In my 1990 Chevy 1500, 5.7 liter water is coming from radiator cap when truck is not running. The water is not pressurized and leaks back from the cap sometimes actually splashing from the cap when moving. This started suddenly when I checked the water level. It is not running hot and the heater works. Once the cap came off while driving as if it was not on securely . I have tried 4 caps two from other vehicles and two from the parts store. All do the same. What can do this? After awhile the water get low enough that the truck runs hot.

 

sounds like the cap is not seating/locking properly on the radiator neck. Did this 1st happen after removing the cap or just show up randomly?

 

check the lip of your radiator neck, sometimes it gets bent and doesn't allow the cap to properly seal