When I replaced my radiator, hoses and poly (turtle) bottle, I had no problems with burping it, I just ran it, cooled it, topped off the poly bottle itself while running and after cooling it, all with the cap off, a couple times and that was that. I never let it get hot enough to boil over with the cap off while purging the air. Is the poly bottle cap also new???? The plastic cap is a 16 lb pressure cap, and if it does not seal on the plastic bottle properly it will not hold pressure and thus will get too hot.
I run a 165 F thermostat on my 87, and the 90 model OEM does list an optional, OEM Jeep, specifically a 180 F thermostat. The 87 to 90 Renix seems to happy with a 180 F thermostat. The newer HO 91 and later models may need the 192-195 F thermostat.
I like the checking the upper radiator hose ideas! All it takes is a large air bubble right under the thermostat to hold up, or delay the coolant flow, and keep the thermostat closed.
" Lower hose springs just keep the hose open when the pump is cavitating from a clogged up radiator that won't flow."
I think it is actually peak flow at WOT, high rpm that colapses the hose, not a clogged radiator.
For those of you that have NOT read the entire thread (Before replying), he has a NEW radiator, water pump, hoses, Jeep Dealer OEM thermostat, and tutle bottle JUST installed, and a verified good working fan clutch. It is overheating at idle in just 4 minutes, in the driveway, in Park, AC off, Electric fan basically hot wired on all the time.
"when replacing the thermostat, i bought it from the Jeep dealer, so im assuming that that is the most reliable source. i did try to burp the cooling system, but didn't have very good luck. and since then the Jeep has been rolled around, and the water level didn't drop in the coolant overflow bottle, so im not sure that that's the problem, but it could be. the exhaust i don't think is the problem, i just had it in the shop for a new cat.and they checked out the whole system. however, i will check the O2 sensor and the ECU. you know the one thing i forgot about was that it might possibly be the head gasket. But, the main things ive heard about the head gasket going out is: water comes out of the exhaust and the oil looks like a milkshake. but my Jeep doesn't do anything like that. what do you think: head gasket? :shiver:"
Ok, the new thermostat could be bad. If you pull it, it can be tested in a hot water pan, along with your new IR thermo toy to test the water temperature!
You can test the lean fuel, partly defective O2 theory by simply disconecting the O2 sensor, which forces the Renix ECU to run the engine rich, and cold, unless one of the other sensors is giving bad data like the CTS, IAT or MAP sensor, which can under certain partial failure modes can also make it run lean, but it would eliminate the O2 sensor and circuit as a possible cause of it running lean. Just for test purposes (as it reduces gas mileage) I know from experience that you can disconnect the CTS (coolant temp sensor as well as the O2 sensor at the same time on Renix, and Renix will run it cooler and richer with out any other problems, other than poor mileage, so it makes an easy test procedure.
Now for that nasty head gasket question, head gasket leaks can take many forms. So yes, even though all your head gasket leak tests are negative so far, it is possible, but I don't think it is probable based on your current symptoms. Head gasket leak (exhaust leak into the coolant system) usually shows up as free combustion gas flow blowing fluid out of the open turtle bottle with the cap off, or a foam over with the cap off shortly after starting the engine.
It is possible to do a pressure a test on the coolant side of the system for a very small leak, but the turtle cap is somewaht unique on the 87-90 Renix, so finding a radiator pressure cap test rig may be very difficult, might be easier to rig up some heater hose and pipe fittings with some custom gadgets and pressure gage than to find a rig to fit the turtle bottle cap.
There is an inline hose adapter available that uses a standard 16 lb radiator cap you can buy (anybody got the link??) and install on the upper radiator hose that simplifies pressure testing the coolant system / head gasket.
Also, the heater core could be leaking, but I would still make sure the thermostat is not bad out of the box, and that the system is fully burped before moving on to the more exotic possibilities.