4kq overheat problem

[ben swann]

Patient is '85 4000 quattro experiencing strange overheating problem.

I noticed earlier around January that the radiator seemed to stay cool
at the bottom and never kick on the fan even when head temps approach
overheating.

I have replaced - Radiator fan switch, thermostat, radiator, and now
water pump which was good afterall.  After spending way more time on
this than I ought, I'm  confused and frustrated at what is going on.

It seems that the coolant is not flowing at all even though it should
be being pumped by the water pump.  I even removed the thermostat and
the problem took longer to happen, but still results in overheating
with the fan not kicking on - radiator stays cool at the bottom, but at
top boiling hot and developing high pressure.  The hose coming off the
thermostat housing gets hot coming out of the block, but is cooler down
toward the radiator intake, and radiator nice and cool at the bottom.

I ran water from hose in both directios through the different
radiators - they don't seem blocked.  I ran hose water through both
directions in the engine, there seems to be no blockage.  The radiators
supposedly are good used radiators and they seem to have good flow
through with a hose and no noticable scaling inside.

I'm at my wits end on this one.

Ben