94 Chevy 350 overheats when temps fall below 0 deg

So I have a 94 Chevy k2500 with the 350. As the title States it overheats when temps are below zero. It was -16 degrees this morning here in Colorado. I have a magnetic block heater for it on the oil pan that I plugged in last night knowing it was going to be this cold. Went out and started it this morning to let it warm up, it idled for about 15 min and when I went outside it was still blowing ice cold air. I took off and drove about 5 blocks and noticed the temp gauge rising alot faster than normal yet my heater was still blowing ice cold. I knew something was up and turned around and headed back home. By the time I got home the temp gauge was in the red. I popped the hood to find that no coolant was flowing. The overflow tank on the passenger side was still on the cold mark and no coolant had flown into it. I shut her down and hopped into my blazer and headed for work.

So a little history on the truck, I did delete the pre heater hose, flapper, and baffles on the stock air intake to make it more free flowing and a true cold air intake, (that's why I have the magnetic heater, to assist in warm-up on cold starts) I also did a complete coolant flush and thermostat swap(changed the stock one for a 210 deg one) about 3 months ago. After the coolant flush I put a 60/40 mix of antifreeze water back in it. I also previously owned a 92 Chevy k1500 with a 305 and all the same mods and had this problem occur with it once before as well. I am a bit stumped as to why this is occurring and I have now had it happen more than once with two different vehicles. I personally think a 60/40 mix should not freeze the coolant in the cold temperatures we see here in Colorado, am I incorrect? I obviously know something isn't/wasn't right but I'm not sure what. By all logic I believe this should not be happening. Any help is appreciated.

Thanks,
Wesley

you did a flush meaning you used 100% water to clean out the coolant system now the question is did you drain the block and heater core ? a 350 GM engine takes about 4.5 gallons of liquid . so you should have used 2.5 gallons of antifreeze to 1.75 gallons of distilled water which should be not problem to -40 deg f... the T stat can stick also but I would expect heater to work.

^^^What he said.

I've seen it over and over...and did it once. Didn't get all the flush water out of the block, thought I had good freeze protection, but did not. The remaining flush water diluted the antifreeze beyond what I expected, and it froze overnight (not enough to break the block, but enough to screw up the heater, and push a core-plug out.) It would be worthwhile to probe your coolant for freeze protection using a trustworthy gauge. I use a refractometer. Some guys like floaty balls.

Another thought: Cold heater, no movement of coolant into the overflow bottle, and hot engine temperature is a sign of an under-filled cooling system. Did you burp all the air out of the system after initially filling the coolant? It's a messy, pain in the ass; made substantially better with one of those big Lisle cooling system funnels. When the thermostat opens, the air and some coolant WILL come out of the rad filler neck; it's nice to capture most of it in a bigass funnel sealed to the rad neck. Then when the air is gone, you top-off the coolant as needed.

https://www.amazon.com/Lisle-24680-Spill-Free-Funnel/dp/B00A6AS6LY/ref=sr_1_1?ie=UTF8&qid=1481281773&sr=8-1&keywords=lisle+coolant+funnel







For the record, magnetic heaters on the oil pan suck. They don't heat the engine worth a crap. (yes, they're better than nothing.) Get a Zerostart block heater that fits into a core-plug hole, put it on a timer so that it's only heating for the hour before you start the engine. Doesn't need to be hot all night. Uses less electricity but does a better job of warming the engine.

ya could be an air bubble >>>. run engine with cap off rev engine a few times until @normal temp. add anti freeze until at the full cold mark ....