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 people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1pressure testing the radiator
- 2replacing the radiator cap and thermostat
- 3performing a leak down test to check for a blown head gasket
- 4checking the rubber overflow hose for cracks or pinholes
- 5Testing the thermostat in boiling water
- 6Bleeding or burping the system to remove air locks
- 7Using a vacuum filler to displace air from the motor
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 ↗
“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 ↗