Cooling System puzzle - 96 Caravan 3.0

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!

 

Sounds like you have air in the cooling system. THe 3.0 is notorious for a
couple things. 1. Head gaskets. Many times they go in a way that allows
combustion gas INTO to coolant with no visible signs i.e. oil in water or
vice versa or visible leaks. 2. They are extremely hard to bleed. You must
run it with the cap off at a faster idle (1500 or so) and allow the
thermostat to open and run a while until it quits "burping". Outside chance
you might have a hose collapsing too.
Larry

 

I had this happen years ago when my coolant "slushed up" & clogged the
radiator. I was driving, the heater went cold and the temperature gage
went way up. But if you are sure the coolant is good to the temperature
(what is that Farenheit, anyway? -10?), then that's not it.

Joseph Osborne