Radiator blockage? Radiator flooding filling reserve reservoir tank with water. Pressure build up almost instant. Pushes water out of radiator when radiator cap off. Replaced thermostat but no result. Any ideas on where a blockage is?

When water gets heated, pressure builds up in the radiator and water vapor is sent to the reserve tank.
And vapor dilutes into the water in the reserve tank.
When the engine cools down, in the radiator, vacuum builds up. To fill the vacuum water is pull from the reserve tank.
If vacuum leaks from radiator, gasket, etc., reserved tank won't able to function

replace the faulty reservoir or radiator
what is happening is that the radiator cap is not raising the pressure in the cooling system because of a leak in the system
the excess water from heating is being pushed out probably onto the ground through the leak
when the cooling system cools down , water is being drawn back into the radiator
probably find a crack in the radiator tank that opens under pressure but seal off as the pressure is lost

Yes, the radiator should be checked first for anti-freeze level. If it is low, top it off and pour remaining fluid into the reserve tank to the proper level. Re-check after driving and then you may just fill the reservoir only if you like.

there is either a pocket of air or the car is burning the coolant if there is no leak. check inside the car for any coolant leaking under the dash if so the heater core has a hole in it.

Did you refill the radiator and reserve tank after the work? If not, the radiator will suck water in from the reserve tank to top the radiator off.

I would think you have a badly leaking engine cylinder head gasket. Have a shop test for that.

Duplicate question.

• There are two hoses on an Antifreeze reservoir tank.
• One is to allow escape in case of unexpected back pressure or overfill of the reservoir.
• The other will be a hose that clamps on to the radiator right below the cap on a little spout coming off the fill neck of the radiator. This hose is the actual path the antifreeze takes when the radiator is low pressure pulls the antifreeze out of the reservoir to use.

• Hope this helps and thank you for using FixYa.

Regards, Tony

fill the rad cold and the reserve tank to the cold mark, i suspect the reserve tank is leaking

fill the reservoir with the coolant. it is a GOOD thing you dont see steam or a leak I hope.YOU DO WANT TO CATCH THE PROBLEM BEFORE IT REALLY OVERHEATS OR YOU MAY HAVE MORE COSTLY PROBLEMS. fill the reservoir. BUT, it may only fill the "radiator" once you do this as it cools off. As the radiator water cools it sucks the reservoir water into the radiator. as it heats up and the water swells (increases in volume) it pushes the excess into the reservoir. when the engine is good and hot and you dont have any coolant in the reservoir, that is a sign that the radiator is low or empty also.
when the engine is cold, find the radiator cap and remove it, add as much coolant you can there. fill the reservoir also. If it has been several years since the radiator cap has been replaced, I would do that now ( it is a cheap investment). if the gasket on the cap is not sealing well anymore or the exspansion valve mechanism on the cap is not working properly anymore, then it will let coolant pump out into the reservoir and not hold radiator pressure. if the reservoir is empty the radiator itself is probably too low or empty.