Why does coolant overflow from the reservoir and not return?
Coolant flows out into the overflow tank as the engine gets hot, but a cooling system leak or air pocket prevents a proper vacuum from forming as the engine cools down. Because the system cannot suck the coolant back into the radiator, the reservoir fills up and spills over while leaving the actual radiator underfilled.
What the numbers show
Counted from the threads quoted below. How we count
- People have been saying this in most months since September 2026.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
Why does coolant overflow from the reservoir and not return?
- 1pressure testing the radiatorDid this help you?
- 2replacing the radiator cap and thermostatDid this help you?
- 3performing a leak down test to check for a blown head gasketDid this help you?
- 4checking the rubber overflow hose for cracks or pinholesDid this help you?
- 5Testing the thermostat in boiling waterDid this help you?
- 6Bleeding or burping the system to remove air locksDid this help you?
- 7Using a vacuum filler to displace air from the motorDid this help you?
- 8Swapping the radiator and installing electric fansDid this help you?
- 9Replacing the radiator cap with one that has a proper pressure ratingDid this help you?
- 10Testing the water for CO2 gases to check for a head gasket failureDid this help you?
- 11Replacing the thermostatDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“VW crafter 2020 sy. I’ve replaced a hose that was leaking and refilled the system. but it’s not flowing through.”source ↗
“So every time i stop in my 1983 fj 45 the coolant over flow tank starts pissing coolant and i have no idea why i just swapped the radiator and swapped to electric fans does anyone know how to fix this the car is also starting to overheat after driving a short distance”source ↗
“I’ve put in 2.5 and it needs 14 but it’s full. Idled with heater on max and the temp went up half way so I turned it off”source ↗
“it has been having an issue where it doesn't suck the coolant in the overflow tank back into the radiator. It will just flow into the overflow tank every time, and overflow out of the tank.”source ↗
Where this came up
People with this problem also raised
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