Last week my van started to near red-line on the temperature gauge. I also had no heat. The thermostat would open finally and temp would come down to normal, but still no heat. I tried to change the thermostat, but the bolts were nearly seized. After paying a shop a small fortune to change the thermo and flush the radiator, I still had the same issue. I noticed in my troubleshooting that the top radiator hose was "collapsed". Nearly out of ideas, I did an online search and several people mentioned the radiator cap. Not really believing it would fix the issue, but I bought a new one and sure enough it fixed it. Now I don't nearly overheat and my heater and vent blow warm air again. Glad it was a 7 dollar part that fixed it, even if I paid a lot to get the thermo replaced, I am sure it didn't hurt.
My van has the 5.7 and I am the original owner. I have low mileage for 18+ years, 126K

 

Excellent!, thanks for sharing your fix, its what makes the forum work

 

I would have never guessed a radiator cap could rob a van of heat, but it must be a pressure related thing. I recently replaced the thermostat, upper and lower radiator hoses and coolant in my 1993 Chevy Van G20. Well worth the time and effort to insure some heat in this bitter cold winter weather.

 

I was just as surprised, and very happy to finally get the issue fixed. I was more worried about overheating the engine, but having reliable heat again is certainly a plus.