Hi all just recently got an 89 4x4 sierra saph cos. it seems to have a problem overheating. The car does not overheat on tickover and the fan kicks in and keeps it at running temp only when on a run ie; in boost. the temperature still remains hi even during a run down the dular as in the middle of N verging the top line. the car never done this when i first purchased it. have recently done a cambelt and replaced the bottom pulley wonderd if the ignition timing could cause this problem? have checked the usual stat ok blead the air out the system, pump appears to be operating fine. there is no oil in water water in oil. should mention that the expansion bottle cap was leaking till i replaced it seemed new one seems fine now. also thot that the head gasket might be on its way out as in it is only letting pressure into the coolant when there is more pressure in the cylinder from the turbo. perhaps a leak in the gasket acting as a 1 way valve when the pressure inreased causing the csr to over heat. any ideas or help would be greatly appreciated. phill