom.. he says its new but you never know it could be clogged somewhere. same thing happend to my f150 it would overheat so i changed the head gaskets, started it up and over headted. the rad was clogged.

 

you do need to bleed the cooling system on these, it is a sealed system.

In my german cars that are sealed I just open a bleeder and use a fluid pump (mityvac) to pump the system full till it pours out the bleeder. Works like a charm. From what I have read here there is a temp sensor on the head somewhere you can open to use as a bleeder.

 

another thing to use if everything checks ok is some rad liquid by royal purple. its like a additive dont remember the name tho. There is also water weater from autozone its all the same. I had a 401 gw that was 30 over and always overheated got some of that stuff mixed and never had a problem.

 

you may have air in the system My XJ was a pain to prime after replacing the Freeze plugs...I found that squeezing the lower radiator hose and filling worked great.

 

There are those few tricks I use, Squeezing the rad hose on the bottom and tops with force bubbles around and hopefully to the top, not at the same time. A bleeder for the air is a good idea. I put one in between the heater core on the highest hose. I pull the hose up as high as I can then fill it up. Every day or two I would check the level. Even if you toss a half a cup of water in there then you know that the air is getting out. One other thing, when the thermostat opens like said above it may not be working. the easiest way to tell is grab the rad hoses to see if one is cold still.

From previous experience I will never use the water additives again. If your cooling system is working right then you dont need it.