The GMC CCKW

Hi, can someone please tell me if the GMC cooling system is pressure system or not. And if it is what is the pressure. My chevy keeps dumping its water out the overflow when it starts to warm up, which I am putting down to a dodgey radiator cap which is not pressurising the system properly (unless any of you have any other suggestions as to the cause). Therefore I need a new cap for my Chevy which is a pressure system at 4lb. I have Jeep cap which fits but this is a 10lb cap. I can get a GMC cap, but need to know if it is a pressure system or not. Thanks

Yes, it is a pressure system - 4 PSI cap

I am sorry to inform you that you may have a blown head gasket or a crack in the head or the block....

They will sometimes blow off coolant when shutdown, but not while running unless they are low on coolant.

start the engine w/o the cap and watch for bubbles. That is a dead giveaway usually for one of the problems I described. Also check the oil for whitness.

Par 106.DESCRIPTION(TM9-801)page 220"Water is drawn from the bottom of the radiator by action of the water pump and is force-circulated through the passages and water jackets in the engine and returned,through the upper connection,to the radiator where it is cooled by the action of the fan drawing air through the radiator core.A pressure cap(on radiator) maintains a pressure of approximately four pounds in the cooling system when the engine is warmed up to normal operating temperature." Par 107.SERVICE(TM9-801)page 220"CAUTION:Do not pour cold water into cooling system when engine temperature is above 200º F.Also cold water poured into the cooling system,regardless of engine temperature,will close the thermostat and not allow the engine water jackets and passages to completely fill.Whenever filling system with cold water,always run engine until normal operating temperature is reached(thermostat opens),then add water until liquid level is visible through filler neck."