What dose it mean when water is coming up through the radiator cap

Hi,lets start at the begining,resons for this can be,firmastat not working properly,fan switch not working properly so fan dose not switch on,waterpump not working properly so not pumping water around properly,radiater got sludge in it so is not cooling properly,or headgasket broken,
first start engine and wait for car to start up then when the tempreture gage gose up to normal running temp lisson for the fan if it dosnt come on proberly temp-switch for the fan,if fan dose come on switch engine off and check that the hoses to the rad are bouth hot,if that is ok,wait for the car to cool down a bit so you can take off the exspantion tank-cap put a cloth or some thing over the cap as there is a lot of presure and hot water,so be carfull not to burn yourself,
when done start car and look to see if the water is moveing around freely if so then the pump must be ok swich engine off,last one take of the oil-filler-cap and look at the oil it should be brown in coulor,if its a musterd coulor then it means the head-gasket has gone,well although ive sied all this thay have a tool on the market wich the garage use to pump presure in the water system wich shows up eney leeks,if you are loseing water apart from the bubbling water that comes out of the exspantion tank that will show it up,I wish you the best and hope its one of the miner jobs.

Also inspect the radiator pressure cap.

I suspect a crack in the head or block. The cooling system is suppose to be closed so any air has to be coming from somewhere - if the head gasket is good then the air is likely to be coming from a crack. A pressure test will confirm if your system is not air tight. That is if you are seeing bubbles in the radiator water if you just mean you are losing water through the overflow then it might just be the radiator cap needs replacing as it is not sealing the system when it gets hot.

It may be a bad radiator cap or the top radiator hose is loose or worn out. Water may be coming out as steam which is difficult to trace. If you have white smoke at the back or check your engine oil for water content, your head gasket may have gone bad. If you have coolant mix that at 50/50 ratio into the radiator. It is colored green or pink and should be easier to trace to make sure the leaks are not in the other parts of the cooling system. It could be on the other hose, radiator or the engine.

You say water, do you actually mean coolant?
Or are you putting straight water into it?
You may need to check the mixing level.
It should have a mix of 50/50.
50% water and 50% antifreeze/coolant.
Lot's of coolants will come pre-diluted, which is usually always the best way to go.

I hope this helps!

Until you get it figured out use regular water. Replace thermostat and radiator/coolant tank caps. Thermostat regulates pressure in the motor and allows radiator to cool. Caps regulate atmosperic pressure. Either or both being bad will cause you system to siphon and fail.

Yes, these days when someone says radiator cap they mean the cap on the coolant overflow tank.
Hope this was helpful.

First it means get a new radiator cap, When the spring goes bad it releases at a lower pressure. You may need a new thermostat as yours is probably stuck open and the water pump just throws the water through.

Check your radiator cap for rust. Replace that.

water pump