I have a Toyota Noah 2005 and after driving,hot water fills the coolant tank form the radiator hence always emptying the coolant tank and adding more to the radiator.Would like to know the solution to these problem.

  • 1
    It could be normal. Do you note that the coolant gets sucked back into the radiator if you leave the engine to cool?
    – Caius Jard
    Oct 11, 2019 at 17:56
  • Are you saying you deliberately empty the overflow tank?
    – mike65535
    Oct 12, 2019 at 1:52
  • Well it used to behave like that,coolant sucked back to the radiator but stopped forcing me to suck some every day before starting my ride.
    – Thomas
    Oct 15, 2019 at 11:57

2 Answers 2


Could be a number of problems but from simplest to most complicated:

1) Bad radiator cap: The cap is supposed to onto vent coolant when the pressure exceeds the system limit. If it's bad it may allow coolant to leak out at a lower pressure. Try a new cap.

2) Bad thermostat: Can be stuck closed or nearly closed and this overheats the coolant in the engine and it pushes water out into the overflow tank.

3) Bad water pump: Usually you'll see other signs such as a HOT engine indicator but this will generally allow the coolant to get too hot, generate too much pressure and vent the coolant.

4) Similar to #2, but a clogged or failed upper or lower hose will keep enough coolant from flowing and cause overheating. It's not uncommon for the layers of the hose to separate and the inner part can collapse and block the flow of coolant. A quick check is to squeeze them and if they feel "squishy" vs. "firm" there may be a problem. To be sure, remove the hose and inspect.

5) Bad fan or fan thermostat: Doesn't turn on when it should and the engine overheats and vents coolant.

6) Clogged radiator also causes overheating and coolant venting.

7) Head gasket failure: Allows compression gasses from the cylinder to enter cooling system, pressurizing it and forcing coolant out. The head gasket seals the very high pressure combustion chamber against leaking and prevents oil and coolant, which are also present nearby in the block and cylinder head. If a part of the gasket fails it can allow highly pressurized gasses from the combustion chamber to get into the cooling jacket. From there is has noplace to go but out the radiator cap and into the overflow tank.

  • Advice noted,Would like more clarity on point number 4 and 7.
    – Thomas
    Oct 11, 2019 at 12:51
  • @Thomas I updated with additional comments.
    – jwh20
    Oct 11, 2019 at 17:48

Firstly, please, understand that overflow tank (what you called a coolant tank) and radiator is a system. They're meant to work together. Expanded hot coolant is meant to return to the overflow tank from the radiator when the engine is hot.
1. If you are really using water instead of coolant - that may be your problem. It boils faster than a coolant, hence, expands.
2. Stop adding water/anything to a radiator. This is what an overflow tank is for. It has markings. Please, check the levels of your coolant when the engine is cold, ideally it should be between min and max for most cars/climates/engine loads.

  • Well noted though in my case,water flows from the radiator to the overflow tank and the following day before staring the engine,level in the radiator is low hence i add more water and suck some from the overflow tank to maintain the level.Ive been doing that for months after constantly changing the radiator lid.
    – Thomas
    Oct 15, 2019 at 11:45
  • That is the radiator cap.
    – Thomas
    Oct 15, 2019 at 11:47

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