93 Z28 M6. In order to clean my low coolant sensor, I drained out some of the coolant then poured it back in after putting everything back together. In the process, I also replaced the radiator cap with one from Autozone. The pressure rating is the same (18 LBS), it fits, and it's what came up in their computers as a matching part (CST part 7018). When I put the coolant back in, I made sure to unscrew both bleed screws and add the coolant at the radiator until coolant was coming out of both of them (Yes, I had a towel under it to save the Optispark). The problem is that when the car warms up and gets around the point where the fan kicks in, the coolant starts to "boil" and volcanos out the reservoir. The fan is kicking in and I'm not overheating. If I unscrew the bleeder on the hoses, LOTS of air escapes and the boiling quickly goes away. I did some searching in these forums and found a post that said it's not air. It's vaporized coolant and water. What's going on???

 

you need to keep bleeding the lines, i had the same problem after my hose came off after i did the tb bypass. kept doing what urs is doin. u have to belld them a lot to get the air out. my neighbor knew a lot about it and fixed it with ease

 

I had the same problem, took me 3-4 times bleeding the system to get the rest of the air out. If you bled the system right away, you only really bled half, you have to let it warm up so the thermostat opens up.

 

when my hose came off in front of the TB i though for sure my opti was a gonner. surprisingly it still works fine, i even hosed the entire engine out and still no problems. i dont know why they say not to get them wet.

anyways, when the motor heats up, my neighbor took off the radiator cap and covered it with his hand and kept letting air out that way, he's crazy cause it was hot. he has the 2 vents closed when he did that. mine kept heating up past 260, it took a while be it went all the way down. give it time and keep repeating the process until you get there

 

I'm not sure exactly what solved it, but here's what happened. I bled it by having the bleeder screw open on the hoses while the car was running. I'd close it once coolant started to shoot out. I did this two or three times, but there was little to no improvement. I put back on the old radiator cap. Problem solved.