Overheating Issue

[94silveraydo]

My heart dropped today as my pride and joy, my 94 K1500 flashed "check gauges" at me and I noticed my coolant temp approaching the red zone. As soon as I saw the gauge I pulled over and shut it off and called for a tow home so I wouldn't damage anything further than it already was. I popped my hood and heard boiling, and noticed everything around the radiator cap was completely doused in coolant. I have just over 240k miles on this truck and would like to get at least another 10k out of it before performing my LS swap. Once home I finished maintenance on the 92 C1500 to get it up and running good enough to get to work tomorrow, and started troubleshooting the K1500. I added about a gallon and a half of coolant but couldn't tell by looking down in the radiator. I started it and let it idle to check for excessive air flow coming from the radiator cap and everything there seemed normal. I didn't see any coolant running through the radiator apart from what was coming from the heater hose return line as I expected with the low level of coolant currently in the motor. There is no evidence of oil/coolant mixture as well, so I don't think right now that it's an issue with a head gasket. I dropped the thermostat in boiling water and it slowly opened over the course of about 15 seconds, just like every brand new thermostat I've tested prior to putting in a vehicle. At this point I'm at a loss as to what the issue could be. Any input is greatly appreciated as if I have to hurry to do my LS swap this weekend the next couple weeks are going to be a bit financially tight.

Thanks in advance,
Adam

 

[Brother Al]

It sounds like maybe your radiator cap has failed, it was loose, or the rad is cracked up near the cap neck. If everything else was good, and it wasnt low on Antifreeze to start with, and its not leaking anywhere, I would start with the water pump... but if its not pumping, its because the belt broke, its seizing up, another pulley is failing, or the belt tensioner is shot... and belt squeel is a big sign. If the water pump is going south, it will eat its seal and usually drips from the weep hole when it fails, by design. Its also possible you had an air pocket, if the coolant was low, and the water pump cavitated the Antifreeze... this would essentially be a big air pocket trapped in the water pump... rare, but it can happen.
Only other thought goes back to it being a system leak and its not pressurizing, so its boiling at normal temp. Water normally boils at 212 degrees F at sea level... higher you go in elevation, lower the temp is. In a coolant system, its pressurized so it boils at a higher temp... roughly between 255 and 265 F (sea level), depending on coolant mix/type & elevation above sea level.
Another is a clogged radiator, if someone was using stop leak in it...