radiator blows antifreeze - out
I have a 95 mazda millenia.
I changed the radiator.
I filled it with antifreeze 50/50
I let it run, top it off, let it run, top it off etc...
After a few days with no problem, all the
antifreeze blows out the radiators' over flow.
Why? Help!

Several possibilities.

1. Cooling fan not coming on for some reason - engine overheats, coolant turns to steam,...
2. Leaking head gasket - ports cylinder pressure into cooling system - cooling system only designed to handle 13-15 psi.
3. Bad thermostat (least likely, but possible).
4. Radiator cap defective.
5. Something else...?

You need to take it to a mechanic or dealer you trust and have them check it out so you don't damage a good engine.

LOL

1. MOST likely (and the cheapest to fix) a stuck CLOSED thermastat.
A stuck CLOSED thermastat closes the passage TO the radiator, causing the Water/Anti-Freeze to excessivly heat up in the block due to NO circulation and the fluid reverses it's flow.

END RESULT is OVER HEATING AND RADIATOR OVER FLOW.

Think of the system as a closed system as it REALLY is.

Water flows because of the water pump pushing the water thru the enigine out the bottom of the block into the radiator, thru thr radiator cooling fins and out, into the top of the engine and once again the cycle continues.

At the top of the engine, where the top radiator hose meets the engine block there is a valve (THE THERMASTAT). The thermastat assy. has a spring and a plate pluging a hole. When the fluid is COLD the valve is closed, covering the passage. As the fluid heats up and pressure builds up behind the spring is great enough the spring compresses and fluid is allowed to travel thru, hense the cooling process.

This is a defeated when water mineral deposits collect on the valve and fuse the valve closed, hense NO cooling of fluid and OVERFLOW happens.

THIS ALSO APPLIES TO THE RADIATOR CAP.

The cheapest way is to replace BOTH the RADIATOR CAP and the THERMASTAT.

If the problem persists then the impelers (fins) on the water pump are NO longer spinning to cause fluid flow. Thus raising the fluid tempature and OVERFLOW.

WATERPUMP REPLACEMENT/SERVICE CHARGE EXPENSIVE, try the other way 1st.

WOW! what a decisive answer, and you may be right, but easiest to fix? Just ask anyone who has changed a 929 thermostat or tried to! No the water doesn't go into the bottom of the radiator, cool off and go back into the motor at the top past the thermostat! It goes the other way and is trapped in the engine by the thermostat untill it starts to open which is controlled by heat NOT pressure.I think we've had this discussion before as to the direction the water flows and I just can't agree with the reverse flow theory.However, if the thermo is easy to get to then it can be removed simply and tested by boiling a saucepan of water and putting the thermostat in and watch to see if the inner plate opens then fish it out and it should slowly close. If it does it's OK if it doesn't get a new one! But besides all that, has the vehicle got a temp gauge, if yes then if its the thermostat it should get hot shortly after start up or if it's the cap or any thing else it will get hotter the more water it ejects! Go with the first response and take it to a mechanic you can trust and if it's a blown h/gskt or a cracked head he can do a simple test to find out, before he spends all your money looking!