Coolant leaking from exapnsion tank weep hole

I have 1st generation Toyota Echo/Vitz/Yaris which got a engine swapped (2NZ-FE) last year. From few days back the coolant from the breather hole of Coolant expansion tank is dripping. Replaced the Radiator cap but the issue still prevails.

Need advice on troubleshooting and identifying the root cause of this issue


Is the expansion tank pressurized? Or is the radiator cap on the radiator?

Yes radiator cap is on the radiator

So is the coolant just to the ‘full’ mark on the tank? If it’s getting up to the weep hole there must be a lot of coolant in the tank, or a bad head gasket is causing frothing in the tank. What’s going on in the tank when the engine is hot and idling?


There is a fill line HOT and a fill line COLD most of the time, you do not fill coolant in the overflow tank to the top when cold as when it heats up it can/will push the extra out… Had a few customers that keep talking about seeing a coolant leak that was only because they were overfilling the coolant overflow tank…

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That hole in the reservoir cap allows air to enter to replace the coolant that is drawn into the radiator after the engine is shut off, as the engine cools. And to allow the air to leave when the coolant in the radiator gets hot and the overflow goes into the reservoir.

Presuming the replacement radiator cap is good, either there’s a little too much coolant in the system, or the cooling system is getter pressurized beyond what it should, likely due to a head gasket problem. Try removing some of the coolant so the level in the reservoir is no more than half full when engine is warm. If no joy, ask your shop to open the radiator cap (cold engine), idle engine, as engine warms up, maybe rev engine a little, do they see a swarm of really small bubbles coming up through the coolant in the radiator?

Yes i filled the coolant to the mark which denotes, “Full.” At idling there is nothing in expansion tank neither bubbles nor any other thing.

No i did not overfilled the tank

As far as i observed the cap is functioning well (will check it again), checked the coolant level multiple times but it rises and starts leaking when its get to operating temperature.

I have followed that procedure to test the head gasket but no bubbles were coming out from it.

I suggest that you remove the radiator cap when the engine is stone cold, start the engine, and then watch for bubbles at the radiator filler.


Are you sure there is not supposed to be any in the exp tank? Those I know have a Min and a Max line. Once the cold radiator is filled, the tank then gets enough put in to reach the Max line. After a couple driving cycles, see if the rad and/or the exp tank need some topping up.

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Most overflow tanks (that I have seen) that had a fill line was for when it was at operating temp or Hot, not cold, if you fill it to the max line cold when it heats up it will be overfilled and leak out the extra coolant and make it look as it is leaking…


OP, suggest to carefully re-read the “how to check engine coolant level” in your owner’s manual. My truck has marks for both cold full and hot full on the plastic expansion bottle. My Corolla’s bottle has no mark. I fill the Corolla’s bottle to the point it is half-full when engine coolant is at normal operating temperature. When the engine is cold (the next morning) the coolant level is pretty close to the bottom of the plastic bottle; i.e. the bottle is nearly empty.

fyi, I’ve had a couple of problems with the coolant- expansion-bottle-system over the years. One problem, the tube between the bottle and radiator developed a small leak, no visible coolant loss, but it prevented the coolant from getting fully sucked back into the radiator after the engine was turned off. The second problem, the air vent clogged, which dissuaded the hot coolant from moving from the radiator to the bottle. Good idea to check that you have neither of those problems. Might be simpler to just replace the bottle and the tube with new stuff all-around, might get lucky.

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Did’nt saw any bubbles

Here’s my confusion point-if the coolant is only up to the Full mark, how’s it getting to the weep hole at the top of the tank?


I have replaced the coolant before the last winters, and the car did not make any problems. Now in the summers has arrived and this happened

No i did not overfilled above the marking

Yes my Toyota also don’t have any marking for cold and hot coolant, i changed the coolant last November but the problem arises this month.

I will check the bottle and hose for replacement, but please could you guide me about air vent of the radiator; where it is located?

That also confusing me, after every drive car expansion tank starts to leak coolant

In my reply you said (twice) that you didn’t overfill the tank… But then you just said you don’t have any marking for hot or cold???

What do you mean and better yet, if you don’t have any marking then how do you know you are not overfilling the overflow tank???

How about this, fill it up to your normal amount and then drive it till it leaks, let it cool completely down and make sure it still has some coolant in the bottom of the tank, then drive again with out adding any coolant, see if it leaks again, wait until it is completely cooled down again, check to see if coolant is still in the tank, if so do not add but drive again and see if it is still leaking and if coolant is still in the tank… If the coolant is still close to the bottom but not dropping any lower and the radiator is still full, then that is your full cold line… Again check only when 100% cooled down and make sure radiator is full…

That is only after making sure your overflow tank is not leaking and your radiator cap is sealing to the radiator inside the neck as well as the outer part of the neck…