Coolant doesn't go back to radiator

[Ross]

I have a 2004 4Runner with the V6. The overflow reservoir fills up but won't return coolant to the radiator.

I have changed the cap but that didn't fix it. I will change the thermostat next.

Is there anything else that could cause this?

 

[RonJR]

I've not seen this issue before, but I would check the integrity of the tube from the radiator to the overflow tank. The radiator has to pull a vacuum on that line to pull coolant back in. Even a small crack or leak would cause the radiator to suck in air instead of coolant. Like having a small hole in the side of your soda straw.

The other possibility is a plugged vent hole on the overflow tank cap. That cap has to let air back in so the radiator can pull in coolant.

 

[AuSeeker]

I would also make sure you have the correct cap or the one you got isn't defective, cooling systems are pressure rated/sensitive and if you have the wrong cap that's maybe rated higher or it's defective it will not be "opened" at the correct time to let the coolant return to the radiator as it cools down.

I would suggest getting an OEM cap to be certain it works correctly with your T4R.

 

[Ross]

I've not seen this issue before, but I would check the integrity of the tube from the radiator to the overflow tank. The radiator has to pull a vacuum on that line to pull coolant back in. Even a small crack or leak would cause the radiator to suck in air instead of coolant. Like having a small hole in the side of your soda straw.

The other possibility is a plugged vent hole on the overflow tank cap. That cap has to let air back in so the radiator can pull in coolant.

If i take off the overflo cap i can blow air out to the radiator side but not where it would flow out of the reservoir to the ground. Should I be able to blow air out that side through the overflow?

 

[Ross]

I would also make sure you have the correct cap or the one you got isn't defective, cooling systems are pressure rated/sensitive and if you have the wrong cap that's maybe rated higher or it's defective it will not be "opened" at the correct time to let the coolant return to the radiator as it cools down.

I would suggest getting an OEM cap to be certain it works correctly with your T4R.

I got the new cap from Toyota. The old cap was also Toyota and In bad shape when I replace it.

 

[AuSeeker]

Might want to consider having a pressure test done on the cooling system and as others have suggested check the return line and passages to the reservoir.

 

[underscorepatrick]

Wanted to bump this.. any solution ?