overflow?

when i shut my truck off it starts throwing water into my overflow, and then it sprays right out of the overflows overflow tube. its causing me to lose water. it only does it when i shut my truck off what is the problem. i have a 13lbs cap on it, which is stock i believe. any help would be greatly appreciative. oh and the truck is running great.

IS the cap any good they do go bad. they could be bad right out of the box to so have it tested other than that what are your temps

When u stop the engine the water stops circulating and the temp goes up for awhile then cools.Four possibilities: Rad cap is defective as Pont says.Replace.Thermostat is not working properly and truck is getting too hot.Get proper Mazda two stage thermo (160/180) install with air bleed hole at 12 oclock.Rad is stopped up.Flush rad and replace coolant.Fan clutch is defective and not pulling enough cool air thru rad.CHECK FOR PROPER FAN CLUTCH OPERATION.BJ

the temps are good. i have a spare cap ill try and throw it on and see if there area ny changes.

my truck did the exact same thin gyesterday but only once