97 gmc safari van, heating and fuel problem?

engine fully warmed up was getting luke warm heat at idle, raise idle even a little heat gets nice and hot. changed thermostat, coolant , flushed . heat was good for a day, now same as before . temperature guage shows engine is running at normal temp. also on full tank of fuel i was getting 5 to 6 hundred kilometers , which i think is average . now im getting 250 kilometers on full tank . full tank is 75 liters , hundred dollars. this van is killing my wallet! please help
is there a valve on coolant hoses that coul be getting stuck?
gas milelage is half of what it was
 

TOOLMAN

New Member
possible bad head or head gasket have it checked by mechanic.

when radator cap is cold to touch check to make such coolant is full.
 
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