ok check it i just went through this problem. the 87-90 have the old french style cooling system and the 91-01 have the chrysler style. both are ok but im gonna tell u how to fix the problem. listen carefully and dont read anymore posts. my 89 would not run under 220 240 up hill then pop top. i put in an all metal 3 row radiator. new fan clutch. new tstat 160 or 180. and just for the hell of it a went to auto zone and bought a new pressure bottle (reservoir). get the one made by dorman its actually a really nice pressure bottle. mine now runs at 195 up hill in 100 degree heat. in order to get your tstat to work u need the bigger radiator cuz the small one just dont hold enough fluid to allow the tsat to do its job. in te smaller radiator the fluid will just get warm and your tstat will just stick open and eventually all the fluid will get hot. with the bigger radiator the fluid will cool and allow the tstat to open and close. hope u get all sorted.
Somebody has got some real B*LLs for a person with 4 posts. :twak:
You have a few errors ( miss statements, or wrong ideas, not sure which) in your post, but some of your advice is good too. Don't take my statement the wrong way. Nothing wrong with tossing out your thoughts and trying to help (don't stop), just don't go telling people to ignore the rest of us unless you really have big B*ALLs, or really know your sh*t, which you don't, yet.
The 87-90 system is called a closed system, partly I guess because everything is sealed under pressure including the POS plastic bottle / reservoir that we all are so fond of:laugh3:. 91-2002, jeeps, have what is called the open system because the overflow bottle is not under pressure, it is outside the pressurized system.
If you have an engine that has a problem (bad O2 sensor, bad partly clogged injectors, ... making it run lean) , and is running too hot, it won't matter what system you have. IF everything is working perfectly, the Renix (87-90) system is a god system, but the current POS bottles on the market have some issues. The bottle caps are weak. That can be solved with a worm gear clamp on the outside of the bottle cap See my "Renix Bottle" cap thread for details. Lowering the shelf about 1" helps with the crushed bottle issue mentioned below.
But if your engine runs too hot, or the radiator is old and clogged some, or both, the bottle is the week link, and will fail first.
There are a number of outstanding thread discussions about all the causes of overheating, and how to test them. To get up to speed look for threads on overheating, head gasket leak, oxygen sensor, fuel injector problems, compression problems, radiator problems, thermostat problems, temperature sensor problems, ZJ fan clutch upgrades, Efan thermal switch (In the radiator), dying water pumps, or water pump seals, and even bad temperature sensors lying to the dash gauge.
But back to the OP, if all your problem is, is the cap coming loose, the solution is way easy, a worm gear clamp.
To switch to the open system (91-2002), you must buy a 91-96 (not sure if 97-2002 is exactly the same or not) radiator with a filler neck that uses the standard radiator cap, or install an inline filler neck that uses a standard radiator cap, in the upper radiator hose. (one like the ones JEG.com sells), then add a standard open bottle, and connect the hoses at the old bottle with out the bottle.
There are some nice threads with pictures here from prior years that show the closed system to open system swap, in its various forms.
That said, many of the new cars, still use the closed system, they just have better bottles that have better shapes, like the 96 Ford Taurus.
Here is link to the most recent Renix Bottle cap and clamp debate!
http://www.naxja.org