Finished up the head/cam swap today (for the second time) and decided to let it idle to full temp before the first drive. I filled the coolant tank up until it got to the fill-cold line, about 2.5 gallons. I then made sure coolant was coming out of the steam tube hose and hooked it back into the radiator. I added coolant until it stopped needing it and put the cap back onto the tank and let it continue to warm up. It normally runs at around 172F on the highway and around 185F idling with the fans on.

I was monitoring the temp with my scan tool and it got up to 230F before I decided to shut it off, fans were on since 180F. The upper radiator hose was warm, the lower hose into the thermostat was cold... The coolant level in the tank also rose 1-2" above the cold fill line. Like an idiot I took at cap off an a geyser of coolant came out, lost about a gallon

I refilled and had another go. Same thing happened, it got up to about 225F and then I decided to rev the engine a bit, perhaps to help cycle the coolant around. I held it at 1500-2000 RPM for 10-15 seconds and then the coolant temp dropped like a rock. It went down to 170F in about 20-30 seconds. I continued to let it idle and the coolant temp never went above 185F.

Does this mean the thermostat was stuck and/or failing? Does it needs revs to warm up the thermostat fast enough to the point of opening? The thermostat is a 160F Motorad unit with no more than 600 miles on it. Haven't had an issue with it before, but then again, I've never really let my truck idle to full temp before. Only other cooling system mods are an F-body 4-way coolant crossover and the e-fans.

 

Never encountered a problem like that myself. Most probably the spring within got stuck? Strange.

And take care next time, coolant additives burns nasty when exposed to damaged skin.

 

Sounds like maybe you had an airpocket??

Ive always filled the system from the driver side top hose.
Pull it off and fill with the hose higher than the engine, when water starts coming out of the radiator on the driver side put the hose on and top it off in the reservior..
crank it up and never had a problem this way.

have had a airpocket or something like that the way you filled it with the reservior one time.. didnt like that haha.