ok this is gonna be a long explanation but i really need some help. i have a 92 z28 with the 5.0 when i first got the car about a month and a half ago i had to install a new radiator and thermostat right away. i flushed out the new radiator because it was not brand new just new to the car and flushed out the block and it all seemed to flow through just fine. then i "burped" the system a few times to make sure that there was not any air in the system. after all of this my temp guage still did not work. i drove on it a while but it would overheat when i had the car in park and running at idle. when i was driving, it was fine until recently. now that the weather is nicer out, it has overheated on me twice while i was driving it. i replaced the coolant temp sensor thinking my guage would work but it didnt help. the guage would stay at zero until the car would actually over heat and then it would shoot to past 260 deg. when it over heated it just spewed coolant into the overfill but if i turned the car off and let it sit a while. then i would be able to drive it again with no issues. tonight i replaced the temp guage sender because i was told that was the probable solution. after i replaced the sensor i realized that my overfill was dry and my radiator was really low. so i then proceded to burp the system. as the car started to warm up the coolant level in the radiator would rise and fall dramatically and then it slowed down to a more subtle rise and fall. i then added more water. the guage is still not working properlly and is still at zero at this point. once the car got up to full operating temp(well to what seemed like was operating temp) the radiator started to spit coolant out and would rise and fall a little more. coolant would still pour out of the radiator at this point and it seemed like it would not take anymore water. at this point i was frustrated and was just going to shut if off and let it cool down and work on it tomorrow. as i shut the car off the radiator spit a lot of coolant out, it was not just a dribble or a few drops that came flying out but it shot coolant a good 4 feet. i just turned it off, let it cool down, topped off the radiator and overfill, put the new radiator cap on and just let it sit. i am really confused as to what this could be and i would really, really appreciate any help that anyone has to offer.

first thing i would do is get that temp gauge fixed. almost sounds to me that the thermostat is failed closed. what is the exhast color. did you check the oil and see if it has a green tint or any hint of anifreeze? if its over heating so fast. you could just have a blown head gasket. or even worse a cracked head. 1 check the oil color. 2 check exaust color. white exhast color = bad. with the radiator cap off u can check for exessive bubbles. better yet i would pressure test the radiator and see if you have a head gasket leaking. are the fans turning on when it reaches temp?

I think the reason the temp guage sender is not working is
that there is air trapped at the sender.

If you have the old sender un-plug the wires on the engine
and hook them to the (loose) / old sender. Get a piece of wire
and wrap it around the threads of the sender and ground the
other end.

Get hot tap water (130-150 F), a cup of coffee (170-190 F),
boiled water in microwave / on the stove (200-212 F) what
ever you can get and see if the guage / sender works.

If it works first thing to replace is the thermostat.

i have replaced the thermostat twice in the past month because i thought the first new one i put in was stuck closed. the oil color is fine and there doesnt seem to be an excesive white coming out of the exhaust. i have not pressure checked it yet but ill look into doing that soon. it takes a while to overheat when it is sitting in park. it took about 15 to 20 min today to overheat when i just let it run at work. do you think the heater core could be clogged?? if the head gasket is going or the head is cracked do you think it would be smarter to put a bigger engine in since thats what i have planned for it later down the road?? i do just need to last me the summer so i can get a beater car and throw this one in the garage since it is my daily driver.

clogged heater core would have nothing to do with over heating. also sounds like the cooling fans are not kicking on if the car is over heating. do they turn on?

well when i was first running into this issue they were turning on but last time that i tryed to burp the system i dont remember if they were. still when i first ran into this problem they were turning on but it would still over heat.