7MGTE Overheating & Steam from Exhaust - Blown Head Gasket Replacement with OEM Composite Gasket

Engineadvanced✓ Community Verified

Problem

7M-GTE with aftermarket HKS 1.2mm MLS head gasket (installed by previous owner, suspected improper block surface prep) began overheating intermittently during highway driving. Temperature would spike up then cool back down. Steam visible from exhaust with condensation forming on rear bumper even after engine was fully warmed up. Coolant overflow bottle was overflowing.

Intermittent overheating - temperature spikes up then cools back downSteam from exhaust even when engine is fully warmed upCondensation forming on rear bumper above exhaustCoolant overflow bottle overflowingNeeded to add approximately 1/2 gallon of coolant after 35-mile driveOil appeared clean with no coolant contamination

Root Cause

Failed MLS (multi-layer steel) head gasket, likely due to block deck surface not being properly prepared/honed when the aftermarket HKS MLS gasket was originally installed. MLS gaskets require very smooth, properly machined surfaces to seal correctly. The gasket can survive for a long time even when not fully sealing, leading to intermittent symptoms. Additionally, half of the valves were found to not be sealing properly during the head rebuild.

Confidence

Moderate confidence — Multiple community members agreed on this solution

View Original Thread →Posted March 2022 on SupraForums.com