mate of mine has just had his engine built only done 100 miles and it over heats. it will sit idling all day long no problem and will drive for around 40 miles then temp rises and over heats, its a 1994 wrx he's had a sniff tst and comp test done and both ok.

 

could be as simple as an air lock,
thats where to start mate.
Shaun.

 

yeah we tried that and stat got us stumped at min

 

are you loosing any water,
check you havn't got a hole in the hose to the expansion tank
as this wont replenish your water system.
Shaun.

 

will check tomorrow after mate borrowed pressure tester from work to see if its leaking water/ drawing air in somewhere

 

hope its a cheap fix mate.
good luck.
Shaun.

 

pressure tested cooling system and found small leak near turbo. hopefully it just that