So, it is -20 C outside, and my van is overheating. It is a puzzle for the following reasons. After it started to overheat , I have a) flushed the system, b) filled and run Block-seal through the block to seal any leaks c) replaced thermostat d) replaced sensor e) flushed lower rad hose (which does not seem to drain when using the rad drain), f) refilled system. I then run the car for 20 mins, and system is fine - both upper and lower hoses, and rad, are all warm. Then, after car sits for a few hours (in -20c temp), it overheats the next time it is run. When it overheats, the lower rad hose is cold (except for a few inches from the engine), and the rad is ice cold. The upper hose is hot, but I think this is just the coolant moving to the rad from the engine due to convection - not from circulation, as the rad and lower hose are both cold. If coolant were circulating, the lower hose would have to be hot, correct? It is as if sometimes the system is blocked, and the coolant cannot circulate, and other times it is fine. The coolant is good to the temp - it is not freezing. Can a waterpump fail intermittently? Is there anyway to test the waterpump's effectiveness? Aghhhh!
