what causes a 94 previa to run hot as soona as you drive quarter mile/

...well there are a number of considerations...an OBD-II code would reveal nothing because I believe that there was a catastrophic overheat as witnessed underhood....the engine puked there and left coolant up in there~ refilled with hose water after they snipped off the blowin' hose and never drained it, put a new hose on and said "fukit" that's where we stand now~

You've got to drain the rusted coolant, flush or replace the radiator...flush the block look for additional leaks, put all BRAND NEW hoses on perhaps a new radiator (a 14 dollar spot thermal gun would tell you where the hot spots are)~ I'd walk slowly to the nearest EXIT then whistle a happy tune because you needn't deal with this creation....necessarily~

Like judge_roy say could be lots of different things like cracked head,head gasket,cracked block, intake gasket,thermostat,radiator,water pump. need to presure test and see. How many miles on it? may be worth more in scrape than it is to fix. Is there water in the oil?

plus does it have an electric cooling fan that could not be working all so. I think you would have to spend more than $600 to fix it. Personally I would not touch it

I drove a '73 Toyota Celica from Oakland to Los Angeles...would overheat...not sufficent flow....okay for goin' to the store and back, but this was a disaster as every twenty minutes had to let it cool and refill the coolant~ not a good scene, man~