Coolant Overflow Tank Not Siphoning Back - Swollen Hose on Cap Nipple

Coolingbeginner✓ Community Verified

Problem

Coolant overflow (puke) tank level continues to rise drive after drive, with coolant not being siphoned back into the radiator as the engine cools down. In severe cases, coolant spews out of the top of the overflow tank.

Coolant overflow tank level keeps rising after each driveCoolant not being drawn back into radiator when engine coolsCoolant spilling out of overflow tank onto the groundIn severe cases, coolant spewing from the top of the overflow tank

Root Cause

The small hose (straw) fitted to the overflow tank plastic cap has swollen or split over time and no longer makes a good seal with the nipple under the cap. This prevents the vacuum siphon from drawing coolant back into the radiator as the engine cools. Can also be caused by a bad radiator pressure cap that does not seal properly.

Confidence

High confidence — Multiple community members agreed on this solution

View Original Thread →Posted February 2026 on SupraForums.com