Unpressurized Cooling System - Aftermarket Radiator Filler Neck Out of Spec

Coolingbeginner✓ OP Confirmed

Problem

Aftermarket replacement radiator (MTX brand) had an out-of-spec filler neck that prevented the radiator cap from sealing, leaving the cooling system unpressurized for 6 years. The issue became critical when A/C use caused coolant to overflow from the expansion tank.

Coolant bubbling and gurgling like a water heater after engine shutoffUpper radiator hose felt soft/squishy even when engine was hotStumbling and rough running for a few seconds on hot startsBirdcage warning light coming onExpansion tank overflowingA/C condenser fans running excessively

Root Cause

The aftermarket radiator filler neck's lower sealing ledge was at 16.5-17.5mm depth (with a 1mm slope across it) instead of the correct 15mm. The radiator cap plunger extends to 16.5mm, leaving a 1mm+ gap that prevented any pressure seal. The cooling system was effectively open to the overflow bottle at all times. Cap and radiator both passed bench pressure tests because the test fixture had correct dimensions, masking the defect.

Confidence

High confidence — Original poster confirmed this resolved the issue

View Original Thread →Posted July 2021 on SupraForums.com